GlossaryKind of term: Manufacturing termCoaxiality and concentricity
What are coaxiality and concentricity?
Coaxiality limits how far the axis of a cylindrical feature may lie from a datum axis; concentricity does the same for a centre point in one cross-section.
In ISO 1101, coaxiality and concentricity share the symbol ◎ and are location tolerances with a datum. Coaxiality concerns an axis: the derived axis of the feature has to lie within a cylinder of the stated ⌀, coaxial with the datum axis. Concentricity concerns the centre point of a cross-section: it has to lie within a circle of the stated ⌀ around the datum point. A diameter between two bearing seats, with coaxiality ⌀0.02 to the common datum A-B formed by those seats, may therefore have its axis no more than 0.01 mm from the datum axis along its whole length.
What does the standard say about coaxiality and concentricity?
In ISO 1101 concentricity applies per cross-section; the current edition marks this with ACS, any cross section, next to the frame. ASME Y14.5 dropped concentricity and symmetry in its 2018 edition; coaxiality is controlled there with position, runout or profile. Concentricity under earlier ASME editions worked with the midpoints of diametrically opposed points and is not the same as the ISO requirement. The standard the drawing invokes, and its edition, therefore matter.
How do coaxiality and concentricity work in practice?
Diameters turned or ground in one chucking usually sit close to the same axis; the remaining error depends on the machine, deflection of a slender part and support from a centre or steady rest. If the part has to be turned round for the second side, clamping decides the error: soft jaws or a collet on the turned diameter, clocking in, or turning and grinding between centres. On a lathe with a sub-spindle, the sub-spindle takes the part over automatically; that is still a reclamp, and the alignment between the two spindles counts. Proving coaxiality unambiguously takes a coordinate measuring machine or a form or roundness tester.
What are coaxiality and concentricity often confused with?
Coaxiality is not runout: runout measures the surface while the part turns about the datum axis, and holds form and location together. A part can be within its coaxiality and still run out too much if the diameter is not round. Nor is coaxiality the same as position, although a position tolerance relative to a datum axis can give a similar zone.
Frequently asked questions
What is the difference between coaxiality and concentricity?
Coaxiality concerns the axis of a cylindrical feature along its length, concentricity the centre point in one cross-section. Same symbol, different zone: a cylinder against a circle.
Can I measure coaxiality with a dial indicator?
Not cleanly. A dial indicator on the turning part reads runout, form error included. If radial runout falls within the coaxiality value, with the same datum, coaxiality is also met as long as the form error is small. Otherwise a coordinate measurement decides.
Why do American drawings no longer show concentricity?
Because the 2018 edition of ASME Y14.5 no longer has it, and symmetry went with it. Drawings to that edition use position, runout or profile instead. Older ASME drawings may still carry the symbol, and ISO 1101 still has it.