3.1 Metal Removal

8/2/2019 3.1 Metal Removal

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3. Metal Cutting

1

1. Turning (forrotational parts)

2. Drilling (forhole making)

3. Milling (fornon-rotationalparts)

Metal cutting removes material

(chips) by a sharp cutting toolby turning, milling, drilling, etc. Major components

Work piece Work piece holder Cutting tool Tool holder Machine tool

+ Variety of materials can bemachined.

+ Variety of part geometries can

be produced.+ Provides good dimensional

accuracy and surface finish.- Wasted material- Time consuming


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Cutting tool classification

Single point tool

One dominant cutting edge

Typically rounded to formnose radius

Turning uses single pointtools

Multi-point tools

More than one cutting edge

Motion relative to workachieved by rotating

Drilling, milling use multi-point tools

2


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1. Turning Process

Performed on a machine tool called Lathe

Single point cutting tool removes material from arotating work piece to generate a cylindrical shape.

3

a) d = Depth of cut, f = Feed rate,b) Fc = Cutting force, Ft = Feed force, Fr = Radial force


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Feed and depth of cut

4


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Lathe operations for round shapes

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a) Straight turningb) Taper turningc) Profilingd) External groovinge) Facingf) Face grooving

g) Form cuttingh) Boring and

internal groovingi) Drilling

j) Cutting offk) Threading

l) Knurling


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6

Engine Lathe


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Simplified two-dimensional cutting

Taylor tool life equationC, n = ConstantSome values of n

* nC v t

Rake angle (degrees) Shear angle (degrees) Friction angle (degrees)


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Forces acting on a chip

9

2 2 0*,sin( )

*sin( ), * cos( )

* cos( ), *sin( )

* sin( ) * cos( )

* ( ) *sin( )

* cos( ) * sin( )

* cos( ) * sin( )

t c s

c t

c t

c t

s c t

s c t

w tR F F A

F R N R

F R F R

F F F

N F cos F

F F F

N F F

Rake angle (degrees)

Shear angle (degrees)

As - Area of shear plane

Ft - Thrust force (N, lbs.)

Fc - Cutting force (N, lbs.)

R - Resultant force (N, lbs.)

F - Friction force (N, lbs.)

N - Normal friction force (N, lbs.)

Fs - Shear force (N, lbs.)

Ns - Normal shear force (N, lbs.)

- Friction angle (degrees)


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Mechanics of chip formation

Independent variable Type of cutting tool

Tool geometry andsharpness

Work piece material

Cutting parameters (feed,speed, depth of cut)

Cutting fluid

Tool and work pieceholding devices

10

Dependent variable Type of chip produced

Force required

Energy dissipated

Work piece, chip, tooltemperature rise

Tool wear

Surface finish


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Feed and SpeedV - Cutting speed (m/min, ft/min)N - Rotation speed (rev/min)f - Feed (mm/rev, in/rev)

fr - Feed rate (mm/min, in/min)D - Cutter diameter (mm, in)l - Length of cut (mm, in)lc - Offset lengthw - Work piece width (mm, in)

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TurningD0 - Original part diameter (mm, in)Df - Final part diameter (mm, in)Da -Diameter average (mm, in)d - Depth of cut (mm, in)

t0 - Chip thickness before cut (mm, in)tc - Chip thickness after cut (mm, in)r - Cutting ratioVc - Chip velocity (m/s, ft/s)Vs - Shear velocity (m/s, ft/s)

0* *

* *

D l lt

f V f N

0

2

f

a

D DD

0

2

fD D

d

0 sin( )

cos( )c

tr

t

*rf N f0

*

VN

D

MRR - Material RemovalRate (mm3/s, in3/s)

MRR = *Da*d*f*N = V*d*f


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Specific Energy (N-m/mm3, in-lb/in3)

Uc -Specific energy (cutting)

Us -Specific energy (shearing)

Uf- Specific energy (friction)Tool Life (min)

C Constant

n Constant

t Tool life

0

0

0

*

**

* *

* ** *

cc

cf

s s ss

c f s

FUw t

F VF VcU

w t V MRR

F Vs F V Uw t V MRR

U U U

0

0

0

* * * *

* * * *

* * * *

c c c

f c f

s s s s

c s f

ct

PW F V U V w t

PW F V U V w t

PW F V U V w t

PW PW PW

PWPW

E

1/* , ( )n nC

C V t t V

Power (N-m/s, W(ft-lb/min)

Hp=(ft-lb/min)/33,000

PWcPower (cutting)

PWsPower (shearing)

PWfPower (friction)

PWt - Total power

E - Mechanical efficiency

Power equations


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13

2 2 0*

,sin( )

*sin( ), *cos( )

*cos( ), *sin( )

*sin( ) *cos( )

* ( ) *sin( )

* cos( ) *sin( )

* cos( ) * sin( )

*tan( )

*tan( )

tan( ),

t c s

c t

c t

c t

s c t

s c t

t c

c t

s

s

w tR F F A

F R N R

F R F RF F F

N F cos F

F F F

N F F

F F

F F

F

A

Rake angle (degrees)

Shear angle (degrees)

As - Area of shear plane

Ft - Thrust force (N, lbs.)

Fc - Cutting force (N, lbs.)

R - Resultant force (N, lbs.)

F - Friction force (N, lbs.)

N - Normal friction force (N, lbs.)

Fs - Shear force (N, lbs.)

Ns - Normal shear force (N, lbs.)

- Friction angle (degrees)

- Shear strength

- Shear strain

r - Shear strain rate

- Coefficient of friction

*cos( )tan( )1 *sin( )

45 ( )2 2

45 ( )

tan( ) cot( )

rr

Merchant

Lee

Force Equations


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Example: In an orthogonal cutting operation, the tool has arake angle = 15 degree. 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) theshear strain for the operation.

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0

0

.30, .65, 15

.30 0.462

.65

c

c

t t

tCutting ratio rt

26.85

cot deg( ) tan deg deg( )

atanr cos deg( )

1 r sin deg( )

180

2.18


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Example: In a turning operation, spindle speed is set to provide a cuttingspeed of 1.8 m/s. The feed and depth of cut are 0.30 mm and 2.6 mm,respectively. The tool rake angle is 8 degree. After the cut, the deformed chipthickness is measured to be 0.49 mm. Determine (a) shear plane angle, (b)

shear strain, and (c) material removal rate.

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0

3

0

3 3

.30 , .49 , 8 deg.,

.30 , 2.6

1.8 / 1.8*10 /

.30.612

.49

* ( )tan( ) 33.5 deg.

1 *sin( )cot( ) tan( ) 1.987

* * 1.8*10 *.3* 2.6 1404 /

c

c

t mm t mm

f mm d mm

V m s mm s

tr

t

r Cos

r

MRR V f d mm s


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Example: A 6 long, .5 diameter 304 stainless steelrod is being reduced in diameter to 0.48 by turning

on the lathe. The spindle rotates at 400 rpm and thetool travels at an axial speed of 8 /min. Calculate

the cutting speed, material removal rate, cutting time,power dissipated, and cutting force. Assume specificenergy requirement for 304 stainless steel is ut =1.47hp-min/cubic inch


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17

0

0

0 1 0

6", .50", .48", 1.47, 400 / min, 8"/ min

.50 .48 .50 .48.49", : 0.01"

2 2 2

8: 0.02 /

400

: * * *.50*400 628 / min 52 / min

:

f

f

a

f

l D D Uc N rev fr

D DD Depthof cut d

frFeed f in rev

N

Cutting speed at D V D N in ft

Cutting speed at D

2

3

3

* * *.48*400 603 / min 50 / min

* * * * *0.49 *0.01*0.02 *400 0.123 / min

60.75min

* 0.02* 400

* 1.47 *.123 0.181 0.181*396, 000 71, 700 /

*2*

f

a

c c

c

V D N in ft

MRR D d f N in

lCutting time t

f N

Power PW U MRR hp in lb in

PWTorque

N

71,70029

400*2*

2* 2*29118

.490c

a

lb in

Cutting Force F lbD


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Example: Using the Taylors tool life equation calculate

the percentage increase in tool life when cutting speedis reduced by 50%. Assume n = .5 and C = 400

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Range of depth of cut, d = .5-12 mm, .02-.5 inRange of feed, f = .15-1 mm/rev, .006-.04 in/rev

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Cutting speeds for carbide andceramic tools


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Mounting of inserts in tool holders

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a) Clamping b) Wing lock pins c) Examples


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Properties of cutting tool materials

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Hot hardness of some tool materials

Plain carbon steels High-speed steels (HSS)

Cast cobalt alloys

Cemented carbides Ceramics

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High-speed steels (HSS)

commonly used. Twobasic types

Tungsten-type

Molybdenum-type


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n and C values of some tool materials

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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

*n

C v tTool life


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a) CNC Lathe b) Ten Cutting Tool Turret


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CNC Turning Center

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2 D illi P


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2. Drilling Process(creates round holes)

Through hole (drillexits on oppositeside of work piece)

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Blind hole (drill does not exit onopposite side of work piece)

Upright drill press


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Twist Drill

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Most common cutting tools for hole-making

Usually made of high speed steel

Shown below is standard twist drill geometry


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Drilling Operations

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a) Reaming: Used to slightly enlarge an existing hole, inorder to achieve better tolerances

b) Tapping: Used to provide internal screw threads in anexisting hole

c) Counter boring: Used to provide a stepped hole, largerdiameter follows a smaller diameter


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Drilling Operations

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d) Counter sinking: Similar counter boring, except the

step in the hole is cone shapede) Centering: Used to drill a starting hole.f) Spot facing: Used to provide a flat surface


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Work holding for drill presses

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Workpart in drilling can be clamped in any of the

following workholders:

Vise- general purpose workholder with two jaws

Fixture- workholding device that is usuallycustom-designed for the particular workpart

Drill jig similar to fixture but also provides ameans of guiding the tool during drilling

D - Drill diameter,

MRR - Material Removal Rate for drilling2*

* *4

DMRR f N


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3. Milling ProcessPart Geometries


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Conventional and Climb Milling

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a) Conventional (up milling)vs. Climb ( down milling)

c) Cutter Travel Distanceb) Slab Milling


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Type of Milling Operations

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a) Slab or peripheral milling

Axis of cutter rotation is parallel to the work piecesurface Cutting edges outside the periphery of the cutter

b) Face milling Axis of cutter rotation is perpendicular to the work

surface Cutting edges on both sides of the cutter

c) End milling Axis of cutter is usually perpendicular to the work

piece

PlainMillingCutter

FaceMillingCutter


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Types of slab/peripheral milling

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(a) Slab milling (b) slotting

(d) straddle milling

(c) side milling

(e) form milling


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Types of face milling

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(c) End milling(a) Conventionalface milling

(b) Partial face milling

(d) Profile milling using

an end mill

(f) Contour milling(e) Pocket milling


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Milling Machines

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(a) Horizontal milling machine (b) vertical milling machine


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Machining Centers

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Reconfigurable Machines

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Given the following orthogonal cutting data, determine: a) Shear angle, b) Friction coefficient,c) Shear stress, d) Shear strain, e) Chip velocity, f) Shear velocity, g) Specific cutting energy,h) Specific friction energy, i) Specific shear energy, j) Temperature at tool chip interface.

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Depth of cut (mm) t0 .15 Rake angle (degrees) 5 deg Cutting force (N) Fc 500

Chip thickness (mm) tc .3 Cutting speed (m/sec) V 4 Themal diffusivity(mm^2/s)

K 100

Width of cut (mm) w 4 Flow strength (MPa) Yf 120 Thurst force (N) Ft 300

Volumetric specific heat (N/mm^2C) c 3

Cutting ratio rt0

tc r 0.5

a) Shear angle (degrees) atan

r cos ( )

1 r s in ( )

180

27.51

b) Friction coefficient

Ft Fc tan ( )

Fc Ft tan ( ) 0.726

Friction angle atan ( )

180

35.964

Shear forceFs Fc cos ( ) Ft s in ( ) Fs 304.893

Area of shear planeAs

w t0

s in ( ) A s 1.299

c) Shear stress (MPa)

Fs

As

234.72


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e) Chip velocity (m/s)Vc

V sin ( )

cos ( ) Vc 2

f) Shear velocity (m/s)Vs

Vc cos ( )

sin ( )

Vs 4.313

g) Specific cutting

energy/unit vol (MN-m/m^3)Uc

Fc

w t0 Uc 833.333

h) Specific friction

energy/unit vol (MN-m/m 3) Ufr Fc sin ( ) Ft cos ( )( )

w t0

Uf 285.364

i) Specific shear energy/unit

vol (MN-m/m^3)Us Uc Uf Us 547.97

Cutting speed

(mm/sec)

V1 V 103

V1 4000

j) Temperature C T3.8 Yf

c

3V1 t0

K

T 276.202

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3.1 Metal Removal