CNC Surface Finishing & Post-Processing 2 — Questions and Answers
Question 1: Which type of coolant application method is most effective for achieving the best surface finish in CNC milling?
- Flood coolant at high flow rate
- Mist coolant only
- Through-spindle high-pressure coolant (Correct answer)
- Dry machining with air blast
Correct answer: Through-spindle high-pressure coolant
Through-spindle high-pressure coolant delivers fluid directly to the cutting edge, flushing chips from the cut and reducing heat at the tool-workpiece interface for better finish.
Question 2: What surface finish defect is caused by chips being re-cut between the tool and the workpiece?
- Chatter marks
- Feed marks
- Chip scoring or scratching (Correct answer)
- Built-up edge (BUE) smearing
Correct answer: Chip scoring or scratching
When chips are not evacuated from the cut zone, they get trapped and dragged across the finished surface, causing scoring or scratch marks.
Question 3: In CNC turning, which insert nose radius produces the best surface finish at a given feed rate?
- Smaller nose radius (sharper tip)
- Larger nose radius (Correct answer)
- Neutral (zero) nose radius
- Nose radius has no effect on finish
Correct answer: Larger nose radius
A larger nose radius reduces the theoretical peak-to-valley height for a given feed rate, as the wider arc better bridges the feed grooves.
Question 4: What does an 'Rz' (Rzmax) measurement on a drawing indicate compared to Ra?
- Rz measures only valleys; Ra measures only peaks
- Rz is the average of the five highest peak-to-valley measurements; Ra is the overall arithmetic average (Correct answer)
- Rz is always smaller than Ra for the same surface
- Rz and Ra are identical measurements with different units
Correct answer: Rz is the average of the five highest peak-to-valley measurements; Ra is the overall arithmetic average
Rz averages the five largest peak-to-valley heights over the evaluation length, making it more sensitive to extreme surface events than Ra.
Question 5: Which post-processing treatment is most commonly applied to aluminum CNC parts to improve corrosion resistance and surface hardness?
- Electroless nickel plating
- Type II or Type III hard anodizing (Correct answer)
- Black oxide coating
- Phosphate coating
Correct answer: Type II or Type III hard anodizing
Anodizing converts the aluminum surface into an aluminum oxide layer that is harder and more corrosion-resistant, and is the standard post-process for aluminum parts.
Question 6: When deburring a part after CNC machining, which edge condition is specified when a drawing notes 'break all sharp edges'?
- A large chamfer of 0.030 inches minimum
- A small chamfer or slight radius to remove the burr and prevent injury or stress concentration (Correct answer)
- A full radius equal to half the edge thickness
- No action required — it is a courtesy note only
Correct answer: A small chamfer or slight radius to remove the burr and prevent injury or stress concentration
'Break all sharp edges' requires a light chamfer or slight radius (typically 0.005–0.015 in) to remove burrs and eliminate stress risers without removing significant material.
CNC Surface Finishing & Post-Processing 2
Which type of coolant application method is most effective for achieving the best surface finish in CNC milling?