GD&T Surface Finish & Assembly Requirements 3 — Questions and Answers
Question 1: A shaft and hole assembly uses an interference fit. Which surface finish condition on the shaft would MOST likely cause the interference to be less than intended?
- Surface that is too smooth
- Surface peaks (asperities) that are flattened during pressing (Correct answer)
- Surface that has deep valleys
- A surface with a lay perpendicular to the shaft axis
Correct answer: Surface peaks (asperities) that are flattened during pressing
When pressing an interference fit, surface peaks are crushed and flattened, effectively reducing the interference and the resulting holding force.
Question 2: The 'lay' of a surface texture is best described as:
- The average roughness depth across the surface
- The predominant direction of surface marks or tool paths (Correct answer)
- The maximum waviness height on the surface
- The spacing between surface peaks
Correct answer: The predominant direction of surface marks or tool paths
Lay refers to the predominant direction of the surface pattern, which is determined by the manufacturing process used.
Question 3: A drawing callout reads 'surface finish Ra 1.6 μm max'. A measured surface has Ra 1.2 μm. This surface is:
- Out of tolerance because it is too smooth
- Within tolerance (Correct answer)
- Only acceptable if Rz is also measured
- Requires re-machining to reach Ra 1.6 μm
Correct answer: Within tolerance
'Max' means the Ra value must not exceed 1.6 μm; a measured value of 1.2 μm is smoother and therefore within tolerance.
Question 4: Which GD&T symbol, when applied to a surface, requires every point on that surface to fall within two parallel planes separated by the tolerance value?
- Straightness
- Flatness (Correct answer)
- Profile of a surface
- Parallelism
Correct answer: Flatness
Flatness requires all surface points to lie within two parallel planes spaced by the tolerance value, with no datum reference required.
Question 5: In a bolted joint assembly, if the mating surfaces have excessive waviness (long-wavelength deviations), the primary risk is:
- Increased friction during assembly
- Uneven clamp load distribution and potential fatigue cracking (Correct answer)
- Higher surface roughness measurement
- Improved sealing without a gasket
Correct answer: Uneven clamp load distribution and potential fatigue cracking
Waviness creates gaps and high-pressure contact zones in bolted joints, causing uneven clamping forces that accelerate fatigue failure.
Question 6: A drawing specifies a surface profile tolerance of 0.05 mm bilateral. This means each surface point must fall within:
- 0.05 mm above the true profile
- 0.025 mm on each side of the true profile (Correct answer)
- 0.05 mm below the true profile
- 0.10 mm total zone offset to one side
Correct answer: 0.025 mm on each side of the true profile
A bilateral profile tolerance of 0.05 mm is split equally, allowing 0.025 mm inward and 0.025 mm outward from the true profile.
Question 7: When a surface finish symbol includes a value above the check mark line and a different value below it, they represent:
- Maximum Ra and minimum Ra limits (Correct answer)
- Ra value and Rz value respectively
- Roughness and waviness separately
- Metric and inch equivalents
Correct answer: Maximum Ra and minimum Ra limits
Per ISO 1302, values above and below the check mark line represent the upper (maximum) and lower (minimum) Ra limits for the surface.
GD&T Surface Finish & Assembly Requirements 3
A shaft and hole assembly uses an interference fit.
Which surface finish condition on the shaft would MOST likely cause the interference to be less than intended?