The fundamental rules of GD&T (ASME Y14.5) define how dimensions and tolerances must be interpreted. Understanding these rules is essential to avoid design and inspection errors.
Rule #1 – Envelope Principle (Perfect Form at MMC)
Also known as the Taylor Principle.
- A feature at Maximum Material Condition (MMC) must have perfect form
- As the feature departs from MMC, form error is allowed within size limits
Example:
A shaft at its largest diameter must be perfectly straight and round.
If it becomes smaller, slight form errors are acceptable.
✅ Ensures fit and assembly in worst-case conditions.
Rule #2 – RFS Default Rule
- All geometric tolerances are Regardless of Feature Size (RFS) by default
- Applies unless a modifier like MMC or LMC is explicitly specified
Means geometry must meet tolerance independent of size
✅ Prevents unintended bonus tolerance.
Other Key Rules
1. Datums Must Be Clear and Complete
- Features controlled by GD&T must reference appropriate datums
- Missing or unclear datums lead to ambiguity in inspection
2. Each Tolerance Controls Only What It Specifies
- Size, form, orientation, and location are independent unless stated
- Do not assume one tolerance controls another
3. Bonus Tolerance Only with MMC/LMC
- Additional tolerance is allowed only when MMC or LMC is specified
- No bonus tolerance under RFS
4. Units and Limits Apply to All Dimensions
- All dimensions must have defined tolerances (directly or via general notes)
Rule #1 is often misunderstood—many engineers forget that size tolerance already controls form at MMC
- Rule #2 is often forgotten—leading to accidental over-tolerancing or under-tolerancing
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