6.3 Rule #1, Form, and Size

6.3 Rule #1, Form, and Size

Quick Answer: ASME Y14.5-2009 Rule #1 (the Envelope Principle, also "perfect form at MMC") requires that the surface of a feature of size not violate a boundary of perfect form at MMC—a perfect cylinder or slab at the MMC size. As the feature departs from MMC toward LMC, form is allowed to vary within the size limits. Perfect form at LMC is not required unless explicitly specified. Rule #1 is overridden by an Independence note, by applying a straightness control to a feature of size, and by certain other explicit form controls. Senior questions test when Rule #1 applies, when it's overridden, and what fit/function consequences follow.

Rule #1 — the envelope principle

For an individual feature of size (a cylindrical surface, a planar width, a sphere, or a set of opposed parallel surfaces), Y14.5-2009 §2.7.1 states that where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which features of the surface can vary, and those limits of size also govern the variations of form allowed on the surface (Rule #1).

The practical statement: at MMC (largest external feature / smallest internal feature), the surface must not violate a boundary of perfect form at MMC. For an external shaft, that boundary is a perfect cylinder (or slab) at the MMC diameter/width; for an internal hole, it's a perfect cylinder at the MMC (smallest) diameter.

Consequences:

• A shaft at MMC must be a perfect cylinder to satisfy the envelope—no barrel, banana, taper, or out-of-round beyond the size limits.
• As the shaft departs from MMC toward LMC, the envelope relaxes: the actual diameter can be smaller, and form error is allowed up to the amount of departure from MMC.
• The entire surface must be within the size limits (every two-point measurement within the ± range) and not violate the MMC boundary. Both conditions must hold.

Why Rule #1 matters for fit

Rule #1 exists so that mating parts assemble by design, not by luck. A Ø20 ±0.1 shaft fits a Ø20.2 ±0.1 bore because the shaft's MMC envelope (Ø20.1) is smaller than the bore's MMC envelope (Ø20.1) only when both are at their MMC boundaries of perfect form. If the shaft were allowed to barrel to Ø20.2 in the middle while "averaging" Ø20.1, it would bind. Rule #1 forbids that: at Ø20.1 the shaft is a perfect cylinder, and only as it shrinks can it pick up form error.

Worked example — does it assemble, and what does Rule #1 catch?

A pin Ø10 ±0.05 (MMC = Ø10.05, LMC = Ø9.95) mates with a hole Ø10.15 ±0.05 (MMC = Ø10.10, LMC = Ø10.20).

• Worst-case assembly gap at MMC: Ø10.10 − Ø10.05 = 0.05 mm clearance, guaranteed because both are perfect cylinders at MMC under Rule #1.
• A supplier ships a pin measuring Ø10.00 across every two-point check (within size limits) but with a three-lobed shape whose circumscribed circle reaches Ø10.07. Even though two-point size is conforming, the pin violates the MMC boundary of perfect form (Ø10.05), because the lobes protrude beyond the envelope. Two-point measurement alone does not verify Rule #1.

The Senior subtlety: Rule #1 catches form error that is not caught by two-point checks, such as a lobed shaft at MMC where every two-point reads Ø10.05 but the circumscribed circle exceeds Ø10.05. Verification of Rule #1 requires a full-surface or envelope check, not just calipers.

Exceptions and overrides

Rule #1 is not automatic in every drawing; it is the default, removed only by explicit action. Y14.5-2009 lists overrides:

1. Independence Principle (IP) note. A drawing-wide note such as "PERFECT FORM AT MMC NOT REQUIRED" (or the ISO-style independence symbol applied to a specific feature) detaches form from size; size is verified independently of form. Used when function depends on a feature's local size but not its envelope (e.g., a cast or non-mating surface).
2. Straightness applied to a feature of size. When a straightness control with a cylindrical zone is applied to the axis (or a straightness control to a center plane) of a feature of size, Rule #1 is overridden for that feature, replaced by the straightness boundary at the specified material condition. This is the classic way to allow a shaft to bow while still assembling at a known worst-case envelope.
3. Other explicit controls that establish a boundary different from the perfect-form-at-MMC boundary similarly replace the envelope. Orientation controls such as perpendicularity at MMC replace the related perpendicularity envelope, but circularity does not override Rule #1, because circularity bounds form within the size limits without relaxing the envelope.

What is NOT an override

A frequent Senior trap: applying flatness to a planar surface does not override Rule #1 for the feature's size, because flatness applies to a surface, not to a feature of size. Rule #1 is overridden only by controls applied to the feature of size itself (its axis or center plane) or by an independence symbol/note. Similarly, circularity does not override Rule #1—it tightens form within the envelope but leaves the envelope intact.

Perfect form at LMC?

Rule #1 mandates perfect form at MMC only. Perfect form at LMC is not required unless the drawing explicitly says so (the Y14.5 phrase is "PERFECT FORM AT LMC REQUIRED"). A typical application is a thin-wall bore that must not become locally oversized at LMC; most features never need it. Senior questions test that you know LMC perfect form is optional and must be specified.

Senior decision moments

• Fit-critical mating feature: leave Rule #1 active (no override) so the MMC envelope guarantees assembly.
• Bent-but-assembling shaft: apply axis straightness at MMC; the override replaces the perfect-form envelope with a virtual condition you can calculate.
• Cast/forged/non-mating surface: apply the independence symbol if local size matters but envelope does not.
• Thin-wall bore needing controlled LMC form: add an explicit LMC-perfect-form note; otherwise the bore is free to depart at LMC.

Worked example — override calculation

Shaft Ø25 ±0.2 (MMC = Ø25.2, LMC = Ø24.8), straightness of axis diameter 0.1 at MMC. The virtual condition (the guaranteed assembly boundary) = MMC + straightness = Ø25.2 + Ø0.1 = Ø25.3. Rule #1 is overridden; the part is acceptable if the axis lies within a diameter 0.1 cylinder at MMC (growing to diameter 0.5 at LMC) and every local size is within Ø24.8–Ø25.2. Without the override, Rule #1 would have required a perfect cylinder at Ø25.2, giving a Ø25.2 assembly boundary; the override trades a 0.1 mm larger assembly boundary for the freedom to bow.

Summary traps

• "Rule #1 = perfect form at LMC." Wrong. MMC only; LMC perfect form must be specified.
• "Any form control overrides Rule #1." Wrong. Only feature-of-size form controls (or an Independence note) override; flatness and circularity do not.
• "Two-point measurement verifies Rule #1." Wrong. Lobed or curved features can pass two-point size and still violate the envelope.
• "A bent shaft at MMC always fails Rule #1." Only if no override is present; a feature-of-size straightness control replaces the envelope with a calculable virtual condition.