chapter 8. material-removal processes: cutting
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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
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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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High speed machining
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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)
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Orthogonal cutting model
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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
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Cutting force (절삭력)
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Machined material (chip) has higher hardness than
uncut material due to strain hardening (shear strain,
~20)
Vickers indentation test
Strain hardening
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Chip morphology (칩형상) (1)
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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.
Chip morphology (칩형상) (2)
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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Chip morphology (칩형상) (3)
Depending on :
cutting speed, temperature,
tool angle, friction, vibration
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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
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Temperature
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Tool wear (공구마멸)
Flank wear
Crater wear
Chipping
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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
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Tool condition monitoring
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Surface finish
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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
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Coated tool (피복공구)
TiN, TiC, TiCN
Al2O3(알루미나)
Diamond
Ion implantation
Multiphase
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Cutting Processes
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Lathe-round shape
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Lathe components
(심압대 )
(주축)
(가로이송대)
(왕복대)
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Drill
(생크길이)
(선단각)
(홈)
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Drilling operations
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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
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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
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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
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Burr Formation
http://lma.berkeley.edu/tools
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FEM Simulation
Demo: EMSIM
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
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Turning conditions
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Drilling conditions
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Milling conditions
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Broaching(브로칭) & Sawing(톱작업)
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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
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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
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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)
Ru
n-o
ut
(㎛)
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
×300
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Micro machined mold
0.753mm
0.993mm
1.005mm
1.221mm
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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
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More Meso/Micro Parts
Micro molding (ABS)
Micro pyramid
300mm
Freeform surface
3mm
Micro rotor
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Micro Tools – Drill
PCBs for semiconductor
PCBs for cell phone
Shape of Micro drill
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Micro Tools – End Mill
Wear of Micro End Mill
Shape of Micro End Mill
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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.
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Changer Arm
Tool
Spindle
ATC : Automatic Tool Changer
Tools are changed automatically.
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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
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Precision machining
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FANUC ROBOnano UiUltraprecision machining
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Battery PackCustom Circuit
Tool Holder
Receiver Circuit
IR LEDRTD
Temperature
Sensor
Indexable
End Mill
Power Sensor
Load Controls TM
Temperature sensor
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Dynamometer (공구동력계)
Dynamometer
Cable
Amplifier
Measured Signal
The whole system configuration
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