What is symmetry?

In ISO 1101, symmetry is a location tolerance with a datum, shown as ⌯: three parallel strokes, the middle one longest. The median plane of the feature, of a keyway for example, has to lie between two parallel planes the tolerance value apart, placed symmetrically about the median plane or axis of the datum. A keyway in a ⌀40 shaft with symmetry 0.03 to A may therefore have its median plane at most 0.015 mm to one side. The width of the keyway falls under its size tolerance.

What does the standard say about symmetry?

ISO 1101 sets out the symbol and the zone, ISO 5459 the datum. Since its 2018 edition, ASME Y14.5 has no symmetry and uses position or profile instead. The symmetry of earlier ASME editions was based on the midpoints of opposed points and does not match the ISO requirement. Which one is meant follows from the standard and edition the drawing invokes.

How does symmetry work in practice?

Milling a keyway or slot on centre starts with finding the centre of the datum: probing both sides of the shaft or block, or workholding that centres the part, such as a V-block or collet. A slot cut to full width in one pass can run off centre because the cutter is loaded unevenly on the climb and conventional flanks and deflects; with a smaller cutter in two passes each flank is set separately, though deflection still counts. With a tight value, probing after the first pass can show the error in time, at the cost of a probing cycle in the program. A coordinate measuring machine checks it, or a gauge block in the keyway with a dial indicator, read from both sides.

What is symmetry often confused with?

Symmetry resembles position, and in ISO it is effectively a special case of it: position with a theoretically exact distance of zero to the datum median plane. The same requirement can therefore appear on a drawing as symmetry or as position. Symmetry is not a size tolerance on the width either: a keyway can be exactly to width and still sit off centre.