Concentricity

Controls

The median points — midpoints of diametrically opposed surface measurements — of a cylindrical feature must all lie within a cylindrical tolerance zone coaxial with the datum axis. It controls mass balance and centre-of-mass distribution, not the actual surface location.

Unlike runout, concentricity ignores surface variation — a perfectly eccentric but round feature could pass runout yet fail concentricity.

Example

A high-speed turbine rotor hub bore relative to the OD — if the mass centre is offset from the rotation axis, imbalance forces increase with speed. In practice, circular runout or true position is almost always specified instead, because concentricity is extremely difficult to verify and the measurement rarely represents the functional requirement.

Inspection

• CMM only — probe pairs of diametrically opposed surface points, compute median points
• A single TIR reading from a dial gauge does NOT verify concentricity — it verifies runout
• Multiple diametrically opposed pairs must be measured at each cross-section
• Practically, this is rarely done — most engineers substitute runout or position

Inspecting concentricity to full rigour is one of the most complex CMM tasks in GD&T verification.

Datum & Modifiers