GD&T Surface Finish & Assembly Requirements 5 — Questions and Answers
Question 1: A threaded fastener hole drawing calls out a perpendicularity tolerance of 0.1 mm for the hole axis relative to the mounting surface. This ensures:
- The thread pitch is correct
- The bolt will not induce bending stress on the mating flange (Correct answer)
- The hole diameter is within tolerance
- The surface roughness of the bore is acceptable
Correct answer: The bolt will not induce bending stress on the mating flange
Perpendicularity of the threaded hole to the mounting surface ensures the bolt pulls straight, preventing bending moments that could cause fatigue failure in the flange.
Question 2: What does the sampling length (cutoff wavelength λc) determine in surface roughness measurement?
- The total scan length of the profilometer
- The wavelength boundary separating roughness from waviness (Correct answer)
- The maximum allowable Ra value
- The speed at which the stylus traverses the surface
Correct answer: The wavelength boundary separating roughness from waviness
The cutoff wavelength λc is a filter setting that separates short-wavelength roughness from longer-wavelength waviness in surface measurements.
Question 3: A sealing surface for an O-ring groove must have Ra between 0.8 μm and 1.6 μm. A measured Ra of 0.4 μm would be:
- Acceptable because smoother is always better
- Out of tolerance because it falls below the minimum Ra (Correct answer)
- Acceptable as it meets the maximum Ra requirement
- Indeterminate without knowing Rz
Correct answer: Out of tolerance because it falls below the minimum Ra
O-ring grooves require a minimum roughness so the elastomer can grip the surface; a surface that is too smooth can allow the O-ring to extrude or roll out.
Question 4: In ASME Y14.5, which condition applies when a feature of size is at its maximum material condition (MMC) and a geometric tolerance is at its least restrictive?
- Least Material Condition (LMC) modifier
- Maximum Material Condition (MMC) modifier (circle M) (Correct answer)
- Regardless of Feature Size (RFS)
- The Envelope Principle
Correct answer: Maximum Material Condition (MMC) modifier (circle M)
The MMC modifier (⊕M) allows the geometric tolerance to increase as the feature departs from MMC toward LMC, providing bonus tolerance for assembly clearance.
Question 5: A component drawing requires a surface finish of Ra 0.8 μm on a mating face. The manufacturing engineer proposes lapping instead of grinding. This is acceptable if:
- The drawing specifies grinding in the process note
- Lapping can achieve Ra 0.8 μm and no process restriction is noted (Correct answer)
- Lapping always produces Ra higher than 0.8 μm
- The customer specifically approves in writing
Correct answer: Lapping can achieve Ra 0.8 μm and no process restriction is noted
Without a specified process restriction on the surface symbol, any process that achieves the required Ra value is acceptable per drawing intent.
Question 6: Profile of a line tolerance differs from profile of a surface tolerance in that it:
- Controls the entire surface simultaneously
- Controls only a single cross-sectional slice at each measuring location (Correct answer)
- Requires more datum references
- Is used only for flat surfaces
Correct answer: Controls only a single cross-sectional slice at each measuring location
Profile of a line controls the profile at individual cross-sections independently, whereas profile of a surface controls the entire surface as one zone.
Question 7: Which surface finish lay symbol indicates that the predominant surface marks are circular relative to the center of the surface?
- = (parallel lay)
- ⊥ (perpendicular lay)
- C (circular lay) (Correct answer)
- M (multidirectional lay)
Correct answer: C (circular lay)
The lay symbol 'C' designates a circular lay pattern, which is typical of surfaces produced by facing operations on a lathe.
GD&T Surface Finish & Assembly Requirements 5
A threaded fastener hole drawing calls out a perpendicularity tolerance of 0.1 mm for the hole axis relative to the mounting surface.
This ensures: