CNC Precision Measurement & Quality Control 3 — Questions and Answers
Question 1: A CNC operator notices that every fifth part measures 0.002" undersized even though the machine returns to the same nominal position. The MOST likely cause is:
- Tool deflection increasing over time
- Systematic thermal expansion of the workpiece
- Backlash in the leadscrew not being compensated (Correct answer)
- Incorrect coolant concentration
Correct answer: Backlash in the leadscrew not being compensated
Uncompensated backlash causes the axis to consistently fall short when reversing direction, producing a systematic undersized error.
Question 2: A profilometer stylus tip radius of 2 µm is specified. How does a larger stylus radius affect roughness measurement?
- It increases the Ra reading
- It underestimates Ra by not reaching into fine valleys (Correct answer)
- It has no effect on the reading
- It causes the stylus to skip over peaks
Correct answer: It underestimates Ra by not reaching into fine valleys
A larger stylus radius cannot penetrate narrow surface valleys, causing it to underestimate the true Ra value.
Question 3: On a coordinate measuring machine, which datum is established first when using the 3-2-1 datum referencing method?
- A line datum requiring 2 points
- The primary datum plane requiring 3 points (Correct answer)
- The origin point requiring 1 point
- The secondary datum axis requiring 4 points
Correct answer: The primary datum plane requiring 3 points
The 3-2-1 method starts with the primary datum (3 points = plane), then secondary (2 points = line), then tertiary (1 point = origin).
Question 4: Which type of fit is specified when a shaft must slide freely in a bore with guaranteed clearance at all times?
- Interference fit
- Transition fit
- Clearance fit (Correct answer)
- Press fit
Correct answer: Clearance fit
A clearance fit guarantees that the shaft is always smaller than the bore, ensuring free sliding motion under all tolerance conditions.
Question 5: A CNC machinist measures an outside diameter with a micrometer and must account for temperature. The part is steel (CTE ≈ 6.5 µin/in/°F) and measured at 85°F instead of 68°F. For a 4.000" part, the thermal growth is approximately:
- 0.00044" (Correct answer)
- 0.00110"
- 0.00026"
- 0.00070"
Correct answer: 0.00044"
ΔL = 4.000 × 6.5×10⁻⁶ × (85−68) = 4.000 × 6.5×10⁻⁶ × 17 ≈ 0.000442".
Question 6: Statistical Process Control (SPC) uses control charts to monitor a process. A process is considered 'out of control' when:
- Any single point falls outside ±3 sigma control limits (Correct answer)
- The Cpk value exceeds 1.33
- The range chart shows zero variation
- More than 50% of points fall above the centerline
Correct answer: Any single point falls outside ±3 sigma control limits
A single point beyond the ±3 sigma (3σ) control limits is the primary Western Electric rule indicating an out-of-control condition.
Question 7: Which GD&T symbol controls both form and orientation of a surface relative to a datum?
- Flatness ⏥
- Angularity ∠
- Parallelism ∥ (Correct answer)
- Cylindricity ⌭
Correct answer: Parallelism ∥
Parallelism controls both the flatness of a surface and its angular relationship to the referenced datum plane simultaneously.
CNC Precision Measurement & Quality Control 3
A CNC operator notices that every fifth part measures 0.002" undersized even though the machine returns to the same nominal position.
The MOST likely cause is: