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Foundation 07

Rule #1 & Features of Size

How size limits and form relate for regular features such as holes, shafts, and opposed parallel surfaces.

A feature of size is a cylinder, sphere, or set of opposed parallel surfaces associated with a size dimension. Rule #1 is the commonly used name for the default boundary relationship that connects the limits of size and form for regular features of size.

What Rule #1 protects

For a regular feature of size, the maximum-material size establishes a perfect-form boundary. A shaft at its largest permitted size cannot bulge beyond a perfect cylinder of that size; a hole at its smallest permitted size cannot violate its perfect cylindrical boundary.

Size and form are connected

As an actual feature moves away from maximum material size, it may have more local form variation while remaining within its size limits and boundary requirement. This is why size limits are not merely two independent diameter readings.

Know when to look closer

Special modifiers and nonstandard feature shapes can change the default interpretation. When a feature is critical, read the applicable drawing notes and current ASME guidance rather than relying on a one-line memory of Rule #1.

Example: a shaft

⌀10.0010.10

A shaft with 10.00–10.10 mm limits must not exceed the perfect-form boundary at 10.10 mm, even if a few individual measurements appear to be within the size limits.

Common mix-up

Rule #1 is not a substitute for a separate straightness, circularity, or cylindricity requirement when the function needs tighter or differently defined form control.

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