Common Beginner Questions
Q1: What is GD&T and why should I use it?
GD&T defines geometry based on function, not just size—improving fit, performance, and clarity.
Q2: When should I use GD&T instead of ± tolerances?
Use GD&T when location, orientation, or form affects assembly or function.
Q3: Do all GD&T tolerances need datums?
No.
- Form tolerances (flatness, straightness) → no datums
- Others (orientation, location, runout) → require datums
Q4: What is the most important GD&T control?
Position tolerance—it’s the most widely used and critical for assemblies.
Q5: What is the difference between MMC and RFS?
- MMC → allows bonus tolerance
- RFS → no bonus, stricter control
Troubleshooting GD&T Issues
Problem 1: Parts don’t assemble properly
- Cause: Missing or incorrect position tolerance
- Fix: Define datums and apply proper location control
Problem 2: High manufacturing cost
- Cause: Overly tight or unnecessary tolerances
- Fix: Review function and relax non-critical tolerances
Problem 3: Inspection results are inconsistent
- Cause: Poor or unclear datum definition
- Fix: Redefine datums based on functional setup
Problem 4: Parts pass size but still fail function
- Cause: Missing geometric controls (form/orientation)
- Fix: Add appropriate GD&T (flatness, perpendicularity, etc.)
Problem 5: Difficulty measuring features
- Cause: Using complex tolerances (e.g., concentricity)
- Fix: Replace with position or profile when possible
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