Size May Matter, But Form Counts Too

Cylindrical parts range from beer cans to bearings to bullets. Therefore, measuring cylindricity has considerable value for manufacturers.

The measurement of cylindricity combines three separate geometric qualities: roundness, straightness and taper. Roundness is a measure of radial error; straightness, a measure of axial error; and taper, a measure of dimensional error. A three-dimensional object can be straight and dimensionally stable but not round; it can be round and dimensionally stable but not straight (banana-like); or it can be round and straight but not dimensionally stable (carrot-like). All manufactured cylinders exhibit roundness, straightness and taper errors. Combining these qualities in a single cylindricity expression gives manufacturers a means of assessing parts based on a specification of form, rather than dimension.

I am frequently asked why this combined measurement of form has value when part errors remain essentially dimensional. A look at some problems that arise with the process will help to answer this question.

First, problems arise when manufacturers try to assess a part’s cylindricity by measuring one or more of its qualities in isolation. For most manufacturers, the first impulse is to measure roundness, incorrectly equating roundness with cylindricity.

For example, some companies may use air gaging or other dimensional gaging to measure the roundness of precision bores. An operator inserts a part onto the air plug, notes the reading, then rotates the part by hand, watching to see if the meter indication remains in the tolerance zone. This is repeated at varying depths until the part is completely checked.

While this procedure may provide a quick go/no-go measurement, all it really indicates is whether the part diameter has stayed within tolerance; no certainty of roundness can be gained from measuring that. Such roundness data from air gaging is almost useless. It cannot provide any straightness data; taper can only be inferred; and lobing is difficult to detect and characterize.

Even when part diameters are in specification, part assemblies may not operate as well as they were designed to do.

Manufacturers who recognize the shortcomings of only using roundness data to assess cylindricity often supplement roundness data with straightness or taper data. This solution is also inherently inefficient, as multiple checks are required. These often involve multiple gages and multiple setups which are time consuming and expensive.

For example, different setups may be required to measure roundness, straightness and taper. Then, once this data is collected, the assemblies may still need to be sorted and matched together.

On the other hand, a form system designed to measure cylindricity often “walks” an operator through the entire process, including part setup, tilt and centering adjustments, measurement, and data display and storage.

Roundness, straightness and taper are not equivalent expressions. Two cylinders can have the same degree of cylindricity when one is round but banana-shaped, and the other is out-of-round but straight. Regardless, cylindricity measurements provide an overall assessment of conformance to dimensional specifications. It can provide an “all clear” signal for roundness, straightness and taper in a single expression from a single test. When a part doesn’t meet a cylindricity spec, errors of shape, out-of-roundness or straightness can then be found in the cylindricity data.

Manufacturers also collect cylindricity data using coordinate measuring machines (CMMs), but circular geometry gages provide a number of advantages, including ease and speed of use, simplicity of programming, accuracy, and suitability for shopfloor use.

Geometry gages measure roundness more reliably than CMMs because geometry gages capture more data. Although some CMMs are as accurate as precision gages, they capture limited amounts of data—only several dozen points per diameter. A geometry gage, by comparison, can capture thousands of points on a diameter and up to 20 diameters on a part. With more data, a geometry gage can perform statistical analyses, such as harmonic analysis, to predict performance or identify lobing conditions to fine-tune the machining process.

With more data, a geometry gage system can generate detailed three-dimensional traces of a part. A geometry gage enables detection of scratches, burrs and other asperities that might otherwise be missed. Once these details can be seen, we’re able to control them by improving our processes. The results are better parts and assemblies.