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In designing gages to make dimensional measurements in a production environment, speed and high precision are the names of the game. Thus accounts for the popularity of instruments such as snap gages for ODs and air plugs for IDs. They are designed to measure a single size, require virtually no operator involvement and provide precise results in some of the toughest conditions.

The same demands are true for surface finish gaging. Now, with the requirement of more monitoring of surface finish, similar gage designs are needed to ensure fast surface finish measurements with little operator involvement and precise results.

Some of the most common features that require surface finish measurement are machined bores, outside diameters of shafts and flat machined surfaces used for sealing/mating surfaces. Measuring these types of surfaces can be completed rather easily if the surface is fully accessible, unobstructed and large enough to accommodate a surface finish instrument.

But not all machined surfaces are in plain view for an operator to perform these measurements. The feature to be measured is often visually obstructed, small in diameter, or it may have an interrupted surface or be located deep within the part.

Deep bore measurements are particularly difficult. Sometimes measurements can be made by bringing the part over to a surface finish gage, staging the part on the gage, then finding a probing system small enough and long enough to get into the hole to make the measurement. This typically does not meet the requirements of being out on the floor at the manufacturing point, being fast or being easy to use. The results may be good, but it is not a production environment check.

Designing gages to do this type of surface finish measurement requires the same type of thinking that went into the snap gage and air plug. This is counter to the usual way of thinking about surface finish gaging—going from versatility in gage design to dedicated gaging—but just like the air plug, the results can be dramatic.

Just as an air plug is made close to the measuring size, a portable surface finish plug can also be made to nearly the size of the hole to be measured. And, instead of an air orifice being the measuring source, a surface finish probe can be built into the plug for performing the surface finish check at a specific location.

However, there is a bit more involved in getting the best performance and eliminating operator involvement. The plug holding the surface finish probe can be made to expand using an air cylinder. This basically locks the plug into position to make it a hands-free operation. Also, the probe can be protected from hitting the part (and potentially breaking) during insertion. As part of the process of expanding the plug into the bore, the probe can be brought into position only when the plug is in the measuring position.

By taking the concepts of high-precision shopfloor gaging and applying them to surface finish measurement, these measurements can be brought to the point of manufacture, right where they belong.