GlossaryKind of term: Manufacturing termCylindricity
What is cylindricity?
Cylindricity is a form tolerance that limits roundness, straightness of the surface lines and taper together over the whole cylindrical surface.
Cylindricity ⌭ is a form tolerance with no datum. The whole cylindrical surface has to lie between two coaxial cylinders radially separated by the tolerance value. That brings roundness in every section, straightness of the generator lines, taper and barrel shape under a single value. On a ⌀50 mm bore 120 mm long with cylindricity 0.01 mm, the form error over the full length, whether oval, barrel-shaped, tapered or bent, has to stay within a radial zone of 0.01 mm.
What does the standard say about cylindricity?
The zone is defined in ISO 1101; terms and default measurement settings are in ISO 12180. As with circularity, the measured value depends on the reference cylinder, the filter and the measuring strategy, such as the number of sections and lines. Under ISO 8015 the size tolerance does not limit form; a cylindricity or the envelope requirement Ⓔ does.
How does cylindricity work in practice?
On long bores and seats, cylindricity can lengthen the route. Boring or fine turning over a long length leaves taper or barrelling when the boring bar or the part deflects; for a tight value, grinding or honing follows, often subcontracted, with transport and lead time on top. Measuring needs a form tester with a vertical straightness axis or a coordinate measuring machine probing at several heights and along several lines. How much time that takes depends on the value, the length and the quantity.
What is cylindricity often confused with?
Cylindricity is not circularity: circularity limits one cross-section. Total runout ⌰ looks similar, but it is measured about a datum axis and limits the position relative to that axis as well as the form; cylindricity has no datum.
How do you see cylindricity in Blake?
In Blake the cylindricity and the fit of the same bore sit with the part as proposals from the drawing, and you confirm or correct both values. Honing or grinding sent to a subcontractor appears in the estimate as a separate line from your subcontract library, open until it has a price. More on estimating.
Frequently asked questions
Which is stricter, cylindricity or circularity?
At the same value, cylindricity, because it includes roundness in every section plus straightness and taper along the length. A part that meets the cylindricity meets the same circularity; not the other way round.
How do you measure cylindricity?
On a form tester with a vertical straightness axis or on a coordinate measuring machine probing several sections and lines. A few diameters taken with an internal micrometer say little about the form between those points.
Why does a bearing seat carry a cylindricity?
Because a thin bearing ring largely takes on the shape of its seat. An oval or tapered seat distorts the raceway and can shorten bearing life, so the designer specifies the form separately from the fit.