Machining processes

Question 1

Make a simple schematic of the kinematics of turning and draw/label the primary motion and feed motion. On your sketch, include the cutting speed and the feed. What is the relation between cutting speed and the spindle speed?

Answer 1

Answer:

• In turning, the workpiece rotates — this is the primary cutting motion.
• The cutting tool moves linearly along the workpiece — this is the feed motion.
• The combination of these motions removes material in the form of a chip.

Relation:

v=πDN

where

v = cutting speed [m/min]

D = workpiece diameter [m]

N = spindle speed [rev/min]

Explanation:

• The cutting speed increases with both the diameter and spindle speed.
• Feed (f) is the tool’s travel per revolution [mm/rev].

Question 2

Discuss how the theoretical surface roughness (𝑅𝑡ℎ) changes with the feed and the nose radius. What would you do to reduce (𝑅𝑡ℎ) in turning? Are there any negative aspects with respect to using a smaller nose radius?

Answer 2

In short:

To reduce 𝑅𝑡ℎ, lower the feed and/or use a larger nose radius. However, too large a radius may cause chatter or higher cutting forces, so a balance between surface quality and stability is required.

Question 3

State how the uncut chip width and chip thickness depend on depth of cut and feed – what is the role of the entering angle? How do you calculate uncut chip area and the material-removal rate?

Answer 3

Short Explanation

Uncut chip thickness: ℎ = 𝑓xsin(𝜅𝑟)
→ smaller entering angle = thinner chip

Uncut chip width: 𝑏=𝑎𝑝/sin(𝜅𝑟)
→ smaller angle = wider chip

Uncut chip area: 𝐴𝑐=𝑎𝑝×𝑓=hxb
→ same for all angles

Material removal rate:
𝑀𝑅𝑅=𝑎𝑝×𝑓×𝑣𝑐
The entering angle changes the shape of the chip (thin/wide) but not the total material removed.

Question 4

Explain how each of the following tool geometrical parameters affects the turning process. For each parameter, state what it influences and give one practical consequence of increasing it:
(i) Entering angle κᵣ
(ii) Rake angle γ
(iii) Inclination angle λₛ
(iv) Nose radius/minor-edge geometry

Answer 4

Answer:

(i) Entering angle (κᵣ):

• Affects chip thickness and direction of cutting force.
• Increasing κᵣ → higher radial forces, rougher surface.
• Smaller κᵣ → smoother surface, less vibration, better finish.

(ii) Rake angle (γ):

• Affects chip formation and cutting forces.
• Larger γ → easier cutting, lower forces, better chip flow.
• Too large γ → weak cutting edge.

(iii) Inclination angle (λₛ):

• Determines chip flow direction.
• Positive λₛ → chip flows upward, reduces friction.
• Negative λₛ → chip pushed down, better for heavy cuts.

(iv) Nose radius (rε):

• Influences surface finish and tool strength.
• Larger rε → smoother surface but higher cutting forces.
• Smaller rε → lower forces but rougher surface.

Question 5

Schematically illustrate (peripheral) milling process and explain the difference between down- and up-milling operation.

Answer 5

Peripheral milling:

• Rotating cutter with multiple teeth removes material along the surface.
• The workpiece moves in a linear feed motion.

Up-milling (conventional milling):

• Cutter rotation is opposite the feed direction.
• Chip starts thin and becomes thick.
• More friction and heat; good for roughing or uneven surfaces.

Down-milling (climb milling):

• Cutter rotation is the same as feed direction.
• Chip starts thick and becomes thin.
• Less heat, better surface, less tool wear.
• Requires rigid machine (can pull workpiece).

Question 6

Explain how turn-milling differs from conventional turning in terms of tool and workpiece motions. List main advantages of turn-milling compared to conventional turning, and some challenges.

Answer 6

Turn-milling: Both tool and workpiece rotate simultaneously. The tool (a milling cutter) moves across a rotating part.

Conventional turning: Only the workpiece rotates; the tool moves linearly.

Advantages of turn-milling:

• Lower cutting forces.
• Higher material removal rate.
• Better surface finish.
• Can machine complex shapes in one setup.

Challenges:

• Complex tool path and programming.
• Requires multi-axis machines.
• Higher equipment cost.

Question 7

Milling is an intermittent cutting process. Explain in detail why the cutting forces in milling vary cyclically during tool engagement. In your answer, describe the role of the immersion angle, chip thickness, and tooth entry/exit.

Answer 7

• In milling, each cutter tooth engages and disengages once per revolution → intermittent cutting.
• As the tooth enters, the chip thickness increases until it exits, where it drops to zero.
• This cyclic change causes periodic variations in cutting force.

Factors:

• Immersion angle (ϕ): Defines how much of the cutter is in contact with the material.
• Larger angle → more teeth engaged → higher average force.
• Chip thickness: Changes from zero to max → force fluctuates.
• Tooth entry/exit: Periodic impact at entry causes vibration and noise.

Question 8

Explain why the cutting speed in drilling is not uniform across the tool radius. Describe the working conditions of the central insert versus the peripheral insert in an indexable drill, and why they require different insert designs. What fraction of maximum cutting speed does each insert typically operate in?

Answer 8

Cutting speed depends on radius:
v=2πrN.

At the center (r = 0) → speed is zero; at the outer edge → speed is maximum.

Central insert:

• Works at low cutting speed.
• Removes material mainly by extrusion and shearing.
• Needs stronger geometry and tougher material.

Peripheral insert:

• Operates at full speed.
• Performs true cutting action.
• Needs sharp edge and wear resistance.

Typical speeds:

• Central insert: about 20–30% of max cutting speed.
• Peripheral insert: about 100% of max cutting speed.

Question 9

Compare solid carbide drills and indexable insert drills in terms of:

(i) Achievable hole diameter range
(ii) Hole tolerance/accuracy
(iii) Tool cost considerations
Where do exchangeable-tip drills fit in, and what is their main advantage compared to solid carbide drills?

Answer 9

Solid carbide drill: Best accuracy and finish, but expensive for large holes.

Indexable insert drill: Best for large diameters, roughing, and economy.

Exchangeable-tip drill: Middle ground — combines accuracy, cost-efficiency, and flexibility.