Fundamentals of Metal cutting and Machining

Processes

Lecture 6-7

1

Contents

A. THEORY OF METAL MACHINING

B. MACHINING OPERATIONS AND MACHINING TOOLS

C. CUTTING TOOL TECHNOLOGY

Material Removal Processes

A family of shaping operations, the common feature of which is removal of material from a starting workpart so the remaining part has the desired geometry

Machining – material removal by a sharp cutting tool, e.g., turning, milling, drilling

Abrasive processes – material removal by hard, abrasive particles, e.g., grinding

Nontraditional processes - various energy forms other than sharp cutting tool to remove material

Cutting action involves shear deformation of work material to form a chip As chip is removed, new surface is exposed

(a) A cross‑sectional view of the machining process, (b) tool with negative rake angle;

compare with positive rake angle in (a).

Machining

Why Machining is Important

Variety of work materials can be machined Most frequently used to cut metals

Variety of part shapes and special geometric features possible, such as: Screw threads Accurate round holes Very straight edges and surfaces

Good dimensional accuracy and surface finish

Disadvantages with Machining

Wasteful of material Chips generated in machining are wasted

material, at least in the unit operation Time consuming

A machining operation generally takes more time to shape a given part than alternative shaping processes, such as casting, powder metallurgy, or forming

Machining in Manufacturing Sequence

Generally performed after other manufacturing processes, such as casting, forging, and bar drawing Other processes create the general shape

of the starting workpart Machining provides the final shape,

dimensions, finish, and special geometric details that other processes cannot create

Speed and Feed

Speed is rotational motion of spindle which allows the tools to produce cut into blank

OR the relative movement between tool and w/p, which produces a cut

Feed is linear motion of tool which spreads cut on the blank

OR the relative movement between tool and w/p, which spreads the cut

Machining Operations

Most important machining operations: Turning Milling Drilling

Other machining operations: Shaping and planing Broaching Sawing

Single point cutting tool removes material from a rotating workpiece to form a cylindrical shape

Three most common machining processes: (a) turning,

Turning

Used to create a round hole, usually by means of a rotating tool (drill bit) with two cutting edges

Drilling

Rotating multiple-cutting-edge tool is moved across work to cut a plane or straight surface

Two forms: peripheral milling and face milling

(c) peripheral milling, and (d) face milling.

Milling

Cutting Tool Classification

1. Single-Point Tools One dominant cutting edge Point is usually rounded to form a nose

radius Turning uses single point tools

2. Multiple Cutting Edge Tools More than one cutting edge Motion relative to work achieved by rotating

Drilling and milling use rotating multiple cutting edge tools

(a) A single‑point tool showing rake face, flank, and tool point; and (b) a helical milling cutter, representative of tools with multiple cutting edges.

Cutting Tools

Cutting Conditions (parameters) in Machining

Three dimensions of a machining process: Cutting speed v – primary motion Feed f – secondary motion Depth of cut d – penetration of tool into

work piece For certain operations, material removal

rate can be computed as

RMR = v f d

where v = cutting speed; f = feed; d = depth of cut

Cutting Conditions for Turning

Speed, feed, and depth of cut in turning.

Roughing vs. Finishing

In production, several roughing cuts are usually taken on the part, followed by one or two finishing cuts

Roughing - removes large amounts of material from starting workpart Creates shape close to desired geometry,

but leaves some material for finish cutting High feeds and depths, low speeds

Finishing - completes part geometry Final dimensions, tolerances, and finish Low feeds and depths, high cutting speeds

Machine Tools

A power‑driven machine that performs a machining operation, including grinding

Functions in machining: Holds workpart Positions tool relative to work Provides power at speed, feed, and depth

that have been set The term is also applied to machines that

perform metal forming operations

Chip Thickness Ratio

where r = chip thickness ratio; to =

thickness of the chip prior to chip formation; and tc = chip thickness after

separation Chip thickness after cut is always greater than

before, so chip ratio always less than 1.0

c

o

tt

r

More realistic view of chip formation, showing shear zone rather than shear plane. Also shown is the secondary shear zone resulting from tool‑chip friction.

Chip Formation

Four Basic Types of Chip in Machining

1. Discontinuous chip

2. Continuous chip

3. Continuous chip with Built-up Edge (BUE)

4. Serrated chip

Type of chip depends on material type and cutting conditions

Brittle work materials Low cutting speeds Large feed and depth

of cut High tool‑chip friction

Discontinuous Chip

Ductile work materials

High cutting speeds

Small feeds and depths

Sharp cutting edge

Low tool‑chip friction

Continuous Chip

Ductile materials Low‑to‑medium cutting

speeds Tool-chip friction

causes portions of chip to adhere to rake face

BUE forms, then breaks off, cyclically

Continuous with BUE

Semicontinuous - saw-tooth appearance

Cyclical chip forms with alternating high shear strain then low shear strain

Associated with difficult-to-machine metals at high cutting speeds

Serrated Chip

Orthogonal Cutting

- Cutting tool is considered as a wedge- The cutting edge is perpendicular to

cutting speed

Shear plane angle can be calculated using this relation:

r: chip thickness ratio= to/tc

Orthogonal Cutting- Shear Strain

Example 21.1

Φ

1. Shear plane angle: Φ

α= 10 deg

;

;

2. Shear strain:

Cutting Forces

F: Friction force b/w chip and rake faceN: Normal to friction force FFs: Shear force applied by w/p on chipFn: Normal to shear force FsThese force can not be measured directly. These need to be calculated using force diagram

Fc: Cutting force acting in direction of cutting speedFt: thurst force acting perpendicular to Fc. Ft increases with increase in chip thickness b4 cut* Fc & Ft both increase as shear strength of material increasesThese force can be measured using dynamometer

Approximation of Turning by Orthogonal Cutting

Not Inclu

ded

Power and Energy Relationships

A machining operation requires power The power to perform machining can be

computed from:

Pc = Fc v

where Pc = cutting power; Fc = cutting force;

and v = cutting speed

Cutting Temperature

Approximately 98% of the energy in machining is converted into heat

This can cause temperatures to be very high at the tool‑chip interface

The remaining energy (about 2%) is retained as elastic energy in the chip

Tool-Chip thermocouple is used for measuring temperatures in machining

- One wire is linked to tool- 2nd wire is linked to chip- Voltage difference is measured and then converted into

current and temp using appropriate relations

Cutting Temperatures are Important

High cutting temperatures

1. Reduce tool life

2. Produce hot chips that pose safety hazards to the machine operator

3. Can cause inaccuracies in part dimensions due to thermal expansion of work material

B - MACHINING OPERATIONS AND MACHINE TOOLS

1. Turning and Related Operations

2. Drilling and Related Operations

3. Milling

4. Machining Centers and Turning Centers

5. Other Machining Operations

6. High Speed Machining

Machining

A material removal process in which a sharp cutting tool is used to mechanically cut away material so that the desired part geometry remains

Most common application: to shape metal parts Most versatile of all manufacturing processes

in its capability to produce a diversity of part geometries and geometric features with high precision and accuracy Casting can also produce a variety of

shapes, but it lacks the precision and accuracy of machining

Rotational - cylindrical or disk‑like shape Nonrotational (also called prismatic) -

block‑like or plate‑like

Machined parts are classified as: (a) rotational, or (b) nonrotational, shown here by block and flat parts.

Classification of Machined Parts

Machining Operations and Part Geometry

Each machining operation produces a part geometry due to two factors:

1. Relative motions between tool and workpart• Generating – part geometry determined

by feed trajectory of cutting tool

2. Shape of the cutting tool• Forming – part geometry is created by

the shape of the cutting tool

Generating shape: (a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, (e) profile milling.

Generating Shape

Forming to create shape: (a) form turning, (b) drilling, and (c) broaching.

Forming to Create Shape

Combination of forming and generating to create shape: (a) thread cutting on a lathe, and (b) slot milling.

Forming and Generating

TurningA cutting operation in which single point cutting tool removes

material from a rotating work-piece to generate a cylinder Performed on a machine tool called a lathe Variations of turning performed on a lathe:

Facing Contour turning Chamfering Threading

A Turning Operation

Close-up view of a turning operation on steel using a titanium nitride coated carbide cutting insert

Cutting Conditions in Turning

Rotational speed N (rev/min):

Cutting speed at cylinder surface v (m/min)

Final diameter of part:

Feed (mm/rev): f Feed rate (mm/min): fr

Time to machine:

L: Length of cut/part

Alternatively,

Material Removal rate:

v (m/min); f (m/rev); d (m). Neglect rotational xtic; v (m3/min)

Tool is fed radially inward- An operation of reducing length/thickness of stock

Operations Related to Turning: Facing

Instead of feeding tool parallel to axis of rotation, tool is fed at an angle thus creating tapered rotational shape

Operations Related to Turning: Taper Turning

Instead of feeding tool parallel to axis of rotation, tool follows a contour that is other than straight, thus creating a contoured shape

Operations Related to Turning: Contour Turning

The tool has a certain shape that is imparted on the w/p by feeding the tooling radially

Operations Related to Turning: Form Turning

Cutting edge cuts an angle on the corner of the cylinder, forming a "chamfer"

How is the tool motion?

Operations Related to Turning: Chamfering

Tool is fed radially into rotating work at some location to cut off end of part

Operations Related to Turning: Cut Off

Pointed form tool is fed linearly across surface of rotating workpart parallel to axis of rotation at a large feed rate, thus creating threads

Operations Related to Turning: Threading

Drilling is an operation of making a hole. The drill (multi-point cutting tool) is fed parallel to axis of rotation.

Reaming is an operation of making a drilled hole accurate and clean.

Operations Related to Turning: Drilling & Reaming

A single point tool is fed linearly, parallel to the axis of rotation, on the inside diameter of an existing hole in the part.

The purpose of boring is to enlarge the size of an existing hole

Operations Related to Turning: Boring

This is an operation in which regular cross hatched pattern is imparted on the w/p. This pattern facilitates holding of a part

Knurling is not a machining operation, as no cutting takes place. Instead it is metal forming operation done in lathe m/c

Operations Related to Turning: Knurling

Engine Lathe

Called engine lathe?Dates from time when these machines were driven by steam engines

Types of Lathe:Horizontal lathe: Used when length of part is larger than its diaVertical Lathe: Used if part dia is larger than its length and part is heavy Lathe Specification:1.Center to center distance2.Swing dia (2* distance from spindle center to guide-ways)3.Weight holding capacity of spindle

Methods of Holding the Work in a Lathe

Holding the work between centers Chuck Collet Face plate

Holding the Work Between Centers

(a) mounting the work between centers using a "dog”

- Work is held b/w head-stock and tail stock centers

- Tail-stock center can be live or dead center

- Live center is held in a bearing so rotates

- Dead center is fixed on tailstock shaft, does not rotate: Result is friction.

- Used for holding parts having a large length to diameter ratio

Holding the Work in a Chuck

(b) three‑jaw chuck

- Used when length to dia ratio of w/p is low.-Can be used with and without support of tail-stock center-Can hold w/p from outside as well as from inside-Two types: 3 jaws/ 4 jaws-3 jaws is self centering chuck-For 4 jaws, w/p centering along the spindle axes is carried manually. Also, these can handle irregularstocks

Holding the Work in a Collet

- Collet consists of tubular bushing with longitudinal slits running over half of its length; and equally spaced around its circumference

- Due to slits, one end of collet can be squeezed to reduce diameter and provide a secure grasping pressure against the work

Holding the Work in a Face Plate

(d) face plate for non‑cylindrical workparts

- Use to clamp irregular w/p (non cylinders)- The face plate is fastened to the lathe spindle- The face plate has several slots/holes inside with special clamps so that irregular shape can be clamped

Types of Lathe & Turning Machines1. Turret Lathe

Turret: tool post that can hold many tools

Tailstock replaced by “turret” that holds up to six tools-Tools rapidly brought into action by indexing the turret-Conventional tool post is replaced by four sided turret to index four tools‑-Used for high production work that requires a sequence of cuts on the part -It is operated manually

Types Lathe & Turning Machines

2. Tool room lathe: small in size used for making precise tools.3. Speed Lathe: No carriage and cross slide assembly. Tool post is fixed with lathe bed. This provides high speed. Used for wood turning and spinning

4- Chucking Machine: -Uses a chuck to hold a w/p-Don’t uses tail stock to hold work. So it can handle light weight and low length w/p-Operates similar to turret (means have lathe except the feeding of tools is done automatically

NOT IN

CLUDED

Types Lathe & Turning Machines

5. Bar Machine:-Similar to chucking m/c, except that a collect (instead of chuck) is used , which permits long bar stock to be fed through head stock into position. -At the end of each machining operation a cut-off operation separates the new part. The bar stock is then fed forwarded for machining of next part.-Two types: Single spindle & Multi-spindle

Fig.a.Type of part produced on a 6 spindle m/cb.Sequence of operations to produce the part: 6 operations are done simultaneously

NOT IN

CLUDED

Types Lathe & Turning Machines

5. CNC Lathe Machine:-In conventional machines, the machines motions are controlled through cam (a m/c element)-In CNC machines, the motion is controlled through a program of instructions. These instructions are given to servo-motors to further control the m/c motions

Boring

Difference between boring and turning: Boring is an operation of enlarging inside diameter

of an existing hole

(inside operation) Turning is an operation of reducing outside

diameter of a cylinder (outside operation) In effect, boring is internal turning operation

Types of Boring machines Horizontal: The rotational axis of w/p is horizontal Vertical - The rotational axis of w/p is vertical

.

Horizontal Boring Mill

Fig. Horizontal boring m/c

- Used when Length of part is larger than its diameter; and the weight is low

A vertical lathe or boring m/c

Vertical Boring Mill

- Used when Length of part is smaller than its Diameter; and the part is heavy

A boring bar made of cemented carbide

Creates a round hole in a workpart

Compare to boring which can only enlarge an existing hole

Cutting tool called a drill or drill bit

Machine tool: drill press

Drilling

Through‑holes - drill exits opposite side of work

Blind‑holes – does not exit work opposite side

Two hole types: (a) through‑hole, and (b) blind hole.

Through Holes vs. Blind Holes

-Cutting speed (v) : mm/min

-Feed (f): mm/rev (f~ drill dia)

-Since there are 02 cutting edges, uncut chip thickness taken by each cutting edge is half the feed.

-Feed rate (fr) in mm/min: f×N

-Time to machine a through hole:

-

-Time to machine a blind hole:

- Material removal rate:

=A× fr

Cutting Conditions in Drilling

NOT IN

CLUDED

Used to slightly enlarge a hole, provide better tolerance on diameter, and improve surface finish

Operations Related To Drilling: Reaming

Used to provide internal screw threads on an existing hole

Tool called a tap

Operations Related To Drilling: Tapping

Provides a stepped hole, in which a larger diameter follows smaller diameter partially into the hole

Operations Related To Drilling: Counter-boring

Similar to counter-boring except the step in a hole is cone-shaped for the flat head screws and bolts

Operations Related To Drilling: Counter-Sinking

Upright drill press stands on the floor

Bench drill similar but smaller and mounted on a table or bench

Drill Press

Radial Drill: Large drill press designed for large parts

- The arm can move radially

- Gang drill machine: Consists of 2-6 upright drills. Each spindle is controlled and operated separately

- Multiple drill machine: several spindles are connected together. Operated simultaneously to make multiple hole into a w/p

Drill Machines

- Fixture: is work holding device designed for clamping a specific shape

- Jig: is work holding device designed for clamping work as well as for guiding the tool

- Vise: A general purpose work holding device possessing 02 jaws that grasp the work in position

Work Holding Devices

Milling

Machining operation in which work is fed past a rotating tool with multiple cutting edges

Axis of tool rotation is perpendicular to feedCreates a planar surface

Other geometries possible either by cutter path or shape

Other factors and terms:Cutting tool called a milling cutter, cutting edges called "teeth" Machine tool called a milling machine

Diff b/w Drilling & Milling?

Interrupted cutting operation:The cutter teeth enter and exit w/p in each revolution. This interrupted cutting imposes sudden loads and thermal shocks.

So teeth design should be robust

Two forms of milling: (a) peripheral milling, and (b) face milling.

Two Forms of Milling

Peripheral Milling vs. Face Milling Peripheral milling

Cutter axis is parallel to surface being machined Cutting is performed by cutting edges on outside periphery of

cutter Face milling

Cutter axis perpendicular to surface being milled Cutting edges on both the end and outside periphery of the cutter

are used in cutting

Basic form of peripheral milling in which the cutter width extends beyond the work-piece on both sides

Width of cutter larger than width of w/p

Types of Peripheral Milling: Slab Milling

Width of cutter is less than width of w/p, creating a slot in the work

Types of Peripheral Milling: Slotting

If width of cutter is too small, the tool will become a saw and the operations will be called sawing

Cutter machines the side of a w/p

Types of Peripheral Milling: Side Milling

Cutter simultaneously machines the 02 sides of a w/p

Types of Peripheral Milling: Straddle Milling

- Cutter rotation and feed are in opposite direction- Chip length is large- Chip thinner at start than its end- Tool engages in material for long

time- Tool life is smaller- Cutting force is along tangent of

teeth, so force tries to lift the part

Two Forms of Peripheral Milling

Up- Milling Down- Milling

- Cutter rotation and feed are in same direction- Chip length is small- Chip thicker at start than its end- Tool engages in material for short time- Tool life is longer- Cutting force presses the part. Result is

low vibration and better surface finish

The cutter overhangs both side of w/p

Types of Face Milling: Conventional Face Milling

slab milling

High speed face milling using indexable inserts

The cutter overhangs one side of w/p

Types of Face Milling: Partial Face Milling

Any difference b/w partial face milling & side milling?

side milling

face milling

Cutter diameter is less than work width, so a slot is cut into part

Also diameter of tool is smaller than its height

End Milling

Difference b/w face & end milling?- In face milling, cutter dia is larger than its height but in end milling cutter dia smaller than its height

slotting

Form of end milling in which the outside periphery of a flat part is cut

End milling: Profile Milling

Another form of end milling used to mill shallow pockets into flat parts

End milling: Pocket Milling

Ball‑nose cutter fed back and forth across work along a curvilinear path at close intervals to create a three dimensional surface form

End milling: Surface Contouring

Rotational speed:

Feed (f): Feed/tooth in mm/tooth/rev

Feed rate:

RMR: (Area of cut × fr)

If w is width of cut; d is depth of cut:

Cut time:

Cutting Conditions in Milling

nt: no of teeth

NOT IN

CLUDED

horizontal knee-and-column milling machine.

Horizontal Milling Machine

Suitable for peripheral milling

Spindle axis is parallel to the work surface

vertical knee‑and‑column milling machine

Vertical Milling Machine

Suitable for face milling

Spindle axis is perpendicular to the work surface

Machining Centers

Highly automated machine tool can perform multiple machining operations under CNC control in one setup with minimal human attention Typical operations are milling and drilling Three, four, or five axes

Other features: Automatic tool‑changing Automatic work-part positioning

Types:

Horizontal

Vertical

Universal

Universal machining center; highly automated, capable of multiple machining operations under computer control in one setup with minimal human attention

CNC 4‑axis turning center; capable of turning and related operations, contour turning, and automatic tool indexing, all under computer control.

Turning Centers

Mill-Turn Centers

Highly automated machine tool that can perform turning, milling, and drilling operations

General configuration of a turning center Can position a cylindrical work-part at a

specified angle so a rotating cutting tool (e.g., milling cutter) can machine features into outside surface of part

Conventional turning center cannot hold work-part at a defined angular position and does not include rotating tool spindles

Operation of a mill‑turn center: (a) example part with turned, milled, and drilled surfaces; and (b) sequence of operations on a mill‑turn center: (1) turn second diameter, (2) mill flat with part in programmed angular position, (3) drill hole with part in same programmed position, and (4) cutoff.

Operation of Mill-Turn Center

Similar operations Both use a single point cutting tool moved

linearly relative to the workpart

(a) Shaping, and (b) planing.

Shaping and Planing

Shaping and Planing

A straight, flat surface is created in both operations

Interrupted cutting Subjects tool to impact loading when

entering work Low cutting speeds due to start‑and‑stop

motion Typical tooling: single point high speed steel

tools

Components of a shaper.

Shaper

Open side planer.

Planer

Moves a multiple tooth cutting tool linearly relative to work in direction of tool axis

Broaching

Broaching

Advantages: Good surface finish Close tolerances Variety of work shapes possible High material removal rate

Cutting tool called a broach Owing to complicated and often

custom‑shaped geometry, tooling is expensive

Performed on internal surface of a hole A starting hole must be present in the part to

insert broach at beginning of stroke

Work shapes that can be cut by internal broaching; cross‑hatching indicates the surfaces broached.

Internal Broaching

C - CUTTING TOOL TECHNOLOGY

1. Tool Life

2. Tool Materials

3. Tool Geometry

4. Cutting Fluids

Cutting Tool Technology

Two principal aspects:

1. Tool material

2. Tool geometry

Three Modes of Tool Failure

1. Fracture failure Cutting force becomes excessive at the tool

point, leading to brittle fracture

2. Temperature failure Cutting temperature is too high for the tool

material causing softening of tool point. This leads to plastic deformation and loss of sharp edge.

3. Gradual wear Gradual wearing of the cutting edge causes

loss of tool shape, reduction in cutting efficiency. Finally tool fails in a manner similar to temp failure

Preferred Mode: Gradual Wear

Fracture and temperature failures are premature failures (how can u avoid these failures to occur?)

Gradual wear is preferred because it leads to the longest possible use of the tool

Gradual wear occurs at two locations on a tool: Crater wear – occurs on top rake face Flank wear – occurs on flank (side of tool)

Diagram of worn cutting tool, showing the principal locations and types of wear that occur.

Tool Wear

Crater wear occurs because of tool chip flow on top rake face. High friction, temp and stresses at the face/chip interface are responsible. Measured as area or depth of dip Flank wear results from rubbing of flank (& or relief) face to the newly generated surface. Measured by width of wear band called wear land.

Notch wear occurs because of tool rubbing against original work surface, which is harder than machined one

Crater wear

Flank wear

Mechanisms of Tool Wear:Abrasion: This is a mechanical wearing action due to hard particles in w/p. These hard particles cause gouging and remove small portions of the tool. It occurs in both crater and flank wear. Adhesion: When 02 metals are forced into contact under high pressure & temp, adhesion or welding occurs b/w them. This mechanism occurs in crater wear. The chip material welds on rake face and later this welded mass is removed due to subsequent chip flow, hence producing dips into the rake face.Diffusion: This is a process in which an exchange of atoms take place across a close contact boundary (like chip-rake face) . At high temp, the atoms responsible for tool hardness diffuse from tool into chip, thus softening top surface of tool. Later this promotes both abrasion and adhesion at rake face. Diffusion causes crate wear.Chemical Reactions: At high speeds, due to high temp at the chip-rake interface, oxidation layer form. This layer is sheared down and a new layer is formed. This process continues and causes crater wear. Plastic Deformation: At high temp, the plastic deformation of tool nose and cutting edge takes place. This further promotes abrasion. This is major reason for flank wear.

Tool wear as a function of cutting time. Flank wear (FW) is used here as the measure of tool wear. Crater wear follows a similar growth curve.

Tool Wear vs. Time

-The tool performance is dictated by uniform wear rate (or slop of steady state region).-The slop of steady state region changes with change in cutting conditions.- Speed is the major influential parameter

Effect of cutting speed on tool flank wear (FW) for three cutting speeds

Effect of Cutting Speed on Wear

Tool Life

Length of cutting time that the tool can be used.- Time till tool fracture?- If so, tool needs to re-sharp again and again. This is not so easy in

production. Also, re-sharpening will affect surface finish- Better to define a a level of tool wear ( say 0.5)- Tool life against each curve is shown in Fig.

Natural log‑log plot of cutting speed vs tool life.

Tool Life vs. Cutting Speed

Taylor Tool Life Equation

Relationship is credited to F. W. Taylor nvT C

where v = cutting speed; T = tool life; n is the slope of the plot; C is the intercept on the speed axis at one minute tool life

n and C are parameters that depend on feed, depth of cut, work material, tooling material, and the tool life criterion used

n

C

Tool Life Criteria in Production

Practically, it is not always easy to measure flank wear (0.5mm) and time to know TOOL LIFE. Therefore, in shops any of these criterion can be used for changing a tool: 1.Complete failure of cutting edge 2.Visual inspection of flank wear (or crater wear) by the machine operator3.Fingernail test across cutting edge4.Changes in sound emitted from operation5.Chips become ribbon-like, stringy, and difficult to dispose off6.Degradation of surface finish7.Increased power8.Work-piece count: Dispose off tool after certain no of pieces9.Cumulative cutting time

Tool Materials

Tool failure modes identify the important properties that a tool material should possess: Toughness ‑ to avoid fracture failure Hot hardness ‑ ability to retain hardness at

high temperatures Wear resistance ‑ hardness is the most

important property to resist abrasion

Typical hot hardness relationships for selected tool materials. Plain carbon steel shows a rapid loss of hardness as temperature increases. High speed steel is substantially better, while cemented carbides and ceramics are significantly harder at elevated temperatures.

Hot Hardness

Typical Values of n and C

Tool material n C (m/min) C (ft/min)

High speed steel:

Non-steel work 0.125 120 350

Steel work 0.125 70 200

Cemented carbide

Non-steel work 0.25 900 2700

Steel work 0.25 500 1500

Ceramic

Steel work 0.6 3000 10,000

nvT C

High Speed Steel (HSS)

Highly alloyed tool steel capable of maintaining hardness at elevated temperatures better than high carbon and low alloy steels

One of the most important cutting tool materials

Especially suited to applications involving complicated tool geometries, such as drills, taps, milling cutters, and broaches

Two basic types (AISI)

1. Tungsten‑type, designated T‑ grades

2. Molybdenum‑type, designated M‑grades

High Speed Steel Composition

Typical alloying ingredients: Tungsten and/or Molybdenum Chromium and Vanadium Carbon, of course Cobalt in some grades

Typical composition (Grade T1): 18% W, 4% Cr, 1% V, and 0.9% C

Cemented Carbides

Class of hard tool material based on tungsten carbide (WC) using powder metallurgy techniques with cobalt (Co) as the binder

Two basic types:

1. Non‑steel cutting grades - only WC‑Co

2. Steel cutting grades - TiC and TaC added to WC‑Co

Cemented Carbides – General Properties

High compressive strength but low‑to‑moderate tensile strength

High hardness (90 to 95 HRc) Good hot hardness Good wear resistance High thermal conductivity High elastic modulus ‑ 600 x 103 MPa Toughness lower than high speed steel

Non‑steel Cutting Carbide Grades

Used for nonferrous metals and gray cast iron Properties determined by grain size and cobalt

content As grain size increases, hardness and hot

hardness decrease, but toughness increases

As cobalt content increases, toughness improves at the expense of hardness and wear resistance

Steel Cutting Carbide Grades

Used for low carbon, stainless, and other alloy steels

TiC and/or TaC are substituted for some of the WC

Composition increases crater-wear resistance for steel cutting But adversely affects flank wear

resistance for non‑steel cutting applications

Cermets

Ceramic-metal composite

Cemented carbide is a kind of cermet

Combinations of TiC, TiN, and titanium carbonitride (TiCN), with nickel and/or molybdenum as binders.

Some chemistries are more complex Applications: high speed finishing and semifinishing of

steels, stainless steels, and cast irons Higher speeds and lower feeds than steel‑cutting

carbide grades Better finish achieved, often eliminating need for

grinding

Coated Carbides

Cemented carbide insert coated with one or more thin layers of wear resistant materials, such as TiC, TiN, and/or Al2O3

Coating applied by chemical vapor deposition or physical vapor deposition

Coating thickness = 2.5 ‑ 13 m (0.0001 to 0.0005 in)

Applications: cast irons and steels in turning and milling operations

Best applied at high speeds where dynamic force and thermal shock are minimal

Coated Carbide Tool

Photomicrograph of cross section of multiple coatings on cemented carbide tool

WC/TiCCo/Ni

Ceramics

Primarily fine‑grained Al2O3, pressed and sintered at high pressures and temperatures into insert form with no binder

Applications: high speed turning of cast iron and steel

Not recommended for heavy interrupted cuts (e.g. rough milling) due to low toughness

Al2O3 also widely used as an abrasive in grinding

Synthetic Diamonds

Sintered polycrystalline diamond (SPD) - fabricated by sintering very fine‑grained diamond crystals under high temperatures and pressures into desired shape with little or no binder

Usually applied as coating (0.5 mm thick) on WC-Co insert

Applications: high speed machining of nonferrous metals and abrasive nonmetals such as fiberglass, graphite, and wood Not for steel cutting, Why??

Cubic Boron Nitride

Next to diamond, cubic boron nitride (cBN) is hardest material known

Fabrication into cutting tool inserts same as SPD: coatings on WC‑Co inserts

Applications: machining steel and nickel‑based alloys

SPD and cBN tools are expensive

Tool Geometry

Two categories: Single point tools

Used for turning, boring, shaping, and planing

Multiple cutting edge tools Used for drilling, reaming, tapping,

milling, broaching, and sawing

Tool Geometry- Single Point Cutting Tool

Chip breaker

Plain/ Peripheral Milling Cutter

Tool Geometry: Multi-Point Cutting Tool

The "business end" of a twist drill has two cutting edges The included angle of the point on a conventional twist drill is 118°

Margins are the outside tip of the flutes and are always ground to the drill diameter

Tool Geometry: Multi-Point Cutting Tool

Twist dill

Twist Drills

An essential feature of drilling is the variation in cutting speed along the cutting edge. The speed is maximum at the periphery, which generates the cylindrical surface, and approaches zero near the center-line of the drill where the cutting edge is blended to a chisel shape.

Drills are slender, highly stressed tools, the flutes of which have to be carefully designed to permit chip flow while maintaining adequate strength.

Twist Drill Operation - Problems

Chip removal Flutes must provide sufficient clearance to

allow chips to be extracted from bottom of hole during the cutting operation

Friction makes matters worse Rubbing between outside diameter of drill

bit and newly formed hole Delivery of cutting fluid to drill point to

reduce friction and heat is difficult because chips are flowing in opposite direction

Cutting Fluids

Any liquid or gas applied directly to machining operation to improve cutting performance

Two main problems addressed by cutting fluids:

1. Heat generation at shear and friction zones

2. Friction at tool‑chip and tool‑work interfaces Other functions and benefits:

Wash away chips (e.g., grinding and milling) Reduce temperature of workpart for easier handling Improve dimensional stability of workpart

Classification of Cutting Fluids by Functions

Cutting fluids can be classified according to function:

Coolants - designed to reduce effects of heat in machining

Lubricants - designed to reduce tool‑chip and tool‑work friction

Coolants

Water is used as base in coolant‑type cutting fluids

Most effective at high cutting speeds where heat generation and high temperatures are problems

Most effective on tool materials that are most susceptible to temperature failures (e.g., HSS)

Lubricants

Usually oil‑based fluids Most effective at lower cutting speeds Also reduce temperature in the operation

Dry Machining

No cutting fluid is used Avoids problems of cutting fluid contamination,

disposal, and filtration Problems with dry machining:

Overheating of tool Operating at lower cutting speeds and

production rates to prolong tool life Absence of chip removal benefits of cutting

fluids in grinding and milling

Gear Cutting

Gear cutting is the process of creating a gear. The most common processes include hobbing, broaching, and machining; other processes include shaping, forging, extruding, casting, and powder metallurgy.

Hobbing is a machining process for making gears, on a hobbing machine,

The teeth or splines are progressively cut into the workpiece by a series of cuts made by a cutting tool called a hob.

Compared to other gear forming processes it is relatively inexpensive but still quite accurate, thus it is used for a broad range of parts and quantities

Hobbing

Hobbing Process

Hobbing uses a hobbing machine with two non-parallel spindles, one mounted with a blank workpiece and the other with the hob.

The angle between the hob's spindle and the workpiece's spindle varies, depending on the type of product being produced.

If a spur gear is being produced, then the hob is angled equal to the helix angle of the hob; if a helical gear is being produced then the angle must be increased by the same amount as the helix angle of the helical gear

Hobbing Process

The two shafts are rotated at a proportional ratio, which determines the number of teeth on the blank; for example, if the gear ratio is 40:1 the hob rotates 40 times to each turn of the blank

The hob is then fed up into workpiece until the correct tooth depth is obtained.

Finally the hob is fed into the workpiece parallel to the blank's axis of rotation

Hob

The hob is the cutter used to cut the teeth into the workpiece.

It is cylindrical in shape with helical cutting teeth. These teeth have grooves that run the length of the hob, which aid in cutting and chip removal.

The cross-sectional shape of the hob teeth are almost the same shape as teeth of a rack gear that would be used with the finished product

Assignment No. 2

What is Powder Metallurgy? What are its capabilities? What are the common Powder Compacting techniques? How is Sintering performed?

Last date of submission: 01- 06-2014 Any two mutually copied assignments will be cancelled