Measuring the form and finish of cylindrical surfaces used to require leaving key parameters unmeasured because of time and technology constraints. But with today’s optical equipment, an entire surface can be scanned and the data that was once overlooked can now be unearthed.
Cylindrical form tolerances traditionally are separated into roundness and straightness measurements because of the limits of available measurement techniques. Typically, a surface is measured at a handful of locations in the roundness direction, and a similar number of straightness traces. The assumption is that the measurements taken at these locations are “typical” of the surface as a whole. The idea is that all the necessary form information is contained in these samples, with the surface filters for straightness specified in a cutoff length, and the roundness filters specified in undulations per revolution (upr). These one-dimensional filters are set to eliminate the influence of the high frequency surface information, specifically roughness, from form measurements such as roundness, straightness, parallelism and cylindricity. The suppression of roughness features that lay predominantly in the axial direction from the roundness measurement are handled separately, and can be a complex problem.