Form Tolerances and Controls
What form controls do
Form tolerances limit variation of a feature without reference to a datum. They are: straightness, flatness, circularity (roundness), cylindricity, and (for profiles) the form aspect of a profile control. Form controls are always self-referencing; a datum in a form feature control frame is a red flag.
Concepts to command
Know the tolerance zone for each: straightness = two parallel lines or a cylinder; flatness = two parallel planes; circularity = two concentric circles; cylindricity = two coaxial cylinders. Understand that Rule #1 already limits form for features of size, so form controls are used to tighten form beyond size tolerance or to control a surface (non-feature of size). Know straightness applied to a feature of size at MMC, which creates a boundary control and overrides Rule #1.
Traps that appear in questions
- Circularity controls roundness in each cross-section but not taper or straightness; cylindricity controls all three (roundness, straightness, taper).
- Flatness on a surface that is also a datum feature: the flatness tolerance must be tighter than any orientation or location tolerance referencing it.
- Applying a datum to a form control frame (invalid).
- Confusing straightness at MMC (a boundary control) with straightness RFS (a form control).
Rule of thumb
Form controls never reference datums. If a feature control frame for flatness, straightness, circularity, or cylindricity contains a datum, the callout is wrong.
Worked example
A shaft Ø20.0 ±0.1 with cylindricity 0.05: the size tolerance already limits form to 0.2 via Rule #1, but the cylindricity callout tightens it to 0.05 across all cross-sections, controlling roundness, straightness, and taper simultaneously.