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Measuring instruments such as scales, calipers and micrometers are used throughout a variety of manufacturing processes because they are inexpensive and versatile. These measuring tools have built-in reference scales to which they compare a part, and then the measurement is presented as a numerical result. Hand tools generally have a long measurement range and, in most cases, provide adequate resolution and performance. If you are just looking for a quick indication of part size, they do a great job. On the other hand, they also require the operator to have the proper skills and make the proper alignments to get reliable results. Thus, their accuracy, performance and measurement speed can be limiting.

Here, gaging equipment starts to shine. Gages compare the part to an external standard (not a built-in scale) and tell the user whether the part is smaller or larger than this master. While the measuring range of a gage cannot hold a candle to that of a measuring tool, gages tend to require less operator involvement, can work significantly faster and tend to be more accurate with higher resolution than measuring tools. Gages also tend to cost a lot more than measuring tools (although, with a little care and maintenance, they can last for decades).

Variable gages are those that provide actual measurement deviations. In this way, they provide the data a shop needs to make decisions about process trends, to diagnose manufacturing issues and to learn a little about the form of a part. While fixed gages such as go/no-go devices also can provide good or bad classification, they still indicate how much smaller or larger a part is than the master being used. 

There are two distinct types of variable gages: adjustable-variable and fixed-variable. The first type can be adjusted over a range and set to a new master size. Adjustable snap gages, inner/outer diameter gages and bench stands fall into this adjustable-variable gage group. These gages tend to be faster than hand tools and are fairly easy to use. They are also less susceptible to operator influence and provide significant improvement in the measuring process in a shop environment. 

The other type of variable gage can be referred to as the fixed-variable variety. These gages are the ultimate in shopfloor measuring. They are by far the most accurate and require virtually no special user skills. 

The term fixed-variable is not an oxymoron but rather a good description for how the gages are constructed. They include air plugs and rings, or mechanical bore plugs made to measure a specific size. Once the bore tooling is inserted, the built-in clearance aligns the plug to the bore. The clearance is so small that it is virtually impossible for the user to influence the reading. 

It is easy to see how fixed-variable gages can be the fastest and still the most accurate. It’s also easy to see, with each gage dedicated to one size and each requiring its own dedicated setting standard, that this type of gage is going to be the most expensive solution as well. But if you have a tight tolerance, and need to measure thousands of parts quickly and with excellent performance, a fixed-variable gage should be your gage of choice. 

Now, if measuring tools are good, variable gages are better, and fixed-variable gages are best, what is the best of the best: fixed-size air tooling or fixed mechanical bore gages? Performance-wise, air gages probably have a slight advantage, since they, in effect, have no moving parts. There are no physical contacts passing their motion through transfer rods to the indicating device. And air gaging offers more versatility as well. The tiny air jets can be used for unique multiple-diameter applications or to generate basic form indications. There are limits on range and surface finish, however. 

Mechanical plug gages, on the other hand, offer a bit more portability, because they are not tied to a display unit with an air hose. They also tend to require less initial investment since they do not require a constant supply of clean air in their operation. 

Either way, gages are a better solution than measuring tools for higher-volume production runs and increasingly tighter tolerances. Choosing the right type depends on a combination of performance, ease of use and overall cost.