⌀ Diameter

How to verify

An outside diameter is measured with a micrometer in several orientations and at several heights (at least two crossed diameters per section and several sections along the part) so as not to confuse size with ovality or taper. For inside diameters, use a three-point bore gauge or an inside micrometer; a caliper with inside jaws carries form error and feel error, so it is fine for a quick check but not for an H7. On a CMM the diameter is calculated by fitting a circle to the probed points, and the number of points affects the result: with only three points you always get a perfect circle even if the part isn't one.

Workshop guideline values

General

±0.1–0.2 mm (around IT11–IT12) on ordinary machined diameters with no fit function.

Accuracy

H7/h6, about 0.015–0.03 mm on diameters from 10 to 50 mm, for ordinary assembly fits.

High precision

0.003–0.008 mm, ground, on bearing seats and spindles.

Guidance only: the drawing's value always wins.

Common mistakes when interpreting it

• Reading the value in a frame with ⌀ as if it were the allowance on each side: ⌀0.1 is a zone 0.1 in diameter, that is, a maximum radial deviation of 0.05.
• Leaving the ⌀ off a position frame for a hole: without it the zone is no longer cylindrical and the acceptance criterion changes completely.
• Confusing ⌀ with S⌀: the latter is spherical and defines a sphere-shaped zone, not a cylinder.