Chapter 1: Theory of Metal Machining

Question 1

What is the definition of Machining?

Answer 1

Cutting action involves shear deformation of work material to form a chip, and as chip is removed, a new surface is exposed.

Question 2

What are the advantages and disadvantages of machining?

Answer 2

+ve:
1. Variety of work materials can be machined
2. Variety of part shapes and special geometric features possible
3. Good dimensional accuracy and surface finish

-ve:
1. Wasteful of material (e.g. chips)
2. Time consuming

Question 3

What are the 6 machining operations?

Answer 3

1. Turning
2. Drilling
3. Milling
4. Shaping and Planing
5. Broaching
6. Sawing

Question 4

Whats the difference between roughing and finishing?

Answer 4

Roughing: High f and d, low v -> remove lots of material
Finishing: Low f and d, high v -> Completes part geometry

Question 5

How does chip formation occur? (Explain the Concepts)

Answer 5

Concept 1: Shear Deformation
Concept 2: Crack propagation
Concept 3: Heat generation (from friction & plastic deformation)

Question 6

What are the measurable forces during operation?

Answer 6

Cf, Cutting Force
Ft, Thrust Force also known as Feed Force Ff

Question 7

Is cutting ratio always <1 or >1?

Answer 7

Chip thickness after cut, a0, is always larger than before, ac, hence Rc is always less than 1.

Question 8

What are the effects of rake angle when it becomes greater?

Answer 8

Higher rake angle (+ve rake angle) means higher shear plane angle -> smaller shear plane -> lower shear force, cutting force, power and temperature

Question 9

What are the 4 basic types of chip in machining?

Answer 9

1. Discontinuous Chip
2. Continuous Chip
3. Continuous chip with Built-up Edge (BUE)
4. Serrated Chip

Question 10

How does Discontinuous Chip form?

Answer 10

• Brittle work materials
• Low cutting speeds
• Large feed and depth of cut
• High tool-chip friction

Question 11

How does Continuous Chip form?

Answer 11

• Ductile work materials
• High cutting speeds
• Small feeds and depths
• Sharp cutting edge
• Low tool-chip friction

Question 12

How does Continuous with BUE form?

Answer 12

• 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

Question 13

How does Serrated Chip form?

Answer 13

• Alternating high shear strain then low shear strain
• associated with difficult to machine metals at high cutting speeds
  (Semi-continuous saw tooth appearance)

Question 14

What is the appox. % of energy in machining that is converted into heat energy?

Answer 14

98%

Question 15

How does cutting temperature affect machining?

Answer 15

• High temp at tool-chip
• Remaining 2% is retained as elastic energy in the chip
• Reduced tool life
• Hot chips (safety hazard)
• Potential inaccuracies (thermal expansion of work material)