Diameter (⌀) — Before Every Diameter Value

The ⌀ symbol placed before a value states it is a diameter — the width of a circular feature through its center — and it must precede every diameter value, in dimensions and in feature control frames alike.

Definition

The diameter symbol ⌀ precedes all diameter values without exception: size dimensions (⌀30), tolerance values inside frames, and datum features of size. A diameter is a straight distance across a circular feature passing through its center — the symbol tells the reader the feature is round and the value spans its full width, not a radius, not a side, not an offset.

Where omitting it changes the meaning

The costliest place to drop ⌀ is a feature control frame. ⌖ 0.5 A states a tolerance zone 0.5 wide — two parallel planes. ⌖ ⌀0.5 A states a cylindrical zone 0.5 across. Same number, different geometry, different inspection. The same rule governs straightness of an axis (a Ø zone versus a width) and position at MMC on holes and bosses.

⌀ vs. R

A full or near-full circular contour is dimensioned with ⌀; a partial contour — a fillet, a corner round, an arc that never closes — is dimensioned with R from its center. The dividing test is whether the feature closes around its center: if it does, ⌀.

• ⌀ — a full contour closing around its center: cylinders, holes, bosses
• R — a partial arc dimensioned from its center: fillets, rounds, corners
• SR / S⌀ — the same two measurements taken to a sphere instead of a cylinder
• CR — R plus one added requirement: a fair curve with no reversals

Spherical diameter: S⌀

When the round feature is a ball rather than a cylinder — a ball-end probe tip, a spherical valve seat — the symbol becomes S⌀: a diameter measured across a sphere in any direction, not a cylinder's single-axis width. Spherical radius (SR) is its partial-contour counterpart.