1. THEORY OF METAL MACHINING
SME Video – Machining Processes
Turning & Lathe Basics (vts_05)
Milling & Machining Center Basics (vts_03)
Machining Technology
Theory of Chip Formation
Merchant Equation
Power and Energy Relationships
Cutting Temperature
In-class Assignment
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

2. Classification of Material Removal Processes
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

3. Tool with negative rake angle
Machining
Cutting action involves shear deformation
Cross‑sectional view
Tool with negative rake angle
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

4. Why Machining is Important
Good dimensional accuracy
Good surface finish
Disadvantages
Wasteful of material
Time consuming
Machining performed after other processes
Casting
Forging
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

5. Machining Operations Turning Drilling Peripheral milling Face milling
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

6. Multiple Cutting Edge Tools
Cutting Tools
Single-Point Tools
Multiple Cutting Edge Tools
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

7. Cutting Conditions for Turning
MRR = v f d
v = cutting speed
f = feed
d = depth of cut
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

8. Roughing vs. Finishing Several roughing operations
Followed by one or two finishing cuts
Roughing - removes large amounts of material
High feeds and depths
Low speeds
Finishing - completes part geometry
Low feeds and depths
High cutting speeds
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

9. Machine Tools Power‑driven machine Performs a machining operation
Including grinding
Functions in machining
Holds workpart
Positions tool relative to work
Provides power at speed, feed, and depth
Machine tools – applies to machines that perform metal forming operations
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

10. Orthogonal Cutting Model
Orthogonal cutting: (a) as a three‑dimensional process.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

11. Chip Thickness Ratio & Shear Plane Angle
r = chip thickness ratio
to = thickness prior to chip formation
tc = chip thickness after separation
Chip thickness after cut greater than before
Based on orthogonal model
Shear plane angle 
where r = chip ratio, and  = rake angle
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

12. Shear Strain in Chip Formation
 = tan( - ) + cot 
where  = shear strain
 = shear plane angle
 = rake angle of cutting tool
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

13. In-class Example
In an orthogonal cutting operation, the tool has a rake angle = 15. 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) the shear strain for the operation.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

14. SME Video
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

15. In-class Assignment
In an orthogonal cutting operation, the in wide tool has a rake angle of 5. The lathe is set so the chip thickness before the cut is in. After the cut, the deformed chip thickness is measured to be in.
Calculate (a) the shear plane angle and (b) the shear strain for the operation.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

16. Production Scheduling Machine-Machine Diagrams
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

17. Time Permitting Content
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

18. Approximation of Turning by Orthogonal Cutting
Turning operation
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

19. Chip Formation
Figure More realistic view of chip formation, showing shear zone rather than shear plane. Also shown is the secondary shear zone resulting from tool‑chip friction.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

20. Basic Types of Chip in Machining
Ductile materials
High cutting speeds
Small feeds
Small depths
Sharp cutting edge
Low tool‑chip friction
Brittle materials
Low cutting speeds
Large feed
Large depth of cut
High tool‑chip friction
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

21. Basic Types of Chip in Machining
Ductile materials
Low‑to‑medium cutting speeds
Tool-chip friction causes portions of chip to adhere to rake face
BUE forms, then breaks off, cyclically
Semicontinuous - saw-tooth appearance
Cyclical chip forms with alternating high shear strain then low shear strain
Associated with difficult-to-machine metals at high cutting speeds
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

22. Forces Acting on Chip Friction force F and Normal force to friction N
Shear force Fs and Normal force to shear Fn
Coefficient of friction between tool and chip:
Friction angle related to coefficient of friction as follows:
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

23. Shear Stress Shear stress acting along the shear plane:
where As = area of the shear plane
Shear stress = shear strength of work material during cutting
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

24. Cutting Force and Thrust Force
F, N, Fs, and Fn cannot be directly measured
Forces acting on the tool that can be measured:
Cutting force Fc and Thrust force Ft
Figure Forces in metal cutting: (b) forces acting on the tool that can be measured
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

25. Forces in Metal Cutting
F = Fc sin + Ft cos
N = Fc cos ‑ Ft sin
Fs = Fc cos ‑ Ft sin
Fn = Fc sin + Ft cos
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

26. The Merchant Equation
Work material will select a shear plane angle  that minimizes energy, given by
Derived by Eugene Merchant
To increase shear plane angle
Increase the rake angle
Reduce the friction angle
(or coefficient of friction)
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

27. Effect of Higher Shear Plane Angle
Higher shear plane angle means smaller shear plane which means lower shear force, cutting forces, power, and temperature
Figure Effect of shear plane angle  : (a) higher  with a resulting lower shear plane area; (b) smaller  with a corresponding larger shear plane area. Note that the rake angle is larger in (a), which tends to increase shear angle according to the Merchant equation
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

28. Power and Energy Relationships
Power is traditional expressed as horsepower (dividing ft‑lb/min by 33,000)
where HPc = cutting horsepower, hp
Gross power to operate the machine tool Pg or HPg is given by
where E = mechanical efficiency of machine tool
Typical E for machine tools  90%
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

29. Unit Power and Specific Energy
Power per unit volume rate of metal cut
Called unit power, Pu or unit horsepower, HPu or
where RMR = material removal rate
Unit power is also known as the specific energy U
Units for specific energy are typically N‑m/mm3 or J/mm3 (in‑lb/in3)
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

30. Energy Relationships Distribution of total cutting energy
Correction factor for unit horsepower calculations
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

31. Cutting Temperature Analytical method derived by Nathan Cook
Experimental method derived by Ken Trigger T = K vm
Experimentally measured cutting temperatures
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e