Chapter one

Fundamentals of Cutting

1.1 Introduction

Parts manufactured by casting, forming, and shaping processes, often require further

operations before the product is ready for use. Moreover, in many engineering applications, parts

must be interchangeable to function properly and reliably during their expected service lives, as

in case of automobile parts. Thus we have to obtain certain dimensional accuracy.

Machining is the broad term used to describe removal of material from a workpiece in the

form of chips. In terms of annual dollars spent, machining is the most important of the

manufacturing processes. Machining is necessary where tight tolerances on dimensions and

surface finishes are required.

We can summarize why material-removal processes are desirable or even necessary in

manufacturing operations as follows:

Closer dimensional accuracy may be required than is available from casting, forming, or

shaping processes alone;

Parts may have external and internal profiles, as well as sharp corners and flatness, that

cannot be produced by forming and shaping processe;

Some parts are heat treated for improved hardness and wear resistance. Since heat treated parts

may undergo distortion and surface discoloration, they generally require additional finishing

operations, such as grinding, to obtain the desired final dimensions and surface finish.

Machining the part may be more economical than manufacturing it by other processes,

particularly if the number of parts desired is relatively small.

Against these advantages, material-removal processes have certain limitations:

Removal processes inevitably waste material and generally require more energy, capital, and

labor than forming and shaping operations. Thus they should be avoided whenever possible.

Removing a volume of material from a workpiece generally takes longer time than it does to

shape it by other processes;

Unless carried out properly, material-removal processes can have adverse effects on the surface

quality and properties of the product.

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1.2 The mechanism of cutting

Cutting processes remove material from the surface of a workpiece by producing chips. In

order to analyze this process in detail, Fig.1.1 presents the basic geometry of two-dimensional chip

formation. The model is two-dimensional for simplicity.

Fig.1.1 Schematic illustration of a two dimensional cutting process (orthogonal cutting).

The material immediately in front of the tool is bent upward and is compressed in a narrow

zone of shear which is shaded on the drawing above. For most analyses, this shear area can be

simplified to a plane.

As the tool moves forward, the material ahead of the tool passes through this shear plane.

If the material is ductile, fracture will not occur and the chip will be in the form of a continuous

ribbon. If the material is brittle, the chip will periodically fracture and separate chips will be

formed. It is within the shear zone that gross deformation of the material takes place which allows

the chips to be removed.

The chips are produced by the shearing process and that shearing takes place along a shear

zone, which is usually reffered to as the shear plane. Below the shear plane the workpiece is

undeformed, and above it is the chip, already formed and moving up the face of the tool as cutting

progresses.

Next we can begin to consider cutting forces, chip thicknesses, etc. First, consider the

physical geometry of cutting (Fig.1.2):

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Fig.1.2 Schematic illustration of the basic mechanism of chip formation in cutting.

where,

t1 = undeformed chip thickness;

t2 = deformed chip thickness (usually t2 > t1);

= tool rake angle.

If we are using a lathe, t1 is the feed per revolution.

In the Fig.1.3 are illustrated basic types of chips produced in metal cutting.

Fig.1.3 Basic categories of chips.

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Continuous chips (Fig.1.3 -1) are usually formed with ductile materials at high cutting speeds

and/or high rake angle.

A built-up edge (BUE) may form at the tip of the tool during cutting (Fig.1.3 - 4). This edge

consists of layers of material from the workpiece that are gradually deposited on the tool (hence

the trem built-up). As it become larger, the BUE becomes unstable and eventually breaks up. Part

of the BUE material is carried away by the tool side of the chip; the rest is deposited randomily on

the workpiece surface. The process of BUE formation and destruction is repeated continuously

during the cutting operation.

A built-up edge, in effect, changes the geometry of the cutting edge. Because of work

hardening and deposition of successive layers of material, BUE hardness increases significantly.

Although BUE is generally undesirable, a thin, stable BUE is usually regarded as desirable

because it protects the tool’s surface and reduce wear.

As cutting speed increases, the size of the BUE decreases or, it doesn’t form at all. The

tendency for BUE to form is also reduced by decreasing the depth of cut, increasing the rake

angle, and using a sharp tool and an effective cutting fluid.

Serrated chips (also called segmented or nonhomogeneous chips), are semicontinuous chips,

with zones of long and high shear strain (Fig.1.3 -5). Metals with low thermal conductivity and

strength that decreases sharply with temperature, such as titanium, exhibit this behaviuor. The

chip have a sawtooth like appearance.

Discontinuous chips, consist of segments that may be firmly or loosely attached to each other

(Fig.1.3 -6). Discontinuous chips usually form under the following conditions:

- brittle workpiece materials, because they do not have the capacity to undergo the high shear

strains developed in cutting;

- workpiece materials that contain hard inclusions and impurities or have structures such as

graphite flakes in gray cast iron;

- very low or very high cutting speeds;

- large depths of cut and low rake angles;

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1.3 Chip breakers

As stated earlier, long chips are undesirable because they tend to become entangled and

interfere with cutting operations and can become a safety hazard. This situation is especially

troublesome in high-speed automated machinery and in untended machining cells using computer

numerically controlled machines. If all the independent machining variables are under control, the

usual procedure to avoid this situation is to break the chip intermittently with a chip braker

(Fig.1.4).

Fig.1.4 Chipbreakers, as part of the insert geometry, are designed to work at different feed/depth of cut areas.

Chip can also be broken by changing the tool geometry, thus controlling chip flows, as in the

turning operations shown in Fig. 1.5.

Fig.1.5 Chips are broken on their own accord (A), against the tool (B) or against the workpiece (C).

In interrupted cutting operations, such as milling, chip breakers are generally not necessary,

since the chips already have finite lengths resulting from the intermittent nature of the operation.

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In drilling and boring, chip control is vital because of the limited space inside holes being

machined. Also in modern high-performance drilling, chips have to be of exact form so as to be

evacuated efficiently from the cutting zone - any congestion, quickly leads to tool breakdown.

1.4 Force calculations

Knowledge of the forces and power involved in cutting operations is important for the

following reasons:

Power requirements must be known to enable the selection of a machine tool with adequate

power;

Data on cutting forces is required for:

a) The proper design of machine tools to avoid excessive distortion of the machine

elements and maintain the desired tolerances for the finished part, tooling and toolholders, and

workholding devices.

b) To determine, in advance of actual production, if the workpiece is capable of

withstanding the cutting forces without excessive distortions.

The forces acting on the tool in orthogonal cutting are shown in Fig1.6.

where,

Fc = cutting force;

Ft = tangential force;

R = resultant of Fc and Ft.

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Fig.1.6 Forces acting on a cutting tool in two dimensional cutting.

where,

F = friction force between tool and chip;

N = normal force between tool and chip.

The cutting force Fc acts in the direction of the cutting speed and supplies the energy

required for cutting. The tangential force Ft acts in the direction normal to the cutting velocity,

that is, perpendicular to the workpiece. These two forces produce the resultant force R.

The resultant force can be resolved into two components on the tool face: a friction force

F along the tool-chip interface, and a normal force N perpendicular to it.

The forces and angles involved in cutting are drawn below (Fig.1.7):

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Fig.1.7 Forces and angles in cutting mechanism.

where,

Fs = shear force;

Fn = force normal to shear plane;

= tool rake angle (positive as

shown);

= friction angle.

The resultant force is balanced by an equal and opposite force along the shear plane and

is resolved into a shear force, Fs , and a normal force, Fn.

The shear force Fs can be expressed as follows:

Fs =Fccos -Ftsin (1.1)

and

Fn = Fcsin +Ftcos (1.2)

where is the shear angle, and is tool rake angle.

The resultant force is:

F = Ftcos +Fcsin (1.3)

The ratio of F to N is the coefficient of friction at the tool-chip interface, and cab be expressed as:

= (1.4)

where

N = Fccos - Ftsin (1.5)

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A final note, the forces Fc and Ft, are used to find R, from that two other sets of equivalent

forces are found:

(1.6)

The coefficient of friction in metal cutting generally ranges from about 0, 5 - 2, 0, thus

indicating that the chip encounters frictional resistance while moving up the face of the tool.

Although the magnitude of forces in actual cutting operations is generally on the order of a

few hundred newtons, the local stresses in the cutting zone and the pressures on the tool are very

high because the contact areas are very small. The chip-tool contact length, for example, is

typically on the order of 1 mm. Thus the tool is subjected to very high stresses, which lead to wear,

and sometimes chipping and fracture of the tool.

1.5 Power consumed in cutting

There are a number of reasons for calculate the power consumed in cutting. These numbers

can tell us how fast we cut, or how large the motor on a machine must be. Power is the product of

force and velocity. The power input in cutting is (Vc is the cutting velocity [m/min]):

(1.7)

This power is dissipated mainly in the shear zone (because of the energy required to shear

the material) and on the rake face (because of the tool-chip interface friction).

The Metal Removal Rate (mrr) is:

(1.8)

where, A0 is the Area of Cut.

From these basic relationships we can a simple relationship that is the ratio between the

energy consumed, and the volume of metal removed,

(1.9)

This result is a force over an area, which is a pressure. As a result ps will be called the

Specific Cutting Pressure.

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The cutting force will vary, thus changing ps, as the cutting velocities are changed (Fig.1.8)

Fig.1.8 The cutting forces according with the cutting velocities.

This curve turns downward for two reasons:

1. The tool experiences edge forces that are more significant at lower cutting speeds;

2. As the velocity increases, the temperature increases, and less energy is required to shear the

metal.

Tool hardness is degraded by temperature, as shown in the diagram below:

1.6 Temperatures in cutting

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As in all metalworking operations, the energy dissipated in cutting operations is converted into

heat which, in turn, raises the temperature in the cutting zone. Knowledge of the temperature

rise in cutting is important because:

The rise in temperature adversely affects the strength, hardness, and wear resistance of

the cutting tool;

Increased heat causes dimensional changes in the part being machined, making control

of dimensional accuracy difficult;

Heat can induce thermal damage to the machined surface, adversely affecting its

properties;

There are three main sources of heat when cutting (Fig.1.9):

1. Heat is produced as the tool deforms (works) the metal;

2. Friction on the cutting face;

3. Friction on the tool flank.

 

Fig.1.9 Heat source in orthogonal cutting.

Heat is mostly dissipated by:

1. The discarded chip carries away heat;

2. Coolant will help draw away heat;

3. The workpiece acts as a heat sink;

4. The cutting tool will also draw away heat.

** factors 1 & 2 dissipate 75 to 80%; factors 3 and 4 dissipate 10% each.

1.7 Tool life: Wear and failure

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Tool life is the time a tool can be reliably used for cutting before it must be

discarded/repaired. Some tools, such as lathe bits are regularly reground after use.

A tool life equation was developed by Taylor, and is outlined below:

(1.10)

where,

V = cutting velocity in m/min;

T = tool life in minutes;

n = a constant based on the tool material;

C = a constant based on the tool and wear.

For example, if we are turning a 25 mm bar, and we have a carbide tool, we want to have the tool last for 1 shift (8

hours) before a change is required. We know that for carbide tools n = 0, 2, and when the bar was cut with a velocity of

120 m/min, the tool lasted for 2 hours. What RPM should the lathe be set at?

First find the C value for the equation,

Next, find the new cutting speed required,

Finally, convert cutting velocity to RPM,

Fig.1.10 Relationship between cutting speed and tool life.

An important relationship to be considered is the relationship between cutting speed and

tool life.  This function can be plotted on log scales as a linear function (Fig.1.10).

 We can find the slope of the line with a two point interpolation:

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Some examples of values are, (note that this is related to “n”):

- High Speed Steel Tool: n = 0, 10 to 0, 25;

- Carbide tool: n = 0,125 to 0, 25;

- Ceramic tool: n > 0, 25.

Although the previous equation is fairly accurate, we can use a more complete form of

Taylor's tool life equation to include a wider range of cuts.

(1.11)

where,

d = depth of cut:

f = feed rate;

x, y = calculated constants.

   Tool wear is still a significant problem in cutting. This wears controls tool life, and will change

work dimensions. Typical types of tool wear include:

1. Flank wear;

2. Crater wear.

 Flank wear – the point of the tool degrades (Fig.1.11).

 

Fig.1.11 Flank tool wear. 

Crater wear also decreases tool life (Fig.1.12). 

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Fig.1.12 Crater tool wear. 

Tool failure can typically grouped under one of the following categories:

1. Complete Failure - the tool is unusable;

2. Flank Failure - this can be estimated with maximum lw values:

a) Roughing Cuts:

- 0,76 mm for carbide tools;

- 1, 52 mm for high speed steel.

b) Finishing Cuts:

- 0,25 mm for carbides;

- 0, 38 mm for high speed steel.

3. Work surface finish is inadequate;

4. Work dimension outside tolerance;

 Flank wear can be discussed as a function of time (Fig.1.13),

where, V1, V2, V3 = cutting velocities

where V3 >V2>V1; #1 – In this region the tool

point is starting to dull; #2 – A typical tool wear

region; #3 – This zone is temperature

sensitive.

Fig.1.13 Flank wear as a function of time.  General notes of concern are:

1- The main factor in tool wear is temperature;

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2 - The main factor in tool life is cutting speed;

3 - Critical temperatures for High Speed Steels are 1150°F and for carbides it is 1600°F;

4 - A higher velocity will increase temperature more than an increase in feed for the same mrr;

5 - A higher feed will increase the tool forces.

1.8 Cutting tool materials

These materials generally need to withstand high temperatures, high forces, resist corrosion,

etc. The list below shows some commercial tool materials:

 

1. CBN - Cubic Boron Nitride;

2. Ceramic;

3. HSS - High Speed Steel;

4. PCD - PolyCrystalline Diamond;

5. WC - Tungsten Carbide;

6. Coated WC - Tools coated with Tungsten Carbide.

 

1.8.1 A short list of tool materials

1. Carbon Steels

- Limited tool life. Therefore, not suited to mass production;

- Can be formed into complex shapes for small production runs;

- Low cost;

- Suited to hand tools, and wood working;

- Carbon content about 0.9 to 1.35% with a hardness about 62°C Rockwell;

- Maximum cutting speeds about 7, 8 m/min. dry;

- The hot hardness value is low. This is the major factor in tool life.

 

2. High Speed Steel

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- An alloyed steel with 14-22% tungsten, as well as cobalt, molybdenum and chromium, vanadium;

- Appropriate heat treating will improve the tool properties significantly (makers of these steels

often provide instructions);

- Can cut materials with tensile strengths up to 75 tons/sq.mm, at speeds of 15-18 m/min;

- Hardness is in the range of 63-65°C Rockwell;

- The cobalt component gives the material a hot hardness value much greater than Carbon Steels;

- Used in all type of cutters, single/multiple point tools, and rotary tools.

 

3. Stellite

- A family of alloys made of cobalt, chromium, tungsten and carbon;

- The material is formed using electric furnaces, and casting technique, and it cannot be rolled, or

worked;

- The material has a hardness of 60-62°C Rockwell without heat treating, and the material has good

hot hardness properties;

- Cutting speed of up to 25- 30 m/min can be used on mild steels;

- The tools that use this method either use inserts in special holders, or tips brazed to carbon steel

shanks.

 

4. Tungsten Carbide

- Produced by sintering grains of tungsten carbide in a cobalt matrix (it provides toughness);

- Other materials are often included to increase hardness, such as titanium, chrome, molybdenum,

etc.

- Compressive strength is high compared to tensile strength; therefore the bits are often brazed to

steel shanks, or used as inserts in holders;

- These inserts may often have negative rake angles;

- Speeds up to 90 m/min are common on mild steels;

- Hot hardness properties are very good;

- Coolants and lubricants can be used to increase tool life, but are not required;

- Special alloys are needed to cut steel.

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5. Ceramics

- Sintered or cemented ceramic oxides, such as aluminum oxides sintered at 1800°F;

- Can be used for turning and facing most metals, except for titanium. Mild steels can be cut at

speeds up to 450m/min:

- These tools are best used in continuous cutting operations;

- There is no occurrence of welding, or built up edges;

- Coolants are not needed to cool the workpiece;

- Very high hot hardness properties;

- Often used as inserts in special holders.

 

6. Diamonds

- A very hard material with high resistance to abrasion;

- Very good for turning and boring, producing very good surface finish;

- Operations must minimize vibration to prolong diamond life;

- Also used as diamond dust in a metal matrix for grinding and lapping. For example, this is used

to finish tungsten carbide tools.

 

7. Cemented Oxides

- Produced using powder metallurgy techniques;

- Suited to high speed finishing;

- Cutting speeds from 90 to 2250 m/min;

- Coolants are not required;

- High resistance to abrasive wear and chattering.

1.9 The Economics of Metal Cutting

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 As with most engineering problems we want to get the highest return, with the minimum

investment. In this case we want to minimize costs, while increasing cutting speeds.

 Efficiency will be the key term - it suggests that good quality parts are produced at reasonable

cost.  Cost is a primarily affected by:

1. - tool life;

2. - power consumed.

 

The production throughput is primarily affected by:

1. - accuracy including dimensions and surface finish;

2. - mrr (metal removal rate).

 The factors that can be modified to optimize the process are:

1. - cutting velocity (biggest effect);

2. - feed and depth;

3. - work material;

4. - tool material;

5. - tool shape;

6. - cutting fluid.

 

Fig.1.14 Tool life as a function of cutting velocity.

We previously considered the log-

log scale graph of Taylor's tool life

equation, but we may also graph it

normally to emphasize the effects

(Fig.1.14).

This graph is representative for

most reasonable cutting speeds. The

velocities at the high and low ranges do not

necessarily exhibit the same relationship.

There are two basic conditions to trade off (Fig.1.15):

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1. - Low cost - exemplified by low speeds, low mrr, longer tool life;

2. - High production rates - exemplified by high speeds, short tool life, high mrr;

*** There are many factors in addition to these, but these are the most commonly considered.

  

Fig.1.15 Total cost as a function of cutting cost and tool cost. 

Dictionary

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Cutting – aschiere

Shear – forfecare

Casting – turnare

Chip - aschie

Hardness – duritate

Wear – uzura

Grinding – rectificare

Bending – indoire

Ductile – maleabil, elastic

Brittle – casant, fragil

Rake angle – unghi de degajare

Lathe – strung

Feed rate – avans

Depth of cut – adancimea de aschiere

Build-up edge – depunere pe tais

Serrated – in zig-zag

Strain – solicitare, efort

Sawtooth – dinte de fierastrau

Flake – lamela, foita

Cast iron – fonta

Entangled – încolacit

Machining cell – celula de fabricatie

Chip breaker – spargator de aschii

Mild steel – otel carbon (nealiat)

Insert – placuta amovibila

Drilling – gaurire

Boring – largire, alezare

Workholding device – dispozitiv de prindere

a piesei

Toolholder – suport de scula

Cutting velocity – viteza de aschiere

Tool life – durabilitatea sculei

Lathe bit - cutit de strung

Reground – reascutire

Carbide – carbura metalica

Rpm - rotatii pe minut

Lapping – lepuire

Rough cut – degrosare

Finishing cut – finisare

High speed steel – otel rapid

Dull – tocit

Tensile strength – rezistenta la intindere

Alloy – aliaj

Furnace – cuptor

Brazed - lipit

Chapter two

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Turning

2.1 Introduction

Turning is one of the most common of metal cutting operations that produces cylindrical

parts. In turning, a workpiece is rotated about its axis as single point cutting tools are fed into it,

shearing away unwanted material and creating the desired part (Fig.2.1). Turning can occur on

both external and internal surfaces to produce an axially-symmetrical contoured part.

Fig.2.1 Turning operation.Fig.2.2 Engine lathe.

The starting material is usually a workpiece that has been made by other processes, such as

casting, forging, extrusion, and drawing. Turning processes are versatile and capable of

producing a wide variety of shapes.

Turning produces solids of revolution which can be tightly toleranced because of the

specialized nature of the operation. Turning is performed on a machine called a lathe in which the

tool is stationary and the part is rotated. The Fig.2.2 illustrates an engine lathe. Lathes are

designed solely for turning operations, so that precise control of the cutting results in tight

tolerances. The workpiece is mounted on the chuck , which rotates relative to the stationary tool.

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2.2 Lathe operations

During the process cycle, a variety of operations may be performed to the workpiece to

yield the desired part shape. These operations may be classified as external or internal. External

operations modify the outer diameter of the workpiece, while internal operations modify the inner

diameter. The following operations are each defined by the type of cutter used and the path of that

cutter to remove material from the workpiece.

Turning is used to produce rotational, typically axi-symmetric, parts that have many

features, such as holes, grooves, threads, tapers, various diameter steps, and even contoured

surfaces. Parts that are fabricated completely through turning often include components that are used

in limited quantities, perhaps for prototypes, such as custom designed shafts and fasteners.

Turning is also commonly used as a secondary process to add or refine features on parts

that were manufactured using a different process. Due to the high tolerances and surface finishes

that turning can offer, it is ideal for adding precision rotational features to a part whose basic shape

has already been formed.

External operations

Straight turning: A single-point

turning tool moves axially, along

the side of the workpiece, removing

material to form different features,

including steps, tapers, chamfers,

and contours. These features are

typically machined at a small radial

depth of cut and multiple passes

are made until the end diameter is

reached.

Facing: A single-point turning tool

moves radially, along the end of

the workpiece, removing a thin

layer of material to provide a

smooth flat surface. The depth of

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the face, typically very small, may

be machined in a single pass or may

be reached by machining at a

smaller axial depth of cut and

making multiple passes.

Grooving: A single-point turning

tool moves radially, into the side of

the workpiece, cutting a groove

equal in width to the cutting tool.

Multiple cuts can be made to form

grooves larger than the tool width

and special form tools can be used

to create grooves of varying

geometries.

Cut-off (parting): - Similar to

grooving, a single-point cut-off tool

moves radially, into the side of the

workpiece, and continues until the

center or inner diameter of the

workpiece is reached, thus parting

or cutting off a section of the

workpiece.

Thread cutting: - A single-point

threading tool, typically with a 60

degree pointed nose, moves axially,

along the side of the workpiece,

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cutting threads into the outer

surface. The threads can be cut to a

specified length and pitch and may

require multiple passes to be

formed.

Forming: Uses a cutting tool

ground with the form or geometry

of the desired shape; this forming

tool is advanced perpendicular to

the axis of the work to reproduce

its shape on the workpiece.

Contour turning, or profiling:

Uses a single-point cutting tool to

reproduce a surface contour from a

template. This operation has been

almost entirely replaced by

numerically controlled or NC

programming.

Taper turning: produces a taper

along the axis of the workpiece.

Tapers are produced by either

offsetting the tailstock from

centerline or by using a “taper

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attachment”. Some short, steep

tapers can be obtained by using the

compound rest alone.

Internal operations

Drilling: A drill enters the workpiece

axially through the end and cuts a hole

with a diameter equal to that of the

tool.

Boring: A boring tool enters the

workpiece axially and cuts along an

internal surface to form different

features, such as steps, tapers,

chamfers, and contours. The boring

tool is a single-point cutting tool,

which can be set to cut the desired

diameter by using an adjustable boring

head. Boring is commonly performed

after drilling a hole in order to enlarge

the diameter or obtain more precise

dimensions.

Reaming: A reamer enters the

workpiece axially through the end and

enlarges an existing hole to the

diameter of the tool. Reaming

removes a minimal amount of

material and is often performed after

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drilling to obtain both a more accurate

diameter and a smoother internal

finish.

Tapping: A tap enters the workpiece

axially through the end and cuts

internal threads into an existing hole.

The existing hole is typically drilled

by the required tap drill size that will

accommodate the desired tap.

Knurling: Is an operation used to

produce a texture on a turned

machine part. Handles are often

knurled in order to provide a gripping

surface. The two wheel inserts shown

on the tool contact the workpiece, and

with pressure, cold-form a pattern

into the surface of the part.

2.3 Turning parameters

Cutting speed (V): The speed of the workpiece surface relative to the edge of cutting tool during

a cut, measured in surface meters per minute (m/min).

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MECH152-L23 (1.0) - 3

Turning

MRR = vfd

http://www.youtube.com/watch?v=86-xKFbdnOo&feature=PlayList&p=3AFB507B668AF162&index=37

Fig.2.1 Schematic illustration of turning operation.

The cutting speed is obtained from the expression:

V = , [m/min] (2.1)

where Dm is the machined diameter [mm], n is the rotational (spindle) speed [rpm].

It is expressed in surface meters per minute (m/min), and it refers only to the workpiece. Every

different diameter on a workpiece will have a different cutting speed, even though the rotating

speed remains the same. The cutting speed is only constant for as long as the spindle speed and/or

part diameter remains the same. In a facing operation, where the tool is fed in towards the centre,

the cutting speed will change progressively if the workpiece rotates at a fixed spindle speed. On

most modern CNC-lathes, the spindle speed is increased as the tool moves in towards the centre.

Table 1. Cutting speed for turning operations.

Material Cutting ToolMaterial

Rough Cut

Finishing Cut

m/min m/min

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Free Cutting Steel

H.S.S 35 90

Cast Alloy 75 145

Carbide 125 205

Low Carbon Steel

H.S.S 31 80

Cast Alloy 65 130

Carbide 106 190

Medium Carbon Steel;

H.S.S 30 69

Cast Alloy 58 107

Carbide; 92 152

High Carbon Steel

H.S.S 24 61

Cast Alloy 53 91

Carbide 76 137

Cast Iron Grey

H.S.S 24 41

Cast alloy 43 76

Carbide 69 125

Brass / Bronze Free Cutting

H.S.S 53 110

Cast Alloy 105 170

Carbide 175 275

Aluminium

H.S.S 40 90

Cast Alloy 55 115

Carbide 75 185

Plastics

H.S.S 30 75

Cast Alloy 45 115

Carbide 60 150

Cutting feed (f): The distance that the cutting tool or workpiece advances during one revolution

of the spindle, measured in millimeters per revolution (mm/rev). In some operations the tool feeds

into the workpiece and in others the workpiece feeds into the tool.

The value of feed influences, not only how thick the chip is, but also how the chip forms against

the insert geometry.

Feed rate (fn): The speed of the cutting tool's movement relative to the workpiece as the tool

makes a cut. The feed rate is measured in millimeters per minute (mm/min) and is the product of

the cutting feed (f) and the spindle speed (n).

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(2.2)

Depth of Cut (d) is practically self explanatory. It is the thickness of the layer being removed

from the workpiece or the distance from the uncut surface of the work to the cut surface,

expressed in mm. It is important to note, though, that the diameter of the workpiece is reduced by

two times the depth of cut because this layer is being removed from both sides of the work.

[mm] (2.3)

Spindle speed (n) - The rotational speed of the spindle and the workpiece in revolutions per minute

(rev/min). In order to maintain a constant cutting speed, the spindle speed must vary based on the

diameter of the cut. If the spindle speed is held constant, then the cutting speed will vary.

Time of machining Tm (min)

[min] (2.4)

29

Axial depth of cut - The depth of

the tool along the axis of the

workpiece as it makes a cut, as in a

facing operation. A large axial

depth of cut will require a low

feed rate, or else it will result in a

high load on the tool and reduce

the tool life. Therefore, a feature is

typically machined in several

passes as the tool moves to the

specified axial depth of cut for

each pass.

Radial depth of cut - The depth of

the tool along its radius in the

workpiece as it makes a cut. A

large radial depth of cut will

require a low feed rate, or else it

will result in a high load on the tool

and reduce the tool life. Therefore,

a feature is often machined in

several steps as the tool moves

over the step-over distance, and

makes another cut at the radial

depth of cut.

Material removal rate. The material removal rate (MMR) is the volume of material

removed per unit time, such as mm3/min. For each revolution of the workpiece we remove a ring-

30

shaped layer of material whose cross-sectional area is the product of the distance the tool travels

in one revolution (the feed, f) and the depth of cut (d). The volume of this ring is the product of

the cross-sectional area, that is, (f .d ), and the average circumference of the ring, that is, π·Dm.

Thus, the material removal rate per revolution will be π·Dm·d·f. Since we have n

revolutions per minute, the removal rate is:

MMR = π·Dm·d·f·n, [mm3/min] (2.5)

This time does not include the time required for tool approach and retraction. Because the

time spent in noncutting cycles of a machining operation is nonproductive and affects the overall

economics, the time involved in approaching and retracting tools to and from the workpiece is an

important consideration. Machine tool are now being designed and built to minimize this time –

one method is first by rapid traverse of the tools, then a slower movement as the tool engages the

workpiece.

Generally, cutting speed is high for soft materials, such as aluminum and brass, and is low

for hard and abrasive materials, such as cast iron and stainless steel. Depth of cut, feed rate, and

cutting speed will vary, depending on the type of cut, material, and geometry of the workpiece.

2.4 Tool geometry

For cutting tools, geometry depends mainly on the properties of the tool material and the

work material. The standard terminology is shown in the following figure. For single point tools,

the most important angles are the rake angles and the end and side relief angles (Fig.2.1).

31

Fig.2.1 Designations for a turning tool geometry.

Rake angle (γ) is important in controlling the direction of chip flow and the strength of

the tool tip. The cutting tool’s rake angle is the angle between the cutting edge and the cut itself. It

may be positive, negative, or neutral. The back rake angle affects the ability of the tool to shear

the work material and form the chip. Positive rake angles reduce the cutting forces resulting in

smaller deflections of the workpiece, tool holder, and machine. If the back rake angle is too

large, the strength of the tool is reduced as well as its capacity to conduct heat. In machining

hard work materials, the back rake angle must be small, even negative for carbide and diamond

tools. The higher the hardness, the smaller the back rake angle. For high-speed steels, back rake

angle is normally chosen in the positive range.

Relief (clearance) angle (α) control interference and rubbing at tool-workpiece interface.

If the relief angle is too large, the tool may chip off; if too small, flank wear may be excessive.

The End Relief Angle prevents friction on the flank of the tool.

The holder for the bit is often angled, and the end relief angle must be larger than the tool

holder angle to prevent rubbing. The relief angles are usually between 50 and 100 but may be

adjusted according to given cuttings.

 Back rake angle – Angle between the tool face and the horizontal plane 900 to the axis of the lathe;

Side rake angle – Angle between the tool face and the horizontal plane parallel to the axis of the

lathe;

End relief angle – Angle between the end of the tool and a line drawn 900 to the centre line of the

lathe;

Side relief angle – Angle between the tool flank and the original side of the tool.

Nose radius (rε) is the angle formed by the point of the tool- 0, 4 mm for heavy cuts, 0, 4-1, 5

mm for finishing cuts (Fig.2.2). This radius may be large for strength, or sharp for fine radius

turning. Also, the sharper the radius the rougher will be the surface finish of the workpiece and

the lower will be the strength of the tool. Large nose radii can lead to tool chatter.

In some turning operations, the single-point

32

Fig.2.2 Nose radius in turning.

tool used for finishing has a rounded front

corner or "nose". The radius of the tool nose,

along with the cutting feed, will determine the

surface roughness formed by the finishing

operation. For a given cutting feed, a larger

nose radius will provide a better finish.

Fig.2.3 Rake (γ) and inclination (λ) angles of a turning

tool.

Inclination angle ( ) is a measure of at

what angle the insert is mounted in the

toolholder (Fig.2.3). When viewed from the side

or front is the angle of insert seat or pocket in the

toolholder from front to back. This inclination

can be either positive, negative or neutral.

Table 2. Cutting angles for different

materials.

33

Workpiece

material

Back rake angle

(0)

Side rake angle

(0)

End relief angle

(0)

Side relief angle

(0)

Aluminum 0 to 10 10 to 20 6 to 14 6 to 14

Brass -5 to 0 -5 to 8 6 to 10 6 to 12

Cast iron -7 to 5 -7 to 12 6 to 8 5 to 10

Mild steel -7 to 12 -7 to 12 5 to 10 5 to 10

Stainless steel -7 to 7 -7 to 10 5 to 10 5 to 10

Plastic -5 to 5 0 to 15 20 to 30 15 to 20

2.5 Tool material

All cutting tools that are used in turning can be found in a variety of materials, which will

determine the tool's properties and the workpiece materials for which it is best suited. These

properties include the tool's hardness, toughness, and resistance to wear.

High speed steel (HSS) tools have been used for some time but are being replaced by

carbide, ceramic and diamond tooling. Carbide inserts are easily replaceable and long lasting.

Ceramic tools can resist high temperatures so high speed machining is possible.

Ceramics are brittle, tough, and often fracture if cuts are interrupted.

Diamond tools produce superior surface finish and are used for nonferrous and

nonmetallic materials.

The material of the tool is chosen based upon a number of factors, including the material

of the workpiece, cost, and tool life. Tool life is an important characteristic that is considered when

selecting a tool, as it greatly affects the manufacturing costs. A short tool life will not only require

additional tools to be purchased, but will also require time to change the tool each time it becomes

too worn.

34

Tool material Applications

High speed steel (HSS) - Special tool shapes

- Low production

- Best for interrupted cuts

Carbides (inserts) - High production

Ceramics (inserts) - High speed machining

- Avoid interrupted cuts

- Avoid positive rake angles

Diamonds (inserts) - Abrasive machining

- High surface qualities, fine tolerances

- Nonferrous or nonmetallic materials

2.6 Lubrication and cooling

The chart shows most of the typical cutting fluids used for a variety of work materials.

Cutting fluids cool the tool, which reduces tool wear and makes higher cutting speeds and feed

rates possible. Secondary functions include lubrication, flushing chips, and rust prevention.

Cutting fluids also prevent a build-up edge on the tool and thus contribute to a better surface

finish.

Work material Cutting fluid Flood, spray

Aluminum None, water-soluble oil, synthetic

oil, kerosene

Flood

Brass None, water-soluble oil, synthetic

oil

Flood

Cast iron Non -

Steel, all types Non, water-soluble oil, synthetic

oil, sulfurized oil

Flood

Plastics None, emulsifiable oil, synthetic

oil

Flood, spary

35

2.7 Workholding methods

Workholding includes any device used to present and hold a workpiece to a cutting tool.

The decision about how to hold a part influences:

- which surfaces or holes can be designated as reference surfaces;

- which surfaces can be machined in a single setup;

- the overall accuracy of the machining process;

- allowable cutting forces, which may include speeds and feeds;

- the tool path;

- possibly the tool size and shape;

Other important factors include:

- cutting tool access to work;

- ease of loading and unloading from workholding device;

- simplicity of workholding setup;

- use of standard catalog workholding components for economy;

To correctly machine a part it must be held in a setup that guarantees a definite location

and orientation. This setup must be repeatable throughout the production run. Additionally, the

workholding device must hold the part securely in position while cutting forces, vibrations,

centrifugal force, and gravity act to dislodge it.

Accuracy and productivity demand that as many machining operations as possible occur

for a single workholding procedure. Reclamping, rechucking, and other repositioning of work

compromises the accuracy of the work, increasing non-value added time to the manufacture of

the part. Sometimes multiple set-ups are needed when datum surfaces must be machined first, or a

through hole has steps on both sides, or a part has blind holes on opposite sides.

Chucks are the primary workholding tools in turning operations and they are best suited to

grip circular or hexagonal cross-sections when very fast, reasonably accurate centering is desired

(Fig.2.4 a-f).

36

More jaws confer more secure grip (if the work is truly cylindrical) and thin-walled work

will deform less. Four jaws are used for holding irregular shapes.

Chucks:

- may have 2, 3, 4, or 6 jaws to hold work by external or internal surfaces;

- many chucks operate manually, but in CNC lathes they operate automatically, others may

operate hydraulically or electrically;

- indexing chucks can index to different positions to present multiple surfaces of a workpiece to the

cutting tool;

- for longer turnings, a tailstock support is used with the chuck;

- chucks may also be magnetic, to grip irregularly-shaped ferrous workpieces.

13

Three-jaw Universal Chuck

a)

15

Four-Jaw Independent Chucks

b)

15

Four-Jaw Independent Chucks

c)

d) e) f)

Fig.2.4 a) Three jaws chuck; b,c) Four jaws chuck; d) four jaws on face plate; e) six jaws chuck; f) Independent four-jaw

chuck.

Faceplates are used to hold workpiece too large or shaped so it cannot be held in chuck or

between centers (Fig.2.5). Usually equipped with several slots to permit use of bolts to secure

workpiece.

37

MECH152-L23 (1.0) - 6

Work Holding

Dog and centre / Between centres Three-jaw chuck

ColletFace plate

Fig.2.5 Faceplate on a lathe.

Angle plate can be used so axis of

workpiece may be aligned with lathe centers.

It is necessary to use counterbalance

fastened to faceplate when workpiece is

mounted off center to prevent imbalance and

resultant vibrations.

A lathe dog, also known as a lathe carrier, is a device that clamps around the workpiece

and allows the rotary motion of the machine's spindle to be transmitted to the workpiece.

A carrier is most often used when turning between centers on a lathe, but it may be used on

dividing heads or any similar situation. It is used in conjunction with a drive plate and drive pins:

the plate is mounted directly on the machine spindle (as with a chuck) and the drive pin is

attached to the plate. In use the carrier and workpiece are inserted between centers and the leg of

the carrier rests against the drive pin.

a)

MECH152-L23 (1.0) - 6

Work Holding

Dog and centre / Between centres Three-jaw chuck

ColletFace plate

b)

Fig.2.6 a) Straight leg carriers; b) Bent tail lathe dog hooked on chuck jaw.

Carriers may be of the straight leg (Fig.2.6a) or bent leg (Fig.2.6b) type. The straight leg

requires the drive pin; the bent leg fits into a slot machined into the drive plate. The bent leg type

is considered safer as there are (slightly) fewer protruding parts to cause accidents.

38

Spindle speeds are reduced when working with carriers, due to the unbalanced nature of

the setup. Care must also be taken by the operator when using carriers, as it is easy to get snagged

on one.

A dead center (one that does not turn freely, i.e., dead) may be used to support the

workpiece at either the fixed or rotating end of the machine (Fig.2.7).

Fig.2.7 Live center (top). Dead center with carbide insert (bottom)

6

Revolving Tailstock Centers

Fig.2.8 Live center (section).

Soft centers are identical to dead centers except the nose is deliberately left soft

(unhardened) so that it may be readily machined to the correct angle prior to usage. This operation

is performed on the headstock center to ensure that the centers axis is aligned with the spindles axis.

A live center or revolving center is constructed so that the 60° center runs in its own

bearings and is used at the non driven or tailstock end of a machine (Fig.2.8). It allows higher

turning speeds without the need for separate lubrication, and also greater clamping pressures.

CNC lathes use this type of center almost exclusively and they may be used for general machining

operations as well. Spring loaded live centers are designed to compensate for center variations,

without damage to the workpiece or center tip.

This assures the operator of uniform constant tension while machining. Some live centers

also have interchangeable shafts. This is valuable when situations require a design other than a

60° male tip.

A collet, one type of chuck, is a sleeve with a (normally) cylindrical inner surface and a

conical outer surface (Fig.2.9). The collet has 3 kerf cuts along its length to allow it to expand and

contract. Depending on the collet design, it can be either pulled (via a threaded section at the rear

39

of the collet) or pushed (via a threaded cap with a second taper) into a matching conical socket to

achieve the clamping action.

a)

b)

Fig.2.9 Collets for round (a) and square (b)

workpieces.

MECH152-L23 (1.0) - 6

Work Holding

Dog and centre / Between centres Three-jaw chuck

ColletFace plate

Fig.2.10 Collet manually acted.

As the collet is forced into the tapered socket, the collet will contract, gripping the

contents of the inner cylinder. Typically collets offer higher levels of precision and accuracy than

self-centering chucks, and have a shorter setting up time than independent-jaw chucks. The penalty

is that most collets can only accommodate a single size of workpiece.

Collets usually are made to hold cylindrical work, but are available to hold square,

hexagonal or octagonal workpieces.

Mechanical face drivers (or drive center) might be the best way to turn parts compared to

other types of traditional chucking methods (Fig.2.11). It consists of a dead center surrounded by

hardened teeth. These teeth bite into the softer workpiece allowing the workpiece to be driven

directly by the center.

40

Fig.2.11 Turning using face drivers.

They allow turning applications to have

increased flexibility to lower cycle times, turn

both the smallest and largest of parts and even

allow interrupted and heavy cuts. The major

benefit of a face driver is found in its ability to

allow the part to be completely turned from one

end to the other in one operation.

Face drivers have four main parts: the flange/shank, nose cone/carrier body, center pin and

drive pins. Three different mounting positions are also available.

Drive pins are used that act as teeth that bite into the part’s face. These are replaceable and are

available in sets of three, five or six…depending on the size of the driver and the part they’re

turning. Some applications require drive pins that are coated with materials such as carbide or

diamond.

Mandrel holds internally machined workpiece between centers so further machinig

operations are concentric with bore. There are two parts per mandrel; a tapered arbor and a

matched flexible sleeve (Fig.2.12b). The large end has a short, straight shank allowing it to be

mounded into a collet. The flat can be clamped by a lathe dog when turning in-between centers.

41

36

Plain Mandrel

a)

b)

38

Gang Mandrel

c)

d)

Fig.2.12 a) Plain mandrel; b) expanding mandrel; c) gang mandrel; d) stub mandrel.

Plain mandrel (Fig.2.12a) has a body with a slight taper (1-2 mm per 100 mm length) for

gripping of the workpiece. With plain mandrel both the cylindrical as well as the end faces of the

workpiece can be machined, whereas with gang mandrel only cylindrical surfaces can be

machined.

An expanding mandrel shown in Fig.2.12d mounted using a collet and a live center.  All of

them can also be mounted in-between centers and driven with a lathe dog.

Gang mandrel (Fig.2.12.c) is used when only the cylindrical surface of a workpiece is to be

machined. This has a fixed collar at one end and a movable collar at the threaded end that may be

adjusted to its position by a nut.

A steady rest, is clamped to a fixed point on the ways, usually near the end, and has

adjustable 'fingers' that are adjusted so that they lightly contact the outside of a long workpiece to

42

keep it from wobbling or thrashing (Fig.2.13). Three jaws tipped with plastic, bronze or rollers

may be adjusted to support any work diameter with steady rest capacity.

Fig.2.13 A steady rest.

On this model, the fingers are brass and are

adjusted by means of thumb screws and then

locked in place by means of lock nuts. Use a

few drops of oil to lubricate the contact surface

between the work and the fingers to keep the

work from heating up and binding.   The steady

rest clamps to the ways with a clamp much like

that on the tailstock, and with the same

frustrating tendency to rotate to an orientation

which does not fit between the ways

Fig.2.14 A follower rest.

Follower rest is mounted on saddle and

travels with carriage to prevent workpiece from

springing up and away from cutting tool

(Fig.2.14). Cutting tool is generally positioned

just ahead of follower rest.

It provides smooth bearing surface for two

jaws of follower rest.

43

2.8 Toolhoders

Toolholders are either right or left handed, or neutral. In turning, chipbreaking is critical to

efficient work processing and good finishing qualities. Proper chipbreaking results from balancing

the depth of the cut and the geometry of the tool. Many insert have chipbreaker grooves molded

into them.

A heavy duty roughing operation on large workpieces makes considerably different demands

to those of finishing operations in small part machining. Toolholder types are defined by the

entering angle, the shape and size of the insert used. For stability during machining, the largest

possible toolholder size should be chosen to suit the application. This provides the most

advantageous tool-overhang ratio and the most rigid base for the insert. In the Fig.2.15 are shown

some methods of attaching inserts to toolholders.

a) b) c)

Fig.2.15 Methods of attaching inserts to toolholders; (a) Rigid clamp design; (b) Lever design; (c) Wedge clamp

design.

Composite operations should be divided into basis cuts for assessment of which toolholder

type is most suitable (Fig.2.13): longitudinal (1), facing (2), profiling (3) plunging (4), and

external machining of small, longitudinal slander components.

44

Fig.2.16 Toolholders for external turning.

In external turning the tool overhang is not affected by the length of the workpiece, and the

size of the toolholder is selected to withstand the forces and stresses which arise during operation

(Fig.2.16).

In internal turning the choice of tool is very much restricted by the component’s hole diameter

and length, as depth of the hole determines the overhang (Fig.2.17).

Fig.2.17 Tool overhang is the most preeminent factor in internal turning (boring operations).

A general rule, which applies to all machining is to always minimize tool overhang and to

select the largest possible tool size in order to obtain the best possible stability and thereby

accuracy. The stability is increased when a larger boring bar diameter is used, but possibilities are

often limited, since the space allowed by the diameter of the hole in the component must be taken

into consideration for swarf evacuation and any radial movement.

In Fig.2.18 are shown some toolholders for internal turning used for longitudinal, facing and

profiling turning.

45

Fig.2.18 Toolholders for internal turning.

Chip evacuation during boring is critical to performance and the security of the operation.

Relatively short, spiral shaped chips should be aimed for internal turning because these are easy to

evacuate. On the other hand, long chips make chip evacuation more difficult and present a risk of

swarf-clogging.

2.8.1 Toolholders for threading

Fig.2.19 Turning thread operation.

Various infeed types. There are three different methods of feeding the insert in during each pass

(Fig.2.20). All arrive at the same profile but the cuts are made differently, with varying influences

on: chip formation, tool wear and quality.

46

Turning screw threads are

common operations on CNC machinery

and is today performed with high

productivity and production security

mainly through the use of indexable inserts

(Fig.2.19). Inserts are available with

cutting edges in the shape of the

appropriate thread forms, for example

Metric, Whitworth. The feed rate of the

machine is the key factor for turning

threads as this has to equal the pitch of the

thread.

Fig.2.20 Infeed types: (A) radial; (B) modified flank; (C) incremental.

The radial infeed (A) is the conventional way, used widely where the insert is fed in at a

right angle to the workpiece and the chip is formed stiffly into a V on both sides of the profiled

cutting edge. Tool wear is more even on both sides of the insert and the method is more suitable

for fine pitches and work-hardening materials.

The modified flank wear (B) is an advantageous method for modern thread turning and

CNC lathes are programmed to have this method in cycles. The insert is fed in at the angle of the

profile less a clearance angle. Clearance behind the cutting point, as in ordinary turning, has to be

provided for in the feed direction. Chip control is better, the process being more similar to

ordinary turning and for using chipbreaker threading inserts, type geometry C. Less heat is

generated in the insert point and production security is generally high with this method.

Incremental type feed (C) is the method used mainly for large profiles. The insert cuts in

varying increments into the profile. This gives rise to more even insert wear. One side of the

thread profile is turned in a few increments, the tool is then advanced and the other side of the

profile is turned in a few increments and so on until the full profile has been generated.

2.9 Inserts

Inserts are individual cutting tools with a number of cutting points. A square insert, for

example, has eight cutting points and a triangular insert has six. Inserts are usually clamped on the

tool shank with various locking mechanisms.

47

Although not as commonly used, inserts may be brazed to the tool shank. Clamping is the

preferred method because each insert has a number of cutting edges, and after one edge is worn,

it is indexed (rotated in its holder) to present another cutting edge.

Carbide inserts are available in a variety of shapes, such as square, triangle, rhombic, and

round. The strength of the cutting edge of an insert depends on its shape (Fig.2.21).

Fig.2.21 Scale 1 indicates that as regards cutting edge strength (S), the larger the point angle to the left, the higher the

strength. While as regards versatility and accessibility (A), the inserts to the right are superior. Scale 2 indicates that the

vibration tendency (V) rises to the left while power (P) requirement is lower to the right.

The smaller the angle, the lower the strength of the edge. In order to further improve edge

strength and prevent chipping, all insert edges are usually honed, chamfered, or produced with a

negative land.

The insert shape selection is based on the trade-off between strength and versatility. For

example, larger point angles are stronger, such as round inserts for contouring and square inserts

for roughing and finishing. The smaller angles (35 and 55 degrees) are the most versatile for

intricate work. Turning inserts may be molded or ground to their working shape. The molded types

are more economical and have wide application. Ground inserts are needed for maximum accuracy

and to produce well defined or sharp contours.

The nose radius (rε) is a key factor in many turning operations and one that needs

consideration as the right choice affects cutting edge strength to surface finish of the component.

An insert is available in several nose radii where the smallest nose radius is theoretically zero but

where 0, 2 mm is more commonly the smallest. The largest is normally 2, 4 mm, although the full

range is not available for one and the same insert shape or size (Fig.2.22).

48

Fig.2.22 The nose radius of an insert is an important performance factor.

Fig.2.23 Surface finish is largely determined by the

relationship between feed and nose radius.

In turning operations, the surface finish

generated will be directly influenced of nose

radius and feed rate. The surface generated by

a single point tool is made up of how the nose

radius moves along the workpiece surface

(Fig.2.24.

The theoretical maximun profile height is calculated through the formula:

Rmax = [μm] (2.3)

where fn is feed rate [mm/rev] and rε is nose radius [mm].

Alternatively, starting out with a certain nose radius and required profile height, a starting

value for the feed rate can be calculated. Table 2.3 is a guide for maximum feed for various nose

radius.

Table 2.3 Guide for maximum feed for various nose radius.

Nose radius (rε) , [mm] 0,4 0,8 1,2 1,6 2,4

Max recommended

feed (fn), [mm/rev]

0,25-0,35 0,4-0,7 0,5-1,0 0,7-1,3 1,0-1,8

49

Insert grade is selected mainly according to the component material, the type of application

and machining conditions.

The main ranges of tool materials are:

- coated cemented carbides (HC);

- cemented carbides (HW);

- cermets (HT, HC);

- ceramics (CA, CN, CC);

- cubic boron nitrides (CBN);

- polycristallline diamonds (DP,HC).

2.10 Equipment for turning operations

Turning machines typically referred to as lathes, can be found in a variety of sizes and

designs. While most lathes are horizontal turning machines, vertical machines are sometimes used,

typically for large diameter workpieces. Turning machines can also be classified by the type of

control that is offered. A manual lathe requires the operator to control the motion of the cutting tool

during the turning operation. Turning machines are also able to be computer controlled, in which

case they are referred to as a computer numerical control (CNC) lathe. CNC lathes rotate the

workpiece and move the cutting tool based on commands that are preprogrammed and offer very

high precision.

2.10.1 Engine lathe

The engine lathe (Fig.2.25) is one of the most useful and necessary machines in a shop. The

major function of the engine lathe is to change the size, shape or finish of a revolving workpiece

with various cutting tools.

The size or machining capacity of all engine lathes is determined by:

- the swing (chuck capacity);

- length of the bed between head and tailstock centers.

50

Fig.2.25 The engine lathe.

Bed - The bed of the turning machine is simply a large base that sits on the ground or a table and

supports the other components of the machine.

Headstock assembly - The headstock assembly is the front section of the machine that is attached to

the bed. This assembly contains the motor and drive system which powers the spindle. The spindle

supports and rotates the workpiece, which is secured in a workpiece holder or fixture, such as a

chuck or collet.

Tailstock assembly - The tailstock of an engine lathe is used to provide a fixture at the end of the

part opposite from the chuck (Fig.2.26). The tailstock can be used to support a long, thin part so

that more radial cutting force can be applied and higher rotational speeds can be attained without a

"whipping" instability effect. Below is illustrated another use for the tail stock. Drill bits can be

fixtured in the tailstock to cut axial holes in a turned part. These central holes are more accurate

than a drill press or mill could provide since the lathe is dedicated to operations in which an axially-

symmetric part is rotated about its central axis.

51

Fig.2.26 The tailstock.

Carriage - The carriage is a platform that slides alongside the workpiece, allowing the cutting

tool to cut away material as it moves (Fig.2.27). The carriage rests on tracks that lay on the bed,

called "ways", and is advanced by a lead screw powered by a motor or hand wheel.

The carriage allows cross-feed and diagonal movements in addition to axial movement.

Fig.2.27 Engine lathe carriage.

52

Cross slide - The cross slide is attached to the top of the carriage and allows the tool to move

towards or away from the workpiece, changing the depth of cut. As with the carriage, the cross

slide is powered by a motor or hand wheel.

Compound rest - The compound is attached on top of the cross slide and supports the cutting tool.

The cutting tool is secured in a tool post which is fixed to the compound. The compound can rotate to

alter the angle of the cutting tool relative to the workpiece.

Turret - Some machines include a turret, which can hold multiple cutting tools and rotates the

required tool into position to cut the workpiece. The turret also moves along the workpiece, feeding

the cutting tool into the material. While most cutting tools are stationary in the turret, live tooling

can also be used. Live tooling refers to powered tools, such as mills, drills, reamers, and taps,

which rotate and cut the workpiece.

Lead screw – A large screw with a few threads per mm used for cutting threads.

2.10.2 Turret lathes

In a turret lathe, a longitudinally feedable, hexagon turret replaces the tailstock (Fig.2.29).

The turret, on which six tools can be mounted, can be rotated about a vertical axis to bring each

tool into operating position, and the entire unit can be moved longitudinally, either annually or by

power, to provide feed for the tools. When the turret assembly is backed away from the spindle by

means of a capstan wheel, the turret indexes automatically at the end of its movement thus bringing

each of the six tools into operating position.

The square turret on the cross slide can be rotated manually about a vertical axis to bring

each of the four tools into operating position. On most machines, the turret can be moved

transversely, either manually or by power, by means of the cross slide, and longitudinally through

power or manual operation of the carriage. In most cased, a fixed tool holder also is added to the

back end of the cross slide; this often carries a parting tool.

53

Fig.2.29 Turret lathe.

Vertical Turret Lathes. Vertical turret lathes are designed for considerably larger and heavier

work than is commonly associated with either type of horizontal turret lathes (Fig.2.30). Vertical

lathes are utilized solely for complex chucking work, particularly for boring operations, and are

not adapted to bar work.

 Vertical turret lathe closely resembles a vertical boring mill. It commonly has a rotating table

ranging from 600 to 1200 mm in diameter, which is equipped with both removable chuck jaws and

T-slots for clamping the work.

Fig.2.30 Single column vertical turret lathe.

54

The main tool head is mounted on the cross rail along which it travels horizontally and

with which it travels vertically. The five sided turret is mounted on a ram which travels vertically

in the cross rail tool head. This turret can be easily and quickly indexed from hole to hole and

clamped with a lever.

Tools are clamped in the tool holes of the turret by the same type of holders as those used in

horizontal turret lathes. Frequently, a second tool holder also is mounted on the cross rail. Each

motion for successive tools can be controlled by means of stops so that duplicate work pieces can be

machined with one tooling setup.

All the cross rail tool heads and the side tool head have power feeds in the forward and

reverse directions, as well as rapid traverse motions for tool approach and return motions. A five

sided turret is available so the operations must be completed by five indexing of the turret. This

necessitates multiple tools setting arranged as follows (Fig.2.31):

Fig.2.31 Tool head of a vertical turret lathe.

1. There are four tools in the holder No.1, first tool, left rough faces the top and then with a vertical

feed rough turn the rim. The next two tools finish top and radius corner and the fourth tool skins the

vanes which are at an angle.

2. Operation, 2, involves three tools to rough face: the boss at three different heights.

3. Operation number 3, finish machines the same faces;

4. Operation 4, one tool finish turns the outside rim with a vertical traverse, while the comers of the

inside boss are radiused and chamfered by the two remaining tools.

55

5. The 5th operation is to rough and finish bore and to size the top recess of the bore.

2.10.3 Screw machines

Screw machines are automated lathes which can quickly mass-produce turned parts. A

screw machine uses cutting methods similar to that of a lathe but is highly automated. Screw

machines are typically used for high-volume, low-cost turned parts. Feed stock for a screw

machine is a long cylindrical rod of material.

The screw machine automatically turns/faces the part, parts it off, and advances the rod for

the next part. A screw machine is illustrated below (Fig.2.32).

Fig.2. 32 A screw machine.

The name screw machine is somewhat of a misnomer, because screw machines spend much

of their time making things that are not screws and that in many cases are not even threaded.

However, the archetypal use for which screw machines were named was screw-making.

All screw machines are fully automated, whether mechanically (via cams) or by CNC

(computerized control), which means that once they are set up and started running, they continue

running and producing parts with very little human intervention.

56

A screw machine may have a single spindle or multiple spindles. Each spindle contains a

bar of material that is being machined simultaneously. A common configuration is six spindles.

The cage that holds these six bars of material indexes after each machining operation is complete.

In a single-spindle machine, these machining operations would most likely be performed

sequentially, with four cross-slides each coming into position in turn to perform their operation.

In a multi-spindle machine, each station corresponds to a stage in the production

sequence through which each piece is then cycled, all operations occurring simultaneously, but on

different pieces of work, in the manner of an assembly line .

2.11 Design of Parts for Turning

The following are general guidelines for design of turned parts:

1. Where possible, turned parts should be designed so that a tailstock is not required. This is

done by designing the part to be stubby rather than long with a high aspect ratio. The figure

below shows the difference.

2. Chuck-clamped cylindrical surfaces should not contain parting lines so that flash does not

introduce errors.

 

3. For cast parts with surfaces to be faced, cast-in relief allows for tool clearance, as shown

below.

  

57

4. On cast parts that are subsequently turned down, burrs can be avoided by avoiding surfaces

perpendicular to the turned-down surface, as shown below.

 

5. Be as specific as possible when referring to removal of burrs. Blanket specifications such as

"break all corners" are not recommended since removing all burrs is expensive. Only certain

burrs are gross enough and compromise safety and functionality enough to warrant removal.

 

6. Keyways should be able to be milled with the endmill traversing the part axially. Radii at the

ends of the keyway are those of the endmill.

 

7. Avoid turning in the areas of weldments, parting lines, and flash. This will tend to extend

cutter life.

 

8. Minimize the number of set ups required. Milling should be grouped into sets of parallel

planes.

 

9. Design for the largest diameter cutters possible. Larger cutters are less prone to breakage and

require lower speeds when compared to smaller cutters. Larger cutters also can accomodate

58

carbide cutting inserts.

 

10. Carbide cutting surfaces require fewer tool changes and have higher cutting performance.

 

11. Blending of radii into existing surfaces should be avoided, even with a ball end mill.

Cosmetic-quality blending is expensive to achieve.

 

12. If a surface is to be faced, it preferably should be angled in order to provide tool clearance.

 

Sharp inside corners need to conform to the cutter radius that is used in that vicinity. If possible,

inside corner radii should be left to the discretion of the fabricator.

13. For the cutter perpendicular to the turning axis, rules of thumb for angles on the part are

shown below:

 

 

59

14. For the cutter at fifty-five degrees to the turning axis, rules of thumb for angles on the part

are shown below:

1. Boring is more expensive than drilling, so drilling should be used if possible.

 

2. Deep holes with aspect ratios greater than 3:1 should be avoided since accuracy and cutting

time will suffer.

 

3. Use through holes instead of blind holes where possible.

 

4. As with all machining operations, the part must be as rigid as possible while being machined.

With boring, this applies to the boring bar itself as well as to the part.

 

5. As shown below, relief for the bottoms of blind holes should be provided.

  

60

Dictionary – 2

Turning – strunjireCasting – turnareForging – forjareExtrusion – extrudareDrawing – trefilareParting – retezareSlug – bloc, baraBlank – semifabricatTailstock – papusa mobilaCompound rest – port-cutitOffset – deplasare, abatereGroove – canelura, renuraRecess – cavitate, canal circularThreading – filetareTap – tarodKnurling – zimtare, striere, randalinareEntering angle – unghi de atacRake angle – unghi de degajareRelief angle – unghi de asezareNose radius – raza la varfCutting edge – taisul sculeiInserts – placuta amovibila din carbura metalicaPoint - varf, taisRoughing - degrosare Tool holder – suport de sculaChipbreaker – spargator de aschiiOverhang – iesirea in consolaPlunging – strunjire de patrundereDeflection – abatereThrough hole – gaura strapunsaSwarf- aschii Pitch of the thread - -pasul filetuluiInfeed – avans transversalCirclip – inel elasticTaper - conic

Boring - largireReaming - alezareTapping – filetare cu tarodulGrip – a apucaPattern - modelRub – a frecaChatter – a trepidaToughness – tenacitateRust – ruginaLathe dog – antrenor (inima de strung)Saddle – sanie transversalaClogging – infundareBind – a lipiSteady rest – luneta (la m.u.)Wobble – miscare neuniformaOverhang – lungime in consolaSqueeze – strangereFace drivers – antrenare frontalaSleeve – bucsa, mansonCollet – bucsa extensibilaThrough hole – gaura strapunsaBlind hole – gaura infundataChuck – mandrina de strangereJaw – bac de strangereBrazed – lipit cu alamaInfeed – avans transversalWipping – oscilatie, bataieTurret lathe – strung revolverBurr – bavuraRam – sanie principalaCross rail – sanie transversalaFeed stock – semifabricatWay - ghidaj

61

Chapter three

Drilling

3.1 Introduction

Drilling is one of the most common of all machining processes. One estimate is that

75% of all metal-cutting material removed comes from drilling operations. It covers the

methods of making cylindrical holes in a workpiece with metal cutting tools most tipically by

using a twist drill (Fig.3.1).

Drilling is associated with subsequent machining operations such as trepanning,

counterboring, reaming and boring. Common to all these processes is a main rotating

movement combined with a linear feed. There is a clear distinction between short hole and

deep hole drilling, the latter being a specialist method for making holes that have depths of

many times (up to 150 times) the diameter.

1

The chips must exit through the flutes to

the outside of the tool. As can be seen in the

Fig.3.1, the cutting front is embedded within

the workpiece, making cooling difficult. The

cutting area can be flooded, coolant spray

mist can be applied, or coolant can be

delivered through the drill bit shaft.

Fig.3.1 Schematic illustration of a cross section

of a hole being cut by a twist drill.

The drilling process can in some respects be compared with turning and milling but the

demands on chipbreaking and the evacuation of chips is critical in drilling. Machining is

restricted by the hole dimensions, the greater the hole depth, the more demanding it is to

control the process and to remove the chips.

3.2 Typical drill press operations

MECH152-L23 (1.0) - 11

Drilling Operations

Reaming Tapping Counterboring

Centre drilling

Spot facing

Countersinking

http://www.youtube.com/watch?v=vCHQLFZHHJc&feature=PlayList&p=3AFB507B668AF162&index=38

Counter Bores - Allows the head of cap screws to be sunk beneath a surface.

Threaded Holes - Taps can be used to add threads to holes.

Counter Sink - Allows counter sunk head screws to be sunk beneath a surface.

Center Drilling - Allows parts to be mounted between centers, on lathes typically.

Spot Face - Allows the head of a bolt to be sunk beneath the surface. This is basically a

shallow counter bore.

2

 Typical parameters for drill bits are:

1. Material is High Speed Steel;

2. Standard Point Angle is 118°.

 

Harder materials have higher point angles; soft materials have lower point angles. The helix

results in a positive cutting rake.

  Drill bits are typically ground (by hand) until they are the desired shape. When done

grinding, the lips should be the same length and at the same angle, otherwise and oversized hole

may be produced.

Drill sizes are typically measured across the drill points with a micrometer.

3.3 Typical drill bits

 

Drill can be defined as a rotary end cutting tool having one or more cutting lips, and

having one or more helical or straight flutes for the passage of chips and the admission of a

cutting fluid (Figs.3.2-3.3).

Fig.3.2 Tip view of the drill.

Fig.3.3 Side view of the drill bit.

3

Some standard drill types are:

1. Straight Shank - this type is held in a chuck;

2. Taper shank - this type is held in a sleeve, and a machine spindle. A drift may also be

used.

  Some other types of drills used are:

1. Core drills - a drill with a small helix, and 3 or 4 flutes. This is used for light drilling,

such as opening holes in castings;

2. High helix - When drilling a deep hole in a soft material these drills are used to help

remove chips;

3. Centre drills - A drill with a small entry tip, and a widening profile. The result is a hole

that has a conical shape on the outside, that may be used to mount the part between

centres, or to act as a guide for a larger drill.

Fig.3.4 A typical twist drill.

Some Glossary of Terms:

DRILL - A rotary end cutting tool having one or more cutting lips and having one or more

helical or straight flutes for the passage of chips and the admission of a cutting fluid.

BACK TAPER - A slight decrease in diameter from point to back in the body of the drill.

CHISEL EDGE - The edge at the end of the web that connects the cutting lips.

4

FLUTES - Helical or straight grooves cut or formed in the body of the drill to provide cutting

lips, to permit removal of chips, and to allow cutting fluid to reach the cutting lips.

not indicate the usable length of the flutes.

HELIX ANGLE - The angle made by the leading edge of the land with a plane containing the

axis of the drill.

LAND - The peripheral portion of the body between adjacent flutes.

LAND WIDTH - The distance in a transverse plane between the leading edge and the heel of the

land measured at a right angle to the leading edge.

LIPS - The cutting edges of a two flute drill extending from the chisel edge to the periphery.

(CORE DRILLS) - The cutting edges extending from the bottom of the chamfer to the periphery.

LIP RELIEF ANGLE - The axial relief angle at the outer corner of the lip. It is measured by

projection into a plane tangent to the periphery at the outer corner of the lip.

MARGIN - The cylindrical portion of the land which is not cut away to provide clearance.

POINT - The cutting end of a drill, made up of the ends of the lands and the web. In form it

resembles a cone, but departs from a true cone to furnish clearance behind the cutting lips.

SHANK - The part of the drill by which it is held and driven.

TANG - The flattened end of a taper shank, intended to fit into a driving slot in a socket.

WEB -The central portion of the body that joins the lands. The extreme end of the web forms the

chisel edge on a two-flute drill.

WEB THINNING - The operation of reducing the web thickness at the point to reduce drilling

thrust.

 3.4 Process Parameters

The cutting speed, or surface speed (vc) for drilling is determined by the periphery speed

and can be calculated from the spindle speed (n) which is expressed in number of revolutions per

minute (Fig.3.5a). During one revolution, the periphery of the drill will describe a circle with a

circumference of π x Dc, where Dc is the tool diameter. The cutting speed also varies depending

upon which cutting edge across the drill-face is being considered. A machining challenge for

drilling tools is that from the periphery to the centre of the drill, the cutting speed declines in

value, to be zero at the centre.

5

Recommended cutting speeds are for the highest speed at the periphery.

a) b)

Fig.3.5 a) Cutting speed, penetration rate, spindle speed and feed

per revolution; b) Main hole machning factors.

Vc = (3.1)

The feed per revolution (ƒn)

in mm/rev expresses the axial

movement of the tool during one

revolution and is used to calculate the

penetration rate and to express the

feed capability of the drill.

The penetration rate or feed

speed (vf) in mm/min is the feed of

the tool in relation to the workpiece

expressed in length per unit of time.

This is also known as the machine feed or table feed. The product of feed per revolution

and spindle speed gives the rate at which the drill penetrates the workpiece.

Vf = fn n, [mm/min] (3.2)

The main factors that characterize a hole from a machining point of view are: diameter,

depth, quality, material, conditions, reliability, productivity (Fig.3.4b).

3.5 Cutting forces and power

To produce a hole requires a certain amount of energy. Cutting forces act on the drill as it

penetrates through the workpiece removing metal and generating a certain amount of power. A

specific cutting force for the material in question also needs to be established.

The specific cutting force value (kc) in N/mm2 , has been worked out and tested for most

materials and is available in a table relating to the effective rake angle of the tool and the

average chip thickness. It is defined as the tangential cutting force needed for a chip with a

6

certain cross-section (one square mm) or the effective cutting force divided by the theoretical

chip area. Values are indicated for a certain feed per tooth.

In addition to the material factor, the power (Pc) in kW required for a drilling operation

depends upon the diameter, feed rate and cutting speed. Most holes with a moderate diameter

are no problem for modern machines but for large diamters with depths of several times the

diameter, it is wise to check the power.

Torque (Mc) in Nm is another value which may be critical for some large-diameter

drilling operations, especially trepanning, as regards the total drilling moment that the drill is

subjected to during machining. The feed, diameter and material are the main factors that affect

the torque value. The torque is the sum of the moments on each cutting edge and the product of

the tangential force and radius from the centre.

Fig.3.6 Point angle and cutting

angle.

Applying an excessive feed force can affect the hole quality, tool reliability and stall the

machine. On the other hand, applying a sufficient feed force is important for the cutting action

and also from producitivity point of view.

Most drill troubles arise from inaccurate

pointing. Correct drill pointing will to a great

extent eliminate drill breakage and

inaccurate holes. The point angle of a twist

drill is the angle made by the cutting lip and

the axis of the drill.

Drill pointing should be varied depending

upon the materials to be drilled, but for

general use, drills leave the factory sharpened

to a 59° point angle (118° included angle), 9° -

7

The feed force (Ff) in N is usually the most important in

driling from a performance point of view. This is the axial

force acting on the drill as it penetrates the material. It needs to

be considered in order to ensure that the spindle power and

strength is sufficient for the drilling operation. The cutting edge

angle of the drill (κr), of the cutting edges, also influences the

feed force (Fig.3.6). The point angle of the drill is φ.

Fig.3.7 Drill pointing.

15° clearance angle and with a chisel edge

angle of 120° to 135° (Fig.3.7).

3.6 Chip control and cutting fluid

Chip control and cutting fluid are important factors in drilling. Generating suitable chip

forms and sizes and evacuating them are vital to the succes of any drilling operation. Without

satisfactory performace in this regard, all drills will rapidly become ineffective due to clogging

up the hole. Cutting speeds and feeds are high with modern drills but this has only been made

possible through efficient evacuation of chips with cutting fluid.

Most short hole drills have two chip channels through which the chips are evacuated.

With modern machines and drilling tools, this is be done very effectively by supplying cutting

fluid internally through the tool coolant holes. The cuting fluid is ejected at the point of the

drill during machining to lubricate the drill and flush out chips throught the channels.

Fig.3.8 Chip formation, chip evacuation

and cutting fluid supply.

8

Chip formation is influenced by the workpiece

material, tool geometry, cutting speed, feed and to some

extent the choice of cutting fluid (Fig.3.8). Generally,

increased feed and/or reduced cutting speed produces

shorter chips. The chip length and form can be said to

be acceptable if the chips can be flushed out reliably.

The rake angle (γE) of the drill varies along the

cutting edge and decreases from the periphery towards

the centre of the drill, such as with solid and brazed

cemented carbide twist drills (Fig.3.9).

Fig.3.9 Rake angle of the drill.

This pressure gives rise to a relatively high axial-force component. If the machine is

weak in relation to the size of hole to be drilled, and the generated feed force, the machine

spindle may deflect and, as a result, oval holes may be produced.

The most common twist drill has a point of 118 degrees. A more aggressive point angle,

such as 90 degrees, is suited for very soft plastics and other materials. The bit will generally be

self-starting and cut very quickly. A shallower angle, such as 150 degrees, is suited for drilling

steels and other tougher materials. This style bit requires a starter hole, but will not bind or

suffer premature wear when a proper feed rate is set.

3.8 Materials for drill bit construction

High speed steel (HSS) is a form of tool steel where the bits are much more resistant to the

effect of heat. They can be used to drill in metal, hardwood, and most other materials at greater

cutting speeds than carbon steel bits and have largely replaced them in commercial applications.

Cobalt steel alloys are variations on high speed steel which have more cobalt in them. Their

main advantage is that they hold their hardness at much higher temperatures, so they are used

to drill stainless steel and other hard materials. The main disadvantage of cobalt steels is that

they are more brittle than standard HSS.

9

Since the cutting speed also drops from the periphery towards

the centre, the cutting edge will work ineffectively at the

point of the drill. As the point of the drill presses and scrapes

the material rather than cuts it, plastic deformation tends to

occur where the rake angle is negative and the cutting speed

low.

Fig.3.10 Titanium nitride coated drill.

Coatings. Titanium nitride is a very hard ceramic material, and

when used to coat a high-speed steel bit (usually twist bits), can

extend the cutting life by three or more times.

A titanium nitride bit cannot properly be sharpened, as the

new edge will not have the coating, and will not have any of the

benefits the coating provided.

Exotics. The material referred to as Tungsten carbide is extremely hard, and can drill in

virtually all materials while holding an edge longer than other bits. However, due to its high

cost and brittleness, it is more frequently used only in smaller pieces screwed or brazed onto

the tip of the bit. It is becoming common in job shops to use solid carbide drills.

Polycrystalline diamond (PCD) is among the hardest of all tool materials and is therefore

extremely wear resistant. The material consists of a layer of diamond particles, typically about

0.5 mm thick, bonded as a sintered mass to a tungsten carbide support. Bits are fabricated

using this material by (1) brazing small segments to the tip of the tool to form the cutting edges,

or (2) sintering PCD into a vein in tungsten carbide "nib". The nib can later be brazed to a

carbide shaft and ground to complex geometries that cause braze failure in the smaller 'segments'

The PCD bits are typically used in the automotive, aerospace, and other industries to drill

abrasive aluminum alloys, carbon fiber reinforced plastics and other abrasive materials, on in

places to run extended life and prevent machine downtime.

10

Diamond powder is used as an abrasive, most often for cutting tile, stone, and other very hard

materials. Large amounts of heat are generated, and diamond coated bits often have to be water

cooled to prevent damage to the bit or the workpiece.

The shank is the part of a drill bit grasped by the chuck of a drill. The cutting edges of

the drill bit are at one end, and the shank is at the other. Different styles of shank/chuck

combination deliver different performance, such as allowing higher torque or greater centering

accuracy.

Fig.3.10 Straight drill bit shank.

Large drill bits can have straight shanks smaller than their drill diameter, so that

medium-size chucks can be used to drill large holes.

Characteristics of a straight shank:

Easy to make on a lathe;

Zero manufacturing if the drill bit is made from round bar stock;

Can be held in a collet chuck;

Can be held in a drill chuck, the commonest sort;

Very accurate centering;

Low torque transmission.

Morse taper shank. The Morse taper allows the bit to be mounted directly into the spindle

of a drill, lathe tailstock or (with the use of adapters) into the spindle of milling machines.

11

Straight shank. The straight shank (Fig.3.10) is the

most usual style on modern drill bits, by number manufactured.

It is most often made the same diameter as the drill bit, for

economy. It's then held in a 3-jaw drill chuck.

Very small bits can have straight shanks larger than the

drill diameter, often for holding in standard size collets.

Morse Tapers come in eight sizes identified by a

number between 0 and 7. Morse Tapers can have two

types of ends:

Tang (Fig.3.11) to facilitate removal with a drift;

Threaded to be held in place with a draw bar.

Fig.3.11 Morse Taper Drill Bit Shank.

It is a self locking (or self holding) taper, that allows the torque to be transferred to the

drill bit by the friction between the taper shank and the socket. The tang at the end of the taper

is only for ejecting the drill bit from the spindle, with the aid of a drift.

The arbor of a drill chuck is often a Morse taper and this allows the chuck assembly to

be removed and directly replaced with the shank of a Morse taper drill bit. A range of sleeves

may be used to bring the size of the smaller Morse tapers up to the size of the drive spindle's

larger taper. Sockets are also available to extend the effective length of the drill as well as

offering a variety of taper combinations.

Characteristics of a taper shank:

Simple to manufacture on a lathe;

Cannot be held in a chuck or collet;

High torque transmission provided the bit is driven hard into the workpiece;

Very accurate centering.

Drill chucks can be of several types, but are typically three-jaw since three points on the

circumference define a circle in two dimensions.

A standard three-jaw and a multi-jaw chuck are shown in the figures below (Figs.3.12-

3.13).

12

Fig.3.12Three-jaws drill chuck. Fig.3.13 Multiple-jaw drill chuck.

3.7 Other types of drills

Gun drill. Gun drills are straight fluted drills which allow coolant (either compressed air or a

suitable liquid) to be directed through the drill's body, directly to the cutting face. They are

used for deep drilling of which gun barrels are the obvious example (Fig.3.14).

13

Fig.3.14 A gun drill and the cutting/cooling configuration.

The coolant provides lubrication and cooling to the cutting edges as well as ejecting the

swarf or chips back out the drills length.

Fig.3.15 The coolant holes in the carbide

gun drill shank and tip.

Countersink. A countersink is a tapered hole drilled with a wide outer portion. A

common usage is to allow the head of a countersunk bolt or screw, when placed in the hole, to

sit flush with or below the surface of the surrounding material. A countersink may also be used

to remove the burr left from a drilling or tapping operation thereby improving the finish of the

product and removing any hazardous sharp edges (Fig.3.16).

Fig.3.16 A countersink with its cut hole configuration.

14

There are two basic types of gun drills: the internal chip

removal type and the external chip removal type.

The internal type is basically a tube with a cutting bit

on the front. Coolant is forced around the outside of the tube,

and pours around the front of the drill forcing the chips into the

center of the tube, and out of the hole.

The external chip removal type operates similarly except

there is a notch cut in the outside of the drill, through which

the chips exit, and a passage for the coolant goes through the

center of the bar (Fig.3.15).

Fig.3.17 Side and view of a 4 fluted countersink.

Better quality fluted countersink cutters will have the flutes (or at least one flute) at an

irregular pitching. This variation in pitching reduces the chance of the cutting edges setting up

a harmonic action and leaving an undulated surface.

Counterbore. A counterbore can refer to a cylindrical flat-bottomed hole, which enlarges

another hole, or the tool used to create that feature. It is usually used when a bolt or cap head

screw is required to sit flush with or below the level of a workpiece's surface. The uppermost

counterbores shown in the image are the same tool. The smaller top item is an insert, the middle

shows another three-fluted counterbore insert, assembled in the holder.

Fig.3.18 Two types of counter bores.

Center drill (Combined drills and countersinks). Single or double-end cutting tool,

having helical or straight flutes, and having a drill portion and an adjacent integral countersink

portion, primarily used to produce center holes in work that will be held between machine

centers or to provide a starting hole for a larger sized drill bit (Fig.6.19).

15

Fluted countersink cutter. The fluted countersink cutter is used

to provide a heavy chamfer in the entrance to a drilled hole This may

be required to allow the correct seating for a countersunk head screw or

to provide the lead in for a second machining operation such as tapping.

Countersink cutters are manufactured with two common angles, 90° and

82°. This difference provides a choice in seating angles for the mating

part (Fig.3.17).

The shank of this holder is a Morse taper although

there are other machine tapers that are used in the industry.

The lower image is of a plain counter bore designed to fit

into drill chuck, and being smaller, is not economical to

make as one piece (Fig.3.18).These centers are used when turning or grinding

workpieces. A workpiece machined between centers can be

safely removed from one process (perhaps turning in a lathe)

and set up in a later process (perhaps a grinding operation)

with what is often a negligible loss in the co-axiality of

features. Traditional twist drill bits may tend to wander

when started on an unprepared surface. Once a bit wanders

off-course it is difficult to bring it back on center. A center

drill bit frequently provides a reasonable starting point as it is

short and therefore has a reduced tendency to wander when

drilling is started.

Fig.3.19 Center drills, Numbers 1 to 6.

For removal of broken taps, hardened bolts or drilling difficult materials such as

chilled cast irons, stellite and glass, can be used a Hard Cut drill-solid carbide drill (Fig.3.23).

This drill has a regrindable geometry and no cutting fluid is required - drill dry.

Fig.3.23 A hard Cut drill – solid carbide drill.

In Fig.3.24 is shown an example of using solid carbide drill to remove a broken tap.

16

1. Securely clamp the workpiece on the machine table in a vice

or similar rigid work-holding fixture. Centre the drill on the

broken tap.

2. Centre drill in the uneven surface of the fractured tap, with

a larger, more rigid drill than the one which will eventually be

used for drilling out the tap.

3. Select the correct size of Hard-cut drill. The recommended

spindle speeds are 1500-3500 rpm. Drill with a consistent,

steady, manual feed. Stop frequently to clear chips from the

hole.

4. Once the tap has been drilled out it is a relatively simple

matter to remove the remaining parts of the tap using a scriber

or similar pointed tool.

remove a broken tap.

Spade drill. A spade drill is usually a two part drill. The cutting point is being removable and

usually made of high speed steel. Often spade drills will have coolant lines running through the

body. Since the cutting point is removable, one drill can be used for a range of hole sizes.

Spade drills are capable of cutting to a depth of about 10 times the drill diameter. Cut

diameters are typically in the range of about 20 to75 mm.

Fig.3.27 Spade drill with its cut hole configuration.

Trepan. A trepan, sometimes called a BTA Drill (after the Boring and Trepanning Association),

is a drill that cuts an annulus and leaves a center core (Fig.3.28).

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Trepans usually have multiple carbide

inserts and rely on water to cool the cutting tips

and to flush chips out of the hole. Trepans are

often used to cut large diameters and deep holes.

Typical drill diameters are 150 to 350 mm and

hole depth from 300 up to 3000 mm.

Fig. 3.28 A Trepan tool.

A comparison between the cutting depth ap, at solid drilling and trepanning is shown in

Fig.3.29.

Fig.3.29 Cutting depth ap in solid drilling and trepanning (Dc-drill diameter, ap-cutting depth).

Indexable insert drills. The indexable insert drill combines the toughness of a steel

drill-shank with the wear resistance of cemented carbide inserts, without the need for re-

grinding.

Fig.3.30 Indexable insert drill.

The most accurate holes are produced by the following of operations:

Centering;

Drilling;

Boring;

Reaming.

For even better accuracy and surface finish, holes may be burnished or internally ground

and honed.

Jig Boring. Jig boring is used to accurately enlarge existing holes and make their diameters

highly accurate. Jig boring is used for holes that need to have diameter and total runout

controlled to a high degree. A cross section of a hole being jig bored is shown in Fig.3.32.

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The life of the drill is long and can be applied to

suit different machining demands. Reliability and

accuracy is higher than ever, coupled with the ability to

produce good machining economics (Fig.3.30).

Fig.3.32 A cross section of a hole being jig bored .

Typically, a part has holes machined on regular equipment and then the part is

transferred to a dedicated jig boring machine for final operations on the especially accurate

holes. Jig boring can also maintain high accuracy between multiple holes or holes and

surfaces. Tolerances can be held readily within ±.005 mm. Dedicated jig boring machines are

designed to machine holes with the tightest tolerances possible with a machine tool.

When designing a part with holes, it is important to determine what holes must be jig

bored.

The reason for this is that jig boring requires extra time and attention, and the jig boring

machine at the machine shop may have a back log of jobs. Jig boring can therefore have a big

impact on the lead time of a part.

Standard boring can be carried out on a mill fitted with a boring head or on a lathe . Boring

is most accurate on a lathe since a lathe is dedicated to solids of revolution (axially symmetric

parts).

3.9 Drilling Machines

Drill press. Drilling machines are used for drilling holes, tapping, reaming, and other general-

purpose, small-diameter boring operations. These machine tools are generally vertical. The most

common vertical type is the drill press, the major components of which are shown in Fig. 3.37.

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Fig.3.37 Schematic illustration of the components of a vertcal drill press.

The most adjustable part of the drill press is the vertical movement of the drill bit, since this

is the motion that is used in production.

1. The capstan wheel (A) moves the drill head up and down.

2. This movement can be locked (D)

3. and there are point-to-point stops (B) for maintaining a specific length of travel.

4. Gradations marked on the stationary part of the drill press (C) let the operator know where he

is vertically.

5. Both the drilling head and table can move vertically and rotate about the vertical guide post

(b).

6. The base of the drill press incorporates a work surface similar to the table's for oversize

workpieces. The base can be bolted down, but often is not since forces on a drill press do not

typically cause it to tip over. The drill is powered by an electric motor (I).

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The workpiece is placed on an adjustable table, either by clamping it directly into the

slots and holes on the table or by using a vise, which in turn can be clamped to the table. The

workpiece should be properly clamped, both for safety and accuracy, because the drilling

torque can be high enough to rotate the workpiece (Fig.3.38).

Fig.3.38 The work area of the drill press.

Drill press are usually designed by the larger workpiece diameter that can be

accommodates on the table. Sizes typically range from 150 mm to 1250 mm.

A drill press has a number of advantages over a hand-held drill:

- less effort is required to apply the drill to the workpiece. The movement of the chuck and

spindle is by a lever working on a rack and pinion, which gives the operator considerable

mechanical advantage;

- the table allows a vise or clamp to position and lock the work in place making the operation

secure;

- the angle of the spindle is fixed in relation to the table, allowing holes to be drilled accurately

and repetitively.

Geared head drill.

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The drill is lowered manually by

hand wheel or by power feed at preset

rates. Manual feeding requires some

skill in judging the appropriate feed

rate. In order to maintain proper cutting

speeds at the cutting edges of drills, the

spindle speed on drilling machines has to

be adjustable to accommodate different

sizes of drills. Adjustments are made by

means of pulleys, gear boxes, or variable-

speed motors.

The geared head drill is identical to the drill

press in most respects, however they are generally of

sturdier construction and often have power feed

installed on the quill mechanism, and safety interlocks

to disengage the feed on overtravel (Fig.3.39).

The most important difference is the drive

mechanism between motor and quill is through a gear

train (there are no vee belts to tension) this makes these

drills suitable for the larger sizes of drill bits (16 mm

or upwards) which would normally stall in a drill press.

Fig.3.39 Geared head drill.

Radial arm drill.

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A radial arm drill is a geared head drill

that can be moved away from its column along

an arm that is radiates from the column.

These drills are used for larger work where a

geared head drill would be limited by its reach,

the arm can swivel around the column so that

any point on the surface of the table can be

reached without moving the work piece. The

size of work that these drills can handle is

considerable as the arm can swivel out of the

tables area allowing an overhead crane to

place the workpiece on the fixed table. Vices

may be used with these machines but the work

is generally bolted to the table or a fixture.

Fig.3.40 Radial arm drill.

BTA hole drilling machine. In Fig.3.41 is shown a machine for deep hole drilling with drilling

range of 18-150 mm, and drilling depth up to 3000 mm, for centre-line drilling of shafts. These

machines use drilling tool type ejector, where chip removal is back through the centre of the

drill.

Fig.3.41 BTA hole drilling machine for deep holes.

Jig borer machine. The jig borer is a type of machine tool invented at the end of World War I

to make possible the quick-yet-very-precise location of hole centers. It was invented

independently in the United States and Switzerland. It can be viewed as a specialized species of

boring mill or milling machine that provided tool and die makers with a higher degree of

positioning precision (repeatability) and accuracy than those general machines had previously

provided (Fig.3.42).

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Fig.3.42 Jig borer machine.

The revolutionary underlying principle was that advances in machine tool control that

expedited the making of jigs were fundamentally a way to expedite the cutting process itself, for

which the jig was just a means to an end. The jig borer was a logical extension of manual

machine tool technology that began to incorporate some then-novel concepts that would become

routine with NC and CNC control, such as:

coordinate dimensioning (dimensioning of all locations on the part from a single reference

point);

working routinely in "tenths" (ten-thousandths of a mm).

3.8 Drilled Part Design Considerations

The following are guidelines for drilled part design.

1. Advantages of drilled holes include accuracy and sharpness of edges. Since machining is

expensive compared to other manufacturing processes, drilling to create a hole should be justified

by looking at alternatives. Before adding drilled holes to a design, ask yourself whether the hole

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is needed and/or whether it can be cast, molded, or pierced with sufficient accuracy instead of

drilled.

 2. Specify standard drill bit sizes. Unusual hole sizes bring up the cost of manufacturing through

purchasing and inventory costs.

 3. Through holes are preferred over blind holes. This has to do with the fact that a blind hole

does not provide as much leeway for chip exit and cooling. Operations such as reaming and

threading after drilling are more easily conducted on a through hole.

4. Do not specify flat-bottomed holes. Twist drills create cone-bottomed holes and flat-bottom

holes cause problems with reaming, etc.

 5. If possible, do not specify holes that are smaller 4 mm in diameter. Drills for smaller holes

tend to break and for convenient mass production, are not recommended.

 6. For large holes, try to cast in a preliminary hole that must only be bored out to

specification. This saves material, transportation cost, and drilling cost.

7. When dimensioning holes, it is better to use rectangular rather than angular (or polar)

coordinates. Angular coordinates will require the machinist to set up a dividing head or to re-

dimension the part, both of which take time.

 8. Minimize the number of drilled hole sizes so that tool changes are minimized.

 9. Minimize the number of directions on the part that holes must be drilled from.

 10. The entrance and exit surfaces of a drilled hole should be perpendicular to the hole axis.

The reasons for this are as follows:

 a) Upon entrance of the drill, the drill tip will wander if the surface that the tip contacts is not

perpendicular to the drill axis.

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b) Exit burrs will be uneven around the circumference of the exit hole. This can make burr

removal difficult.

Bad and good examples of entrance and exit lands are shown in the figure below.

11. Intersections of drilled holes with other cavities should be avoided if at all possible. If

interesection with a cavity is unavoidable, the drill axis should at least be outside of the cavity,

as shown below.

12. On drawings, multiple holes in a flat surface should be located from the same horizontal

and vertical datums.

13. If there are protrusions surrounding a drilled hole, it may be difficult to bring the drill press

head close to the entrance surface, resulting in a drill bit that is prone to wandering, chatter,

and other instabilities. This problem can be solved by providing a fixture with a drill bushing

close to the drill bit. However, part design must allow for this fixture.

14. Deep, narrow holes with length to diameter ratios of larger than 3 should be avoided.

Deeper holes are possible but the drill will tend to wander and possibly break. One way to avoid

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a deep, narrow hole is to use a stepped entrance.

Blind holes should be drilled to a depth 25% deeper than the actual hole in order to

provide space for chips.

 

15. Deeper holes with external coolant supply. Usually drilling of a hole can be performed in

one single step. But if deep holes are drilled (more than 3 x D), using external fluid supply, one

third of the depth can be drilled continuously followed by a peck drilling cycle (Fig.3.33).

Fig.3.33 Drilling deeper holes with peck drilling cycle.

16. Drilling of non-flat surfaces (Fig.3.34).

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Peck drilling cycle: After drilling

one third of the depth, the drill is

lifted sufficient for chip evacuation,

cleaning of the hole and then

followed by repeated drilling cycles.

Fig.3.34 Drilling non-flat surfaces.

16. Convex surfaces.

Fig.3.35 Drilling convex surfaces.

Dictionary – 3

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Drilling of component surfaces inclined

to a maximum of 10° is acceptable but a

reduction of feed is essential on entry to

prevent drill sliding, and, when the drill exits, to

prevent wear on circular land or even drill

breakage.

Through-holes: when exiting through-holes the

feed must be reduced to 1/3 of normal feed.

Convex surfaces are possible to drill if the

radius is larger than 4 times the drill diameter

and the hole is perpendicular to the surface. The

feed should be reduced to a half of normal rate

when entering (Fig.3.35).

Trepaning – taiere de canale circulareCounterboring – adancireReaming – alezareBoring – largireTwist drill – burghiu elicoidalFlute – caneluraWander – a devia, a se abateDrill press – masina de gauritThrough hole – gaura strapunsa, de trecereCore – miezAllowances – adaos de prelucrare

Cutting speed – viteza de aschiereFeed per revolution – avans pe rotatieFeed speed – viteza de avansRake angle – unghi de degajareStall – oprire, stagnarePoint angle – unghi la varfClogging – blocare, infundareScrape – a razui, a raclaDeflect – a se curba, adevia, a se incovoiaLip – margine, muchieDrill bit – burghiu spiralRipple – ondulatieChisel edge – muchie transversalaClearance – joc, spatiuHeel – calcai, pintenBlunt – tocit, bontBlind hole – gaura infundataShank – coada

Chuck – mandrinGrasp – a strange, a apuca3-jaw drill chuck – mandrin cu trei bacuriLathe – strungCollet chuck – bucsa extensibilaTaper shank – coada conicaLathe tailstock – papusa mobila a strunguluiSocket – mufa, bucsa, mansonTang – coadaDrift – poanson, placa pentru scoaterea burghiului Draw bar – tija, bara de tractiuneSleeve – bucsa, mansonTighten – a strange

Deep hole – gaura adancaNotch – crestatura, taieturaCountersink – tesitor, freza conicaBurr – bavuraCounterbore – adancitor, largitorFlush head – cap inecatCenter drill – burghiu de centrareSpot drill – gaurire preliminaraCentre punch – punctatorCore drill – carotieraGrab – a se intepeniVice, vise – menghinaJig bore machine – masina de gaurit in coordonateLog – grindaQuill - arbore gol

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Peck drilling – gaurire pas cu pas