The trouble with Roundness

09 / 06 / 2013

One of the geometrical tolerance characteristics is ‘Roundness’ (or ‘circularity’ if you are working to ASME Y14.5).

The way in which Roundness has always been defined in the past is that it applies to the line element created in any plane perpendicular to the axis of the feature.

Thus roundness is measured perpendicular to the axis of the feature, and not perpendicular to the surface of the feature. This is inconsistent with other geometrical tolerances (e.g. run-out, profile, etc) which are all measured perpendicular to the surface.

While this works out just fine for cylinders (where perpendicular to the axis is the same as perpendicular to the surface), it does not always work out well for cones. The greater the angle of the cone, the greater the cosine error in any measurement.

For these reasons, a proposal was put forward in the ISO technical committee TC213, several years ago, to change the definition of roundness, so that it would be defined as a tolerance which applied perpendicular to the surface.  The committee flip-flopped over this for a while; there was a strong technical argument to support the change, but changes to something so long established were bound to cause confusion and resistance. At first the change was going to be introduced, then it wasn’t – every time it was discussed, it caused a great deal of debate.

For a while the plan was then to provide an alternative to roundness for cases where it should be applied normal to the surface, consisting of  a Circular Run-out tolerance, with the feature used as its own datum.  On examination however, it appeared that this would lead to a subtly different outcome, and this plan was also abandoned.

In ISO 1101:2017, the direction feature symbol was introduced, along with some new rules.  In cases where roundness is to be applied to a cylinder, it works in the same way that it alwasy did.  In ALL other cases, a direction feature symbol is mandatory, in order to clearly specify in which direction the tolerance applies:

More information about the direction feature symbol can be found in Clause 15 of ISO 1101:2017.

« back