Chapter 8. Material-Removal Processes: Cutting

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Chapter 8. Material-Removal Processes: Cutting

Sung-Hoon Ahn

School of Mechanical and Aerospace Engineering Seoul National University

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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)

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Prof. Ahn, Sung-Hoon Prof. 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)

Prof. Ahn, Sung-Hoon Prof. Ahn, Sung-Hoon Shear plane angle

Tool Chip

Shear-angle relationships – Merchant model

View demo



 





sin 1

tan cos

) cos(

sin

r r

t

OD t

^C

 

 where   1

t

C

r t



sin cos

cos sin

T C

N

T C

R

F F

F

F F

F









(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 (

연속형 칩

)



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 (

톱니형 칩

)



Nonhomogeneous / segmented chips



Zones of low and high shear strain



Ti : sharp decrease of strength and thermal conductivity with increased temperature

 Discontinuous chips (불연속 칩)



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

(13)

Turning tool

(14)

Turning ( 선삭 )

(mm) cut

of depth :

(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

T n

C V

C VT

ⁿ

log log

log  



70m/min

0.25, Carbide)

(Tungsten

50m/min

0.15, (HSS)



V n

(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

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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

 Al

₂

O

₃

(

알루미나

)

 Diamond

 Ion implantation

 Multiphase

(23)

Cutting Processes

(24)

Lathe-round shape

(25)

Lathe components

(심압대)

(주축)

(가로 이송대)

(왕복대)

(26)

Drill

(생크길이)

(선단각) (홈)

(27)

Drilling operations

(28)

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 mm

³

/min

Cutting Tool

- cross section

(30)

Sources of Errors

 Vibration (chatter)

 Tool deflection

 Temperature change

 Run-out

 Form Errors

 Surface Roughness

Form Error

R_a



Ideal Geometry

Surface Roughness

CL

Y

Form Error

X



a. ideal geometry

b. form error

c. surface roughness

(31)

Burr Formation

http://lma.berkeley.edu/tools

(32)

FEM Simulation

 Demo: EMSIM

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

(33)

1-2-3 rule

Modular fixture

Fixture

(34)

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

Prototyping

Injection Molding

(Days)

Typical Feature Size

1

μ

m 1mm 1m

MEMS Micro Machining

10

μ

m 100

μ

m 10mm 100mm

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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

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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) Run-out (㎛)

100㎛

200㎛

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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

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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 tool

Turret

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 Ui

(54)

Battery Pack Custom Circuit

Tool Holder

Receiver Circuit

IR LED RTD

Temperature Sensor Indexable

End Mill

Power Sensor

Load Controls ^TM

Temperature sensor

(55)

Dynamometer ( 공구동력계 )

Dynamometer

Cable

Amplifier

Measured Signal

The whole system configuration