STRAIGHTNESS

FORM

Controls how straight a line element or axis is

✓ NO DATUM NEEDEDASME Y14.5-2018 §12.4

Straightness controls how much a line element on a surface, or an axis of a feature of size, can deviate from a perfect straight line. Applied to a surface it controls each line element independently. Applied to a feature of size (with ⌀) it controls the derived median line.

WHEN TO USE IT

Use surface straightness to control individual line elements on cylindrical or flat surfaces. Use axis straightness (with ⌀) to allow a shaft or pin to bow slightly while still fitting through a mating hole — useful when the assembly allows some bending but not excessive curve.

COMMON MISTAKES

Applying straightness to a surface without understanding it only controls individual line elements, not the full surface

Omitting the ⌀ symbol when controlling the derived median line of a cylindrical feature

Using straightness when cylindricity or flatness is the actual intent

Applying straightness with a datum reference — it is a form control and does not use datums

IS YOUR CALLOUT CORRECT?

Check these before releasing your drawing.

If controlling a cylindrical axis, does the FCF include the ⌀ symbol?

Without ⌀, straightness applies to surface line elements, not the derived median line.

Is the FCF free of any datum reference letters?

Straightness is a form control. No datum reference should appear in the FCF.

Is straightness the right control, or does the function require cylindricity or flatness?

Straightness only controls individual line elements. For full surface form, use flatness or cylindricity.

RELATED SYMBOLS

OTHER FORM CONTROLS

⏥Flatness ○Circularity ⌭Cylindricity

Redprint checks straightness automatically

Redprint checks for missing ⌀ symbols on axis straightness callouts and flags any datum references in straightness FCFs.

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