CNC Precision Measurement & Quality Control 2 — Questions and Answers
Question 1: A CNC machinist measures a bore with an inside micrometer and gets 1.2503". The print calls for 1.2500" ±0.0003". What is the correct disposition?
- Accept — within tolerance (Correct answer)
- Reject — oversize by 0.0003"
- Reject — undersize
- Re-measure with a different tool before deciding
Correct answer: Accept — within tolerance
1.2503" falls within the allowable range of 1.2497"–1.2503", so the part is acceptable.
Question 2: Which surface finish parameter describes the average height of surface irregularities measured from the mean line?
- Rz
- Ra (Correct answer)
- Rmax
- Rt
Correct answer: Ra
Ra (arithmetic average roughness) is the average absolute deviation of the surface profile from the mean line.
Question 3: A technician uses a sine bar with a 5.000" center-to-end distance and needs to set a 15° angle. What gauge block stack height is required?
- 1.294" (Correct answer)
- 4.830"
- 0.500"
- 2.500"
Correct answer: 1.294"
Height = L × sin(θ) = 5.000 × sin(15°) = 5.000 × 0.2588 ≈ 1.294".
Question 4: On a CMM report, the term 'true position' refers to:
- The distance from origin to feature center
- The diameter of the tolerance zone within which the feature axis must lie (Correct answer)
- The deviation in Z-axis only
- The flatness error of the datum surface
Correct answer: The diameter of the tolerance zone within which the feature axis must lie
True position tolerance defines the cylindrical (or circular) zone within which the feature's axis or center point must fall.
Question 5: What is the primary reason gauge blocks are 'wrung' together rather than stacked loosely?
- To prevent oxidation
- Molecular adhesion eliminates the air gap and maintains accuracy (Correct answer)
- To make them easier to handle
- Wringing magnetizes the blocks for stability
Correct answer: Molecular adhesion eliminates the air gap and maintains accuracy
Wringing creates a near-zero air gap through molecular adhesion and surface tension, keeping the combined height accurate to millionths of an inch.
Question 6: A dial test indicator reads 0.0015" TIR when sweeping a turned shaft in V-blocks. This reading represents:
- The shaft diameter
- The total runout from high to low reading (Correct answer)
- The surface roughness
- One-half the actual eccentricity
Correct answer: The total runout from high to low reading
TIR (Total Indicator Reading) is the full swing from the lowest to highest reading, equal to twice the eccentricity.
Question 7: Which material is the ANSI standard for gauge blocks used in calibration labs?
- Hardened tool steel
- Chrome carbide ceramic
- Tungsten carbide
- Chromium steel (grade A2 or better) (Correct answer)
Correct answer: Chromium steel (grade A2 or better)
ANSI/ASME B89.1.9 specifies chromium steel as the standard material for gauge blocks, offering hardness, stability, and low thermal expansion.
CNC Precision Measurement & Quality Control 2
A CNC machinist measures a bore with an inside micrometer and gets 1.2503".
The print calls for 1.2500" ±0.0003".
What is the correct disposition?