chapter 8. material-removal processes: cutting

M2794.001800 M A T E R I A L A N D M A N U F A C T U R I N G P R O C E S S E S

Chapter 8. Material-Removal

Processes: Cutting

Sung-Hoon Ahn

School of Mechanical and Aerospace Engineering

Seoul National University

2

Prof. Ahn, Sung-Hoon

Machining (기계가공)

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

from a workpiece.

Cutting processes (절삭가공)

Abrasive processes (입자가공)

Advanced machining processes (특수기계가공)

3

Prof. Ahn, Sung-HoonProf. Ahn, Sung-Hoon

High speed machining

4


Chip formation (칩형성)

Orthogonal cutting (직교절삭)

Oblique cutting (경사절삭)

Positive Rake angle (경사각)

Chip away from the workpiece

Negative Rake angle

Sturdy

(여유각)

(전단면)

(전단각)

(경사각)

5


Orthogonal cutting model

6


Shear plane angle

Tool

Chip

Shear-angle relationships – Merchant model

View demo

sin1

costan

)cos(sin

r

r

ttOD C

1 where

Ct

tr

sincos

cossin

TCN

TCR

FFF

FFF

http://cybercut.berkeley.edu/merchant/

7


Cutting force (절삭력)

8


Machined material (chip) has higher hardness than

uncut material due to strain hardening (shear strain,

~20)

Vickers indentation test

Strain hardening

9


Chip morphology (칩형상) (1)

10


Continuous chips (연속형칩)

High cutting speeds / high rake angles

(secondary shear zone)

Good for tool life & surface finish (may

need chip breaker)

Ductile materials with low coefficient of

friction

Built-up Edge chips (BUE, 구성인선)

High friction coefficient – workpiece

weld on the tool edge

Periodical separation of material bad

surface

It can be reduced by increasing the

cutting speed and the rake angle,

decreasing the depth of cut, using a

tool with a small tip radius and effective

cutting fluid.


Serrated chips (톱니형칩)

Nonhomogeneous / segmented chips

Zones of low and high shear strain

Ti : sharp decrease of strength and

thermal conductivity with increased

temperature

Discontinuous chips (불연속칩)

Brittle materials (ductile materials with

high friction coefficient): Cast iron,

bronze

Chatter, vibration

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Depending on :

cutting speed, temperature,

tool angle, friction, vibration

12


Oblique cutting (경사절삭)

Inclination angle (기움각), i

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Turning tool

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Turning (선삭)

(mm)cut ofdepth :

(mm/rev) feed :

(m/min) speed cutting:

Where

w

f

V

Vfw removal of Rate

15


Temperature

16


Tool wear (공구마멸)

Flank wear

Crater wear

Chipping

17


Flank wear

F. W. Taylor’s tool life equation

log C

TnCV

CVT n

logloglog

70m/min 0.25,Carbide)(Tungsten

50m/min 0.15,(HSS)

Vn

Vn

18


Tool condition monitoring

19


Surface finish

20


Machinability (절삭성)

Surface finish & integrity

Tool life

Machinability rating: cutting speed per 60min, e.g. 100 100ft/min (0.5m/s)

Force and power requirements

Chip control

Free-machining steel (쾌삭강)

Steel with: lead, bismuth, sulfur, calcium

Easy to machine: aluminum, brass, magnesium

Difficult to machine: wrought-copper, titanium, tungsten

21


Cutting-tool materials

Carbon steels

High Speed Steels (HSS, 고속도강)

(M and T series)

Cast cobalt alloys

Carbides (초경합금)

WC (tungsten carbide)

TiC (titanium carbide)

Inserts

Easy to replace tools

22


Coated tool (피복공구)

TiN, TiC, TiCN

Al2O3(알루미나)

Diamond

Ion implantation

Multiphase

23


Cutting Processes

24


Lathe-round shape

25


Lathe components

(심압대 )

(주축)

(가로이송대)

(왕복대)

26


Drill

(생크길이)

(선단각)

(홈)

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Drilling operations

28


DOC

DOC

WOC

WOC

Milling

Depth of Cut (DOC)

Width of Cut (WOC)

Slab milling (평밀링)

Conventional Milling

(up milling, 상향절삭)

Recommended, clean surface

before machine

Climb Milling (down milling, 하

향절삭)

Efficient cut (larger chip)

Less chatter

Production work

29


Material removal by milling

Cutting speed (m/min)

V = pDN

(D : diameter of cutter(m), N : rotational speed of the cutter(rpm))

Material Removal Rate (MRR)

MRR = WOC * DOC * f

f = feed rate (mm/min) = n * N * t

Example

V = 50 m/min, t = 0.1 mm/tooth, number of tooth (n)= 2,

D = 4 mm, DOC = 0.2, WOC = 3, Cutter RPM (N) = 50000/(πx4) = 3979

f = 2 *3979 * 0.1 = 796 mm/min, MRR = 3* 0.2 * 796 = 4776 mm3/min

Cutting Tool

- cross section

30


Sources of Errors

Vibration (chatter)

Tool deflection

Temperature change

Run-out

Form Errors

Surface Roughness

Form Error

Ra

Ideal Geometry

SurfaceRoughness

CL

Y

Form Error

X

a. ideal geometry

b. form error

c. surface roughness

31


Burr Formation

http://lma.berkeley.edu/tools

http://lma.berkeley.edu/tools

32


FEM Simulation

Demo: EMSIM

http://mtamri.me.uiuc.edu/testbeds/emsim/html/index2.shtml

http://mtamri.me.uiuc.edu/testbeds/emsim/html/index2.shtml

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1-2-3 rule

Modular

fixture

Fixture

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Cost Estimation

35


Turning conditions

36


Drilling conditions

37


Milling conditions

38


Broaching(브로칭) & Sawing(톱작업)

39


Controller

Microscope

Spindle

Main computer

X-Y stage

Vice

Micro Machining System

Tip of 127μm endmill

Micro endmills

Positional resolution : 1 ㎛

Tool diameter : 50 ㎛~1000 ㎛

High speed : 46,000 RPM

Tool material : HSS & TiN coating

Work piece : Metal, Polymer, etc Precision stage

40


Prototyping Size & Time

1

Typical

Prototyping

Time

0 10 100 1000

Rapid

PrototypingInjection

Molding

(Days)

Typical

Feature Size

1μm

1mm

1m

MEMSMicro Machining

10μm

100μm

10mm

100mm

41


1,000

10,000

100,000

0.1 1 10 100

average TiAlN coating

Tool diameter (mm)

Tool Cost (\)

DFM - Micro Milling

10mm endmill

10μm stage error

0.1% for slot cutting

100μm endmill

10μm stage error

10% for slot cutting

Cost structure of micro

machining is different from

that of macro machining.

Tool cost dominates

A tip is not exact edge in micro scale

42


Spindle run-out

< Total Indicator Reading (TIR) Measurement >

Run-out effect on the final geometry is critical in micro machining

Total run-out = TIR (Total Indicator Reading) + Error Terms

(vibration, thermal deformation, etc)

< Concept of run-out >

< Result of Total Indicator Reading (TIR) >

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25

Tool length (mm)

Ru

n-o

ut

(㎛)

100㎛

200㎛

43


Micro walls

Barrier ribs

Rib width: 60㎛Height: 500 ㎛Tool: φ200 ㎛

Spindle: 24,000rpm

DOC: 25㎛Feed rate: 100㎛/s

Time: 3hr 28min

×1200 ×600

×100

Geometric error: ~ 5 ㎛ (including error of microscope)

×100

×600 ×600

×300

×300

44


Micro machined mold

0.753mm

0.993mm

1.005mm

1.221mm

45


From Concept to Part

CAD design Micro machining

Micro injection

molded rotor

Rotor with

micro channel

Aluminum mold

Part for UAV/MAV

ABS plastic

l = 2.9mm,

t= 300mm,

Pivot D=250 mm

Specification

Total time: 5 days

46


More Meso/Micro Parts

Micro molding (ABS)

Micro pyramid

300mm

Freeform surface

3mm

Micro rotor

47


Micro Tools – Drill

PCBs for semiconductor

PCBs for cell phone

Shape of Micro drill

48


Micro Tools – End Mill

Wear of Micro End Mill

Shape of Micro End Mill

49


Spindle

Chuck

Cutting tool Turret

Workpiece

Milling toolTurret

Cutting + Milling

Cutting

CNC Lathe

CNC lathe can achieve multi-functional machining using attached milling turret, sub-spindle,

etc.

50


Changer Arm

Tool

Spindle

ATC : Automatic Tool Changer

Tools are changed automatically.

51


Pallet #1

Pallet #2

APC : Automatic Pallet Changer

Pallet : a flat platform on which a workpiece is installed.

During machining the workpiece on pallet #1, another workpiece

could be installed on pallet #2.

cf. tomb stone

52


Precision machining

53


FANUC ROBOnano UiUltraprecision machining

54


Battery PackCustom Circuit

Tool Holder

Receiver Circuit

IR LEDRTD

Temperature

Sensor

Indexable

End Mill

Power Sensor

Load Controls TM

Temperature sensor

55


Dynamometer (공구동력계)

Dynamometer

Cable

Amplifier

Measured Signal

The whole system configuration

chapter 8. material-removal processes: cutting

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