topic 2 machining 160214

BDA 3052 Manufacturing Technology

Material Removal Process (Metal

Machining Process)

1.1 Theory of Metal Cutting

overview, theory of chip formation, force &

merchant equation, power & energy, cutting

temperature

1.2 Machining Operations and Machine Tools

turning, drilling, milling, machine centers,

cutting tool technology


Material Removal ProcessesIntroduction

It is shaping operations, it remove material from a

starting workpart so the remaining part has the

desired geometry

Divided into three main groups:

1) Machining – material removal by a sharp

cutting tool, e.g., turning, milling, drilling

2) Abrasive processes – material removal by hard,

abrasive particles, e.g., grinding

3) Nontraditional processes - various energy

forms other than sharp cutting tool to remove

material


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


Machining

Nomenclature of single point tool


Seven elements of single-point tool geometry; and

(b) the tool signature convention that defines the

seven elements.


Simplified 2-D model of machining that

describes the mechanics of machining

fairly accurately

Figure 21.6 Orthogonal cutting: (a) as a three-dimensional process.

Orthogonal Cutting Model


Orthogonal and Oblique cutting

Assumptions in orthogonal cutting

(Merchant theory)

01.The tool is perfectly sharp and no contact along clearance face.

02. The shear surface is a plane extending upward from the cutting edge.

03.The cutting edge is a straight line, extending perpendicular to the direction of motion and generates a plane surface as the work moves past it.

04.The chip does not flow to either side.

05.The depth of cut is constant.

06.Width of the tool is greater than the work piece.

07.The work moves relative to the tool with uniform velocity.

08.A continuous chip is produced with no built up edge.


Assumptions in orthogonal cutting

(Merchant theory)

09. Chip is assume to shear continuously across plane AB on which the shear stress reaches the value of shear flow stress.

10.Width of chip is remains equal to the width of the work piece. i.e. Plane strain conditions exist.


Difference between orthogonal and

oblique cutting

Orthogonal cutting Oblique cutting

01.The cutting edge of the tool is perpendicular to the direction of the tool travel.

The cutting edge is inclined at angle with the normal direction of the tool travel.

02. The cutting edge clears the width of the work piece on either ends.

The cutting edge may or may not clear the width of the workpiece.

03. The chip flows over the tool. The chip coils in tight. The chip flows on the tool face making an angle with the normal cutting edge. The chip flows side ways in a long curl.

04. Only two components of the cutting force acting on the tool.

Three components of the forces acting on the tool.

05.Maximum chip thickness occurs at the middle. Maximum chip thickness may not occur at middle.

06. For the given feed rate and DOC, the force which act or shears the metal acts on a smaller area and therefore, the heat developed per unit area due to friction along the tool work interface is less and the tool life is less.

It acts on larger area and thus tool life is more.


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


Cutting Process in Turning


Relationship between chip thickness,

rake angle and shear plane angle

)cos(

sin

)cos(

sin

s

s

l

lr


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 always greater than

before, so chip ratio always less than 1.0

c

o

t

tr


Example of Problem

In a machining operation that approximates orthogonal

cutting, the cutting tool has a rake angle = 10. The chip

Thickness before the cut to = 0.50 mm and the chip

thickness after the cut tc = 1.125 mm. Calculate the shear

plane angle and the shear strain in the operation.

Answer : = 25.4

= 2.386


Example of Problem 2

In an orthogonal cutting operation, the tool has a rake

angle = 15. The chip thickness before the cut = 0.30

mm and the cut yields a deformed chip thickness = 0.65

mm. Calculate (a) the shear plane angle and (b) the

shear strain for the operation.


Shear strain during chip formation: (a) chip formation depicted as a series of

parallel plates sliding relative to each other, (b) one of the plates isolated

to show shear strain, and (c) shear strain triangle used to derive strain

equation.

Shear Strain in Chip Formation


Shear Strain

Shear strain in machining can be computed

from the following equation, based on the

preceding parallel plate model:

= tan( - ) + cot

where = shear strain, = shear plane angle, and =

rake angle of cutting tool

BD

DCAD

BD

AC



In an orthogonal cutting operation, the tool has a rake

angle = 15. The chip thickness before the cut = 0.30

mm and the cut yields a deformed chip thickness = 0.65

mm. Calculate (a) the shear plane angle and (b) the

shear strain for the operation.


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


Brittle work

materials

Low cutting

speeds

Large feed and

depth of cut

High tool-chip

friction

1. Discontinuous Chip


Ductile work

materials

High cutting

speeds

Small feeds and

depths

Sharp cutting edge

Low tool-chip

friction

2. 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

Figure 21.9 (d) serrated.


Higher shear plane angle means smaller shear plane which means lower shear force, cutting forces, power, and temperature

Effect of shear plane angle : (a) higher with a resulting lower shear plane area; (b) smaller with a corresponding larger shear plane area. Note that the rake angle is larger in (a), which tends to increase shear angle according to the Merchant equation

Effect of Higher Shear Plane Angle


Friction force F and Normal force to friction N

Shear force Fs and Normal force to shear Fn

Forces in metal cutting: (a)

forces acting on the chip in

orthogonal cutting

Forces Acting on Chip


Resultant Forces

Vector addition of F and N = resultant R

Vector addition of Fs and Fn = resultant R'

Forces acting on the chip must be in balance:

R‘’ must be equal in magnitude to R

R’ must be opposite in direction to R

R’ must be collinear with R


Cutting Forces


Coefficient of Friction

Coefficient of friction between tool and chip:

Friction angle related to coefficient of friction

as follows:

N

F

tan

-(1)

-(2)


Shear Stress

Shear stress acting along the shear plane:

sin

wtA o

s

where As = area of the shear plane

Shear stress = shear strength of work material during

cutting

s

s

A

FS -(3)

-(4)


F, N, Fs, and Fn cannot be directly measured

Forces acting on the tool that can be measured:

Cutting force Fc and Thrust force Ft

Forces in metal

cutting: (b) forces

acting on the tool that

can be measured

Cutting Force and Thrust Force


Merchant’s circle diagram


Forces in Metal Cutting

Equations can be derived to relate the forces that

cannot be measured to the forces that can be

measured:

F = Fc sin + Ft cos (5)

N = Fc cos - Ft sin (6)

Fs = Fc cos - Ft sin (7)

Fn = Fc sin + Ft cos (8)

Based on th ese calculated force, shear stress

and coefficient of friction can be determined



Cutting force = 1559 N

Thrust force = 1271 N

Width of cutting = 3 mm

Rake angle = 10

Shear plane angle = 25.4

Original Thickness = 0.5 mm

Determine the shear strength of the work

material.

shear stress, S / shear strength, = 247 N/mm2


Forces in Metal Cutting

From equation 3, the force diagram (Merchant ‘s Circle

Diagram), can be used to derived the following

equations:

)cos(

)cos(

)cos(sin

)cos(

so

c

FwStF

)cos(

)sin(

)cos(sin

)sin(

so

t

FwStF


The Merchant Equation

From equation 3, 4 and 7, Merchant Equation for shear

stress can be expressed as,

)sin/(

sincos

o

tc

t

FF


The Merchant Equation

Of all the possible angles at which shear

deformation can occur, the work material will

select a shear plane angle that minimizes

energy, given by

Derived by Eugene Merchant

Based on orthogonal cutting, but validity extends

to 3-D machining

2245


What the Merchant Equation Tells Us

To increase shear plane angle

Increase the rake angle

Reduce the friction angle (or coefficient of

friction)

2245


Power and Energy Relationships

A machining operation requires power

The power to perform machining can be

computed from:

Pc = Fc

where Pc = cutting power (Nm/s); Fc = cutting force

(N); and = cutting speed (m/min)



In U.S. customary units, power is traditional

expressed as horsepower

HPc = Fc/33,000

where HPc = cutting horsepower, hp



Gross power to operate the machine tool Pg or

HPg is given by

or

where E = mechanical efficiency of machine tool

Typical E for machine tools 90%

E

PP c

g E

HPHP c

g


Unit Power in Machining

Useful to convert power into power per unit

volume rate of metal cut

Called unit power, Pu or unit horsepower, HPu

or

where MRR = material removal rate (mm3/s)

MRR

PP c

U MRR

HPHP c

u


Specific Energy in Machining

Unit power is also known as the specific energy U

Units for specific energy are typically N-m/mm3 or

J/mm3

wt

F

wvt

vF

MRR

PPU

o

c

o

ccu


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

The remaining energy (about 2%) is retained as

elastic energy in the chip


Cutting Temperature is 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


Cutting Temperature

Analytical method derived by Nathan Cook

from dimensional analysis using

experimental data for various work materials

where T = temperature rise at tool-chip interface; U =

specific energy; v = cutting speed; to = chip thickness

before cut; C = volumetric specific heat of work

material; K = thermal diffusivity of work material

333040

..

K

vt

C

UT o


Example Problem 6

Cutting speed = 100 m/min

Chip original thickness = 0.5 mm

Thermal diffusivity = 50 mm2/s

Specific Energy = 1.038

Volumetric specific heat work material = 3 x10-3 J/mm3

Find the mean temperature rise at the tool-chip

Interface.


Cutting Temperature

Experimental methods can be used to measure temperatures in machining

Most frequently used technique is the tool-chip thermocouple

Using this method, Ken Trigger determined the speed-temperature relationship to be of the form:

T = K vm

where T = measured tool-chip interface temperature, and v = cutting speed

K and m depend on the cutting conditions and work material

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

characteristic 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

Figure 22.2 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

Turning

Single point cutting tool removes material from a

rotating workpiece to generate a cylinder

Performed on a machine tool called a lathe

Variations of turning performed on a lathe:

Facing

Contour turning

Chamfering

Cutoff

Threading

Turning operation.

Turning

Turning Operation

Close-up view of a

turning operation on

steel using a titanium

nitride coated carbide

cutting insert (photo

courtesy of Kennametal

Inc.)

Tool is fed

radially inward

Facing

Instead of feeding tool

parallel to axis of

rotation, tool follows a

contour that is other

than straight, thus

creating a contoured

shape

Contour Turning

Cutting edge cuts an angle on the corner of

the cylinder, forming a "chamfer"

Chamfering

Tool is fed radially into rotating work at some

location to cut off end of part

Cutoff

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

Threading

The rotational speed in turning related to the desired cutting

speed at the surface of the cylindrical workpiece by the

equation:

N = rotational speed, rev/min; = cutting speed, m/min,

And Do = original diameter of the part, m.

Cutting Conditions in Turning - 1

DN

The change in diameter is determined by the depth of cut,

d:

Do – Df = 2d

Do = original diameter, mm; Df = final diameter, mm

d = depth of cut


The feed in turning is generally expressed in mm/rev. This

feed can be converted to linear travel rate in mm/min by the

formula:

fr = Nf

fr = feed rate, mm/min; f = feed mm/rev


The time to machine from one end of a cylindrical workpart

to the other is given by:

Tm = L/fr

Tm = time of actual machining, minutes; and L = length of

the cylindrical workpart, mm


The volumetric rate of material removal rate can be most

conveniently determined by the following equation:

MRR = vfd

MRR = material removal rate, mm3/min, f = feed, mm


A cylindrical workpart 200 mm in diameter

and 700 mm long is to be turned in an

engine lathe. Cutting conditions are as

follows: cutting speed is 2.30 m/s, feed is

0.32 mm/rev, and depth of cut is 1.80

mm.

Determine (a) cutting time, and (b)

metal removal rate.

Cutting Conditions in Turning

Problem 1

A work materials are to be turned to final size of

175 mm length having diameter of 60 mm.

Total length of the work material is 300 mm. A

single point tool having a certain degree of rake

angle is used. The work material rotates at 1400

RPM. The feed is 0.35 mm / revolution.

Final size of the work material is 51 mm.

Calculate:

1. Cutting velocity,

2. Time taken to machine to the length of 55 mm,

3. Total Material removal rate to get 51 mm diameter.

Cutting Conditions in Turning

Problem 2

Milling

Machining operation in which work is fed past a

rotating tool with multiple cutting edges

Axis of tool rotation is perpendicular to feed

Creates a planar surface

Other geometries possible either by cutter

path or shape

Other factors and terms:

Interrupted cutting operation

Cutting tool called a milling cutter, cutting

edges called "teeth"

Machine tool called a milling machine

Peripheral Milling vs. Face Milling

Peripheral milling

Cutter axis parallel to surface being machined

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

METHODS OF MILLING-1

1) Up milling is also referred to as conventional milling. The direction of the cutter rotation opposes the feed motion. For example, if the cutter rotates clockwise , the workpiece is fed to the right in up milling.

METHODS OF MILLING-2

2) Down milling is also referred to as climb milling. The direction of cutter rotation is same as the feed motion. For example, if the cutter rotates counterclockwise , the workpiece is fed to the right in down milling

Basic form of peripheral milling in which the

cutter width extends beyond the workpiece

on both sides

Slab Milling

Width of cutter is less than workpiece width, creating a slot in the work

Slotting

Cutter overhangs work

on both sides

Conventional Face Milling

Form of end milling in

which the outside

periphery of a flat

part is cut

Profile Milling

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

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

Surface Contouring

Cutter diameter is

less than work

width, so a slot is

cut into part

End Milling

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

Pallet shuttles

Automatic workpart positioning

High speed face

milling using

indexable inserts

(photo courtesy

of Kennametal

Inc.).

Milling Operation

The cutting speed is determined at the

outside diameter of a milling cutter.

N = rotational speed, rev/min; = cutting speed,

m/min,

And Do = outside diameter of a milling cutter,mm.

Cutting Conditions in Milling - 1

DN

The feed, f in milling is usually given as a feed per cutter

tooth; called the chip load, it represents the size of the chip

formed by each cutting edge.

fr = Nnt f

fr = feed rate, mm/min; N = spindle speed, rev/min; nt =

number of teeth on the cutter; f = chip load in mm/tooth


The material removal rate,

MRR = wdfr

w = width; d = depth of cut; fr = feed rate, mm/min;


Peripheral Milling

Approach distance A, to reach full cutter depth given by:

d = depth of cut, mm, and D = diameter of the milling cutter,

mm


)( dDdA

The time to mill the workiece Tm is

therefore;


r

mf

ALT

Face Milling – cutter is centered

Where A and O are each to half the cutter

diameter;

A = O = D/2

D= cutter diameter, mm


Face Milling – cutter is offset

Where A and O are each to half the cutter

diameter;

w= width of the cut, mm


)( wDwOA

The time to mill the workiece in face

milling,Tm is therefore;


r

mf

ALT

2

A peripheral milling operation is performed

on the top surface of a rectangular

workpart which is 400 mm long by 60 mm

wide. The milling cutter, which is 80 mm in

diameter and has five teeth, overhangs the

width of the part on both sides. The cutting

speed is 70 m/min, the chip load is 0.25

mm/tooth, and the depth of cut is 5.0 mm.

Determine (a) the time to make one pass

across the surface, and (b) the maximum

material removal rate during the cut.

Cutting Conditions in Milling

Problem 1

A face milling operation is performed to finish

the top surface of a steel rectangular work

piece 350 mm long by 55 mm wide. The milling

cutter has four teeth (cemented carbide inserts)

and a 85 mm diameter. Cutting conditions are:

v = 600 m/min, f = 0.35 mm / tooth, and d =

3.5 mm.

Determine:

a) the time to make one pass across the surface.

b) the metal removal rate during the cut.

Cutting Conditions in Milling

Problem 2

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

Used to slightly

enlarge a hole,

provide better

tolerance on

diameter, and

improve surface

finish

Reaming

Used to provide

internal screw

threads on an

existing hole

Tool called a tap

Tapping

Provides a stepped

hole, in which a

larger diameter

follows smaller

diameter partially

into the hole

Counterboring

Letting N represent the spindle rev/min,

= cutting speed, m/min; D = the drill

diameter,mm.

Cutting Conditions in Drilling - 1

DN

Feed can be converted to feed rate using the the same

equation as for turning:

fr = Nf

fr = feed rate, mm/min; N = spindle speed, rev/min; f = feed

in drilling, mm/rev


The time to drill through holes;

Tm= machining time, min; t = work thickness,

mm; fr = feed rate, mm/min


r

mf

AtT

The allowance is given by;

A = approach allowance, mm; = drill point angle


290tan5.0

DA

The time to drill blind holes;

Tm= machining time, min; d = hole depth,

mm; fr = feed rate, mm/min


r

mf

dT

A drilling operation is to be performed with a

12.7 mm diameter twist drill in a steel

workpart. The hole is a blind hole at a depth of

60 mm and the point angle is 118. The cutting

speed is 25 m/min and the feed is 0.30

mm/rev.

Determine

(a) the cutting time to complete the drilling

operation, and

(b) metal removal rate during the operation, after the drill bit reaches full diameter.

Cutting Conditions in Drilling

Problem 1

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 and/or

dynamic, leading to brittle fracture

2. Temperature failure

Cutting temperature is too high for the tool

material

3. Gradual wear

Gradual wearing of the cutting tool

Preferred Mode: Gradual Wear

Fracture and temperature failures are premature

failures

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)

Figure 23.1 Diagram of worn cutting tool, showing the principal

locations and types of wear that occur.

Tool Wear

Figure 23.2 Crater wear,

(above), and flank wear (right) on

a cemented carbide tool, as seen

through a toolmaker's

microscope (photos by K. C.

Keefe, Manufacturing Technology

Lab, Lehigh University).

FLANK WEAR and BUE

CRATER WEAR

BUILT UP EDE

Taylor Tool Life Equation

Relationship is credited to F. W. Taylor

CvT n

where v = cutting speed; T = tool life; and n and Care parameters that depend on feed, depth of cut,

work material, tooling material, and the tool life

criterion used

n is the slope of the plot

C is the intercept on the speed axis at one minute

tool life

Tool Life Criteria in Production

1. Complete failure of cutting edge

2. Visual inspection of flank wear (or crater wear) by the machine operator

3. Fingernail test across cutting edge

4. Changes in sound emitted from operation

5. Chips become ribbon-like, stringy, and difficult to dispose of

6. Degradation of surface finish

7. Increased power

8. Workpiece count

9. 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 abrasive wear

topic 2 machining 160214

Engineering

tool technology

manufacturing processes

machining material removal

sharp cutting tool

cutting edge

machining operations

mechanics of machining

manufacturing sequence