Self- And Datum-Referenced Form Measurements

Form or geometry gages are generally divided into two categories depending on what form parameters they are designed to measure. While there is no formal designation to differentiate these two categories—sometimes they are simply referred to as roundness or cylindricity gages—their difference is primarily based on their ability to make self-referenced or datum-referenced measurements.

The basic form parameters of roundness, concentricity, circular runout, circular flatness, perpendicularity, plane runout, face runout, circular parallelism and coaxiality are all self-referenced measurements, and they are done with a basic roundness gage. This type of gage stages the workpiece on a precision rotary table and provides a means of positioning the gaging transducer against the part. As the turntable rotates, the gaging transducer senses and measures the deviation from the reference circle provided by the spindle.

The parameters measured on roundness gages have datums that are self-referenced. Self-referenced measurements do not require any datum to be established on the part independent from the feature being checked. Hence, parameters such as flatness and roundness require only one measurement. With the roundness check, after the workpiece has been centered on the precision rotary table and the gage generates a centered polar trace, the report indicates ideal roundness and also calculates roundness deviation from the reference circle. There are four different standardized methods to mathematically establish the reference circle: radial separations, least squares and maximum and minimum inscribed circles. Based on the method chosen, readings can vary up to 15 percent.

A step up from the basic roundness gage is the cylindricity gage. Cylindricity gages have the gaging transducer supported by a precision vertical slide that also serves as a vertical reference with known straightness and linear positioning accuracy. Besides being capable of measuring the parameters noted above, this category of gage allows measurement of datum-referenced parameters.

With datum-referenced measurements, one or more setup measurements are required to establish a datum on the part. This must be done before features such as concentricity or coaxiality can be measured. To gage single plane concentricity of a bearing ring, the inner diameter (ID) must be measured first to establish a center datum. Then the outer diameter (OD) is measured and its center is established. Eccentricity is the distance between those two centers, while concentricity is double that amount.

Concentricity requires that an axis be established as the datum, as opposed to a center point. At least two roundness measurements are required to establish the reference axis, or datum. Finally, a third measurement is made on the part feature in question to establish the location of its center relative to the established datum.

Cylindricity is yet more complex. Gaging software is needed to lead operators through this measuring process, instructing them to measure the first diameter, then the second diameter and finally the feature in question. The one important thing the computer software does not do is tell the user where to make the datum measurements. This is something that the quality or process engineer must decide.
What is the best way to establish a datum? It depends on the part configuration and how the part is to be used. On a shaft with a journal on each end where you need to establish concentricity of the center journal, the datum could be set up using the two journals—assuming these two journals are coaxial to each other.

No matter what your requirements, you will find geometry gages have become more attractive both in price and ease of use. This allows these types of gages to be used closer and closer to the point of manufacture, and it allows use by operators without a lot of special training.