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MET 33800 Manufacturing Processes
Chapter 20
Fundamentals of Machining
MET 33800 Manufacturing Processes
Chapter 20
Fundamentals of Machining
Unless otherwise indicated, illustrations in this presentation are taken from the 11th Edition of Degarmo’s Materials and Processes in Manufacturing textbook by J.T. Black and Ronald A. Kohser, ©2012, Wiley.
Materials Processing
Chapters 15-17
Chapters 30-33
Chapters 20-27
Chapters 11-13
Chapter 20 ‐ 3
Page 268 Figure 11‐1, The Five materials processing families with subgroups and typical processes
2
Introduction
Machining Process of removing unwanted material in the form of chips.
Metal Cutting (Removal)Machining when material is metallic.
Fundamentals of Machining/ Orthogonal Machining
Chapter 20 ‐ 4
http://crawfordpublications.com/wp‐content/uploads/2013/11/Metalworking‐picture.png
FundamentalsMachining is a complex process
Input variables:
Machine Tool Selection
Cutting Tool Selection
Properties of the Workpiece
Cutting Parameters
Workholding Device
Chapter 20 ‐ 5
Chapter 20 ‐ 6
3
Basic Chip Formation Processes
Turning (chapter 22)
Drilling (chapter 23)
Milling (chapter 24)
Shaping (chapter 25)
Sawing (chapter 25)
Broaching (chapter 25)
Grinding (chapter 26)
Fundamentals
Chapter 20 ‐ 7
Figure 20‐2, Page 535
FundamentalsGenerate Flat Surfaces
Chapter 20 ‐ 8
Generate Cylindrical Surfaces
Chapter 20 ‐ 9
4
Definitions
Machine Tool ‐ Supplies power and support of the workholding device and cutting tool.
Workholding Device ‐ Can be stationary or rotating. (chapter 27)
Cutting Tool ‐ Can be stationary or rotating. (chapter 21)
Tool Holder ‐ Supports tool. Strength and accuracy critical for success of process.
Chapter 20 ‐ 10
Chip Formation ParametersDepth of Cut (d)
Depth of cutting tool in workpiece.
Equation for turning initial diameter1 and
final diameter2
1 2D - Dd =
2
Chapter 20 ‐ 11
1 2D - Dd =
2
Chapter 20 ‐ 12
Figure 20‐12, Page 545
5
d = tool width2 1D - Dd =
2
Chapter 20 ‐ 13
Machining Parameter Reference Handbooks
Chapter 20 ‐ 14
Machining Parameter Reference
Chapter 20 ‐ 15
6
Chip Formation ParametersCutting Speed (V)
For turning, relates velocity of rotating workpiece to stationary tool. Common units:
surface feet/min (sfpm or fpm)
meters/min (m/min)
Less common units:
inches/min (ipm)
meters/sec (m/s)
Chapter 20 ‐ 16
Chapter 20 ‐ 17Figure 20‐12, Page 545
Machining Parameter Reference
Chapter 20 ‐ 18
7
Chip Formation ParametersSpindle Speed (Ns)
Calculated by cutting speed (V) and part/tool size (D).
Equation for turning initial diameter1
D N1 sV = 12
12 V
N = s D
1
Chapter 20 ‐ 19
12 V
N = s D
1
Chapter 20 ‐ 20Figure 20‐12, Page 545
12 V
N = s D
1
Chapter 20 ‐ 21
8
Chip Formation Parameters Feed (fr) – Amount of material removed per
revolution of the tool (turning).
Units = inches per revolution (ipr)
Feed (ft) – Amount of material removed per individual tool tooth (milling).
Units = inches per tooth (ipt)
Feed (fm) – Table movement rate (milling).
Units = inches per minute (ipm)
Chapter 20 ‐ 22
Chapter 20 ‐ 23Figure 20‐12, Page 545
Chip Formation ParametersFeed (fm)
Table movement rate (milling).
where: n = number of teeth
Ns = spindle rpm
m t sf = f n N
Chapter 20 ‐ 24
9
Machining Parameter Reference
Chapter 20 ‐ 25
Chip Formation Parameters
Material Removal Rate (MRR)
Volume of material removed (in3/min).
Equation for turning
r MRR 12 V f d
Chapter 20 ‐ 26
Chip Formation ParametersCutting Time (Tm)
Function of the length of cut (L) and tool approach and/or over travel allowance/clearance (LA and LO).
Equation for turning a diameter.
For cutting to a shoulder Lo = 0.
A O
m
L + L + LT =
f Nr s
Chapter 20 ‐ 27
10
Chapter 20 ‐ 28
Chip Formation Parameters
Parameters – Other Processes
Chapter 20 ‐ 29
Parameters – Other Processes
Chapter 20 ‐ 30
11
Parameters – Other Processes
Chapter 20 ‐ 31
Example Problem
Material: 1020 Hot Rolled Carbon Steel (200 HB) 1.00 diameter bar.
Process: Turn to 0.78 dia x 4.00 length
Tool: Brazed uncoated carbide single‐point
Determine the Machining Parameters
Chapter 20 ‐ 32
1 2D ‐ D 1.00 ‐ 0.78
d = = = 0.1102 2
Chapter 20 ‐ 33
Example Problem
Figure 20‐12, Page 545
12
Chapter 20 ‐ 34
Example Problem ‐ SolutionDetermine the Machining Parameters
Chapter 20 ‐ 35
Example Problem ‐ SolutionDetermine the Machining Parameters
Chapter 20 ‐ 36
13
Energy and Power in MachiningOblique cutting force components:
FC Primary (largest) cutter force in direction of cutting velocity. Requires 99% of total power.
FF Feed force that amounts to ~50% of FC and requires negligible power
FR Radial force that amounts to ~50% of FC and requires negligible power
Chapter 20 ‐ 37
Energy and Power in Machining
Chapter 20 ‐ 38Figure 20‐12, Page 545
Energy and Power in Machining
Chapter 20 ‐ 39
Figure 20‐12, Page 545
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Energy and Power in Machining
Chapter 20 ‐ 40Figure 20‐12, Page 545
cF VHP =
33,000
Energy and Power in Machining
Horsepower (HP)
Power at the machine spindle can be approximated by:
Chapter 20 ‐ 41
Specific Horsepower (HPs)
Relates spindle horsepower and MRR:
s
HPHP =
MRR
Energy and Power in Machining
Chapter 20 ‐ 42
15
Chapter 20 ‐ 43
Motor Horsepower (HPm)
Includes machine efficiency (E) and correction factor (CF) for tool wear and variation in cutting speed:
sm
HP × MRR × CFHP =
E
Typical value of E = 0.80
Energy and Power in Machining
Chapter 20 ‐ 44
Feed Correction Factor (CF) Table
Energy and Power in Machining
Chapter 20 ‐ 45
MAXIMIZING SURFCAM, S.C. Jonathon Lin and Behrooz Lahidji, Scholars International Publishing Corp., ©1997.
16
Cutting Force
Cutting Force (FC)
Can be estimated using MRR, HPs and V:
s
C
H P × M RR × 33 ,000F
V
Chapter 20 ‐ 46
c
s
F V HPHP = and HP =
33,000 MRR
Example Problem Revisited
Material: 1020 Carbon Steel 200 HB
Geometry: 1.00 diameter bar
Process: Turn to 0.78 dia x 4.00 length
Tool: Brazed uncoated carbide
Determine Power and Force Parameters
Chapter 20 ‐ 47
Chapter 20 ‐ 48
MAXIMIZING SURFCAM, S.C. Jonathon Lin and Behrooz Lahidji, Scholars International Publishing Corp., ©1997.
17
Example Problem ‐ SolutionDetermine Power and Force Parameters
Chapter 20 ‐ 49
Cutting Force
Cutting Force (FC)
Maximum depth of cut can be estimated using the motor horsepower equation:
m
max
s r
HP Ed
12 HP V f CF
Chapter 20 ‐ 50
Chip Formation ProcessesOrthogonal Machining
Cutter motion and cutter edge are perpendicular.
Oblique Machining
Cutter motion and cutter edge not perpendicular. Includes shaping, drilling and single point turning.
Chapter 20 ‐ 51http://image.slidesharecdn.com/theory‐of‐metal‐cutting‐120730021018‐phpapp01/95/theory‐ofmetalcutting‐56‐728.jpg?cb=1343614427
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Oblique tool geometry (3‐D) is simplified to 2‐D geometry to allow simple representation of the cutting process.
Orthogonal Machining
Chapter 20 ‐ 52
Orthogonal Machining
Methods for Orthogonal Machining Force Measurement
Chapter 20 ‐ 53
Chip is formed by a localized shear process over a narrow region.
Large‐strain, high‐strain‐rate plastic deformation work hardens material.
When work hardening reaches saturated condition, shearing occurs.
Orthogonal Machining
Chapter 20 ‐ 54
19
Orthogonal Machining
Chapter 20 ‐ 55
Parameters:
V = tool velocity
t = uncut chip thickness
w = width of chip
= back rake angle
= shear plane angle
ψ = shear lamella (microscopic shear planes) angle
Orthogonal Machining
Chapter 20 ‐ 56
Chip Types:
1. Continuous highly ductile materials.
2. Continuous w/built‐up edge material adhering to tool.
3. Discontinuous low ductility materials.
Chip Formation Process
Chapter 20 ‐ 57
http://www.goesslingusa.com/wp‐content/uploads/2013/08/Metal‐Chips‐1.jpg
20
Discontinuous
Continuous
Continuous w/Built‐Up EdgeChapter 20 ‐ 58
Major Heat Sources
1. Shear Process major heat source.
2. Tool‐Chip Interface heat from friction.
3. Tool Flank heat from friction.
Amount of heat affects tool selection.
Control of heat critical tool life.
Heat and Temperature
Chapter 20 ‐ 59
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Heat and Temperature
Chapter 20 ‐ 60
Reference: Page 511 Materials and Processes in Manufacturing, 9th Edition, Degarmo, Black and Kohser, Wiley
21
Heat and Temperature
Chapter 20 ‐ 61
Reference: Figure 21‐30 Page 510 Materials and Processes in Manufacturing, 9th Edition, Degarmo, Black and Kohser, Wiley
Heat and Temperature
Chapter 20 ‐ 62
Reference: Figure 21‐32 Page 511 Materials and Processes in Manufacturing, 9th Edition, Degarmo, Black and Kohser, Wiley
High‐strength materials compared to low‐strength materials require higher cutting forces greater tool deflection, greater work deflection, increased friction (heat), increased heat generation (shear) and greater power.
Highly ductile materials compared to low ductility materials permit extensive plastic deformation increased heat generation, increased friction (heat) and greater power.
Chip Formation Processes
Chapter 20 ‐ 63
22
Machining is a dynamic process.
High strain and strain rates.
Closed‐loop interactive process:
Initial chip load on tool.
Chip load determines forces.
Forces produce deflection and alter chip load on tool.
Altered chip load determines new forces.
Force‐displacement response repeats.
Result can be vibration or chatter.
Vibration and Chatter
Chapter 20 ‐ 64
Vibration and Chatter
Chapter 20 ‐ 65
Vibration and Chatter
Chapter 20 ‐ 66
23
Dynamic Stability Factors:
Cutting process parameters (d, V, fr).
Cutter geometry (rake angles, shape).
Characteristics of the machining process (tooling, machine tool, fixture, work).
Cutting Stiffness (Ks) of the work material.
Vibration and Chatter
Chapter 20 ‐ 67
Cutting Stiffness (Ks)
Cutting Stiffness (Ks) can be equated to Specific Energy (U).
Cutting Stiffness is critical for chatter and vibration.
Cutting Stiffness is a material property.
Cutting Stiffness
Chapter 20 ‐ 68
Cutting Stiffness
Chapter 20 ‐ 69
24
Factors Affecting Chatter:
Higher Cutting Stiffness (Ks) requires more force and results in less stability.
Rotation Speed (Ns ) affects the phase shift between overlapping surfaces and the possible regeneration of vibration. Generally as speed increases chatter becomes more significant.
Increasing Feed Rates (Ft ) has a relatively small effect on chatter but does affect vibration which can affect chatter.
continued on next slide
Vibration and Chatter
Chapter 20 ‐ 70
Factors Affecting Chatter (continued):
Increasing the Depth of Cut (d) has the largest effect on increasing chatter due to the higher cutting forces required.
The Total Width of Chip is based on the depth of cut times the number of cutting edges. Increasing the total width of chip increases the cutting force and possibility of chatter.
Increasing the Back Rake Angle reduces the cutting force (Fc) which generally decreases chatter.
continued on next slide
Vibration and Chatter
Chapter 20 ‐ 71
Factors Affecting Chatter (continued):
Reducing Tool Clearance Angles increases friction between the tool and workpiece. Increased friction can have the affect of process damping which generally decreases chatter. However, additional heat is created which can thermally distort the work and increase the tool wear.
Cutting Tool Geometry including nose radius, shape and lead angle influence chip area which affects chatter.
Vibration and Chatter
Chapter 20 ‐ 72
25
The End – See Oncourse for Videos
Chapter 20 ‐ 73
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