GD&T Surface Finish & Assembly Requirements 4 — Questions and Answers
Question 1: A part drawing specifies flatness of 0.05 mm on a mounting surface. Flatness controls:
- How parallel the surface is to a datum
- How flat the surface is regardless of its location or orientation (Correct answer)
- The surface roughness to within 0.05 mm Ra
- Both orientation and form simultaneously
Correct answer: How flat the surface is regardless of its location or orientation
Flatness is a form control that restricts only the shape of the surface without any datum reference, meaning location and orientation are not controlled.
Question 2: For a press-fit pin in a housing, the designer increases the surface roughness of the pin. What is the likely design intent?
- Reduce assembly force required
- Increase friction and holding force after pressing (Correct answer)
- Improve corrosion resistance
- Reduce contact stress at the interface
Correct answer: Increase friction and holding force after pressing
Higher surface roughness increases the interlocking of asperities in a press fit, which raises the axial and torque holding force of the joint.
Question 3: Which of the following best defines 'waviness' in surface texture measurement?
- Short-wavelength deviations caused by tool vibration
- Long-wavelength deviations exceeding the roughness cutoff wavelength (Correct answer)
- The average peak-to-valley height over 5 sampling lengths
- Surface scratches visible to the naked eye
Correct answer: Long-wavelength deviations exceeding the roughness cutoff wavelength
Waviness consists of longer-wavelength surface irregularities that remain after the roughness (short-wavelength) component is filtered out.
Question 4: A part with a profile of a surface callout referenced to datums A, B, and C controls:
- Form only, since profile is a form control
- Form and orientation only
- Form, orientation, and location simultaneously (Correct answer)
- Location only, since datums are referenced
Correct answer: Form, orientation, and location simultaneously
When datums are referenced with a profile of a surface tolerance, it simultaneously controls form, orientation, and location of the surface.
Question 5: In GD&T, the surface texture symbol (check mark shape) without any modification means:
- No surface finish requirement applies
- Material removal by any process is acceptable (Correct answer)
- Material removal is prohibited
- Grinding only is required
Correct answer: Material removal by any process is acceptable
The basic surface texture symbol (check mark without a bar) indicates that any manufacturing process capable of achieving the specified roughness is acceptable.
Question 6: A hydraulic cylinder bore must maintain an oil film. Which surface texture specification is MOST appropriate?
- Ra 6.3 μm with random lay
- Ra 0.4 μm with cross-hatch (angular) lay from honing (Correct answer)
- Ra 3.2 μm with parallel lay
- Ra 0.1 μm mirror finish
Correct answer: Ra 0.4 μm with cross-hatch (angular) lay from honing
Honed cross-hatch patterns at Ra ~0.4 μm retain oil in the valleys while providing adequate sealing with piston rings, which is the industry standard for cylinder bores.
Question 7: Two mating flat surfaces in an assembly each have a flatness tolerance of 0.03 mm. What is the worst-case combined flatness deviation between them?
- 0.03 mm
- 0.015 mm
- 0.06 mm (Correct answer)
- 0.09 mm
Correct answer: 0.06 mm
In the worst case, both surfaces bow in opposite directions; the combined effect stacks the tolerances, giving a maximum gap variation of 0.03 + 0.03 = 0.06 mm.
GD&T Surface Finish & Assembly Requirements 4
A part drawing specifies flatness of 0.05 mm on a mounting surface.
Flatness controls: