Have You Purchased a Diameter Gage or a Roundness Gage?

If this is your first time going through the cylindrical gage purchasing process, you will be presented with a wide range of choices. When it comes to cylindrical parts, you can choose between a machine that takes radial measurements using a single-point probe (usually a roundness gage) or one that uses contact points for diameter measurement (a diameter gage). Each has specific applications. In both, the measuring transducers run perpendicular to the axis of the part. With the opposite point gage, it is less critical to control the part axis relative to the measuring probes. A relatively large runout in the spindle has less effect on the type of measurement (diameters) it performs. As the part moves toward one probe, it moves away from the other. The relative size measurement of the part therefore remains the same (although vertical movement of the part can affect the result when a rounded follower is used). With such a system, a less expensive spindle with relatively high runout can be used without adversely affecting the measurement.

However, a two-point system is not sufficient for measuring roundness. Here’s why:

Imagine a cylinder with a three-quarter or “cloverleaf” pattern around its circumference. When one of the probes enters a valley, the other approaches a peak. To the two-point diameter gage, this part appears round. This is certainly not what we want when measuring such an important characteristic as roundness! Instead, the single-point contact (radial) gage is required. (Note: this type of gage contacts one point at a time, but the part is rotated to collect the complete data set). The single-point contact gage can offer much more than diameter measurements. However, there is a price to pay. To perform accurate radial measurements, we need to control the axis of the part while it rotates for measurement. This requires a system that can securely clamp the part while introducing very little runout error. A precision spindle is therefore essential with this gage. Factor in the weight of the part and you need a highly specialized type of precision spindle. This accounts for much of the price difference between a diameter gage and a roundness, or profile, gage.

A common mistake is to buy a two-point gage based on price, only to find out later that it cannot inspect important features such as profile, chatter, and roundness.

But where the two-point contact gage cannot give you roundness, the single-point contact can give you a diameter. This is because the entire point cloud is stored relative to the precise center of the part. So we only need to apply a little math to get an average diameter, max-min two-point diameter, etc.

Beware of two-point gages that attempt to mimic the results of single-point gages. Terms such as “diametric roundness” and “ovality” are used as mathematical attempts to approximate a roundness measurement. Nowhere in the roundness standards (ISO, ANSI, or JIS) is an exception made that allows manipulation of the opposite side data. While the ovality measurement may have some value (but there are better ways to quantify such an error), it is NOT a substitute for a true roundness measurement.

Sometimes it is worth investing a little more up front to obtain a gage that can make ALL the measurements your project or a future project will require.