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In industries that require high volumes and tight tolerances, there is a convergence of two manufacturing technologies: fixed-body plug gaging and manufacturing automation.

Fixed-body plug gages are one of the best ways to measure hole diameters. This type of gage has no rocking, unlike an adjustable bore gage, and therefore, it is fast and provides high performance for tight-tolerance holes. Because these gages are so easy to apply, they are typically used to measure a high volume of parts.

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There are two types of fixed-body plug gages. The first is a mechanical type that uses two contacts on a flexible sprint arrangement to squeeze a “V” rod that transfers the diameter change to the indicating readout. The second is air gaging, which typically uses two small air jets restricted by the bore building backpressure, which is converted to a dimensional change. With its small measuring sensor, air gaging may allow for making multiple measurements in a small area, thus increasing productivity. Both are very accurate, fast, easy to use and well suited for parts with tight tolerances.

Automation continues to provide productivity improvements and success for high-volume parts or where parts are heavy and must be moved from various manufacturing stations. Automation of the manufacturing process can take a lot of different forms. One can be a complete dedicated manufacturing line, which includes dimensional gaging built into the process using dedicated gaging stations or integrated robot arms placing gaging into parts as they pass through the manufacturing operations. Or, in the case of heavier industrial parts, robots may move the heavy parts between manufacturing cells while placing the parts on a gaging station between processes.

This seems to be the perfect marriage of tight-tolerance gaging and in-process automated manufacturing. However, with fixed-plug gaging, the body size of the gage is very close to the diameter it is made to measure. Therefore, the plug “locks” the gaging part of the plug in the hole. This requires very little operator skill and results in virtually no operator influence. Obviously, there has to be some clearance between the plug body and the hole diameter so the plug can fit into the hole being measured, and control of this clearance is critical for the gage builder.

This is where there can be a detour in the application of fixed-plug gaging and automation. Body clearance can become an issue, whether a dedicated gaging station or a robot-loaded part on a gage. When fixed-plug gaging is employed, the body clearance between the air plug and the bore typically runs about 0.05 mm. This body clearance is easy for manual handheld gages since the operator can “feel” the plug aligning with the bore, and when aligned, the plug easily goes into place. Unfortunately, robots or dedicated gaging stations may not have this feel for aligning. When not aligned, the gage will jam, possibly damaging the gaging tool, positioning mechanism and the part being measured.

There are solutions to make the marriage work. Mechanical or pneumatic compliance devices can be built into the gaging station to help with any misalignment between the gage and the part to be measured. Compliance devices hold the fixed body tool in place in a very rigid manner, but they allow for a small amount of movement, 360-degrees axially to the plug, which allows the plug to float and self-align to the bore being measured. Often, an increased lead-in is built into the plug to help find the bore and align it. But for difficult applications such as a short land, blind, large diameter, the compliance device is an absolute requirement for the in-process application.

Compliance devices can take the form of a mechanical mechanism with springs and clearances to center and align the part, or they can be pneumatic, providing a floating, friction-free air cushion to allow for the tool to be aligned with the part. They can even be designed to be used horizontally or vertically.

One typical application is for a gaging station that measures parts between manufacturing processes. A robot arm would be used to position heavy parts into a machine tool, and then, once the process is complete, the part would be removed and placed onto a gaging station. Because the part is heavy and robust, industrial robots are required. They may not have the capability to place the part on the fixed-body gage precisely enough for alignment. A compliance device built into the gaging station would help align the part as the robot arm places the part on the gage.

Another application could be in a manufacturing line where parts move through dedicated gaging stations. Compliance devices would be built into the fixture and moved into the gaging position to ensure alignment and prevent potential jamming.

Thus, we have a perfect marriage, with high-precision gaging in an automated process. In addition, fixed-plug gaging can be designed with coatings that offer a long life for millions of parts. And with a pneumatically operated compliance device with virtually no mechanical wear, these marriages can last a long, long time.