Symmetry

Controls

The median points of a feature — midpoints of paired surface measurements taken from opposite sides — must all lie within a tolerance zone of two parallel planes equally disposed about the datum centre plane. Controls mass distribution symmetry rather than surface position.

A feature can be offset as a whole (eccentric) but still pass symmetry if both sides are equally displaced from the datum — making it notoriously confusing to apply correctly.

Example

A keyway milled in a shaft must be centred within 0.05mm of the shaft's diametral midplane so that equal wall thickness is maintained on both sides, preventing the key from applying a side load. In practice, true position (RFS) is now used instead since ASME removed symmetry in 2018.

Inspection

• CMM — probe paired opposing surface points, compute median points at each cross-section
• A dial gauge reading surface position does NOT verify symmetry — it measures location, not median points
• Practically very difficult — each measurement pair must be diametrically opposed
• Most shops substitute position or gap measurement as a practical alternative

Rigorous symmetry inspection is rarely done outside aerospace and defence contracts.

Datum & Modifiers