Snap gages are most frequently used in production environments where precise diameter or thickness measurements need to be made quickly and repeatedly. They can be hand held or mounted on frames. Typically, they consist of a “C” frame, two anvils (one fixed and the other under spring tension) and a backstop on the frame to locate the part.
The earliest snap gages were of the fixed, go/no-go, variety. These were simple, reliable and efficient tools, but separate gages were needed to measure all the different part sizes.
Mounting an indicator onto the moveable, or sensitive, anvil and making the fixed anvil adjustable proved a great leap forward. Not only did the new design provide greater measuring range and eliminate the need for multiple fixed gages, but indicating snap gages also allowed users to learn something from the measuring process. By actually measuring part size (either absolutely or comparatively), users could begin to see how their process was trending.
Today, indicator snap gages are made to different designs based on the indicating device to be used with the gage. As the expected performance of the gage will dictate some of the subtle design features built into the gage, you can expect that a gage that reads out to 10 microinches will be different from one that reads out to 0.001 inch.
The basic indicator snap gage comes in two varieties: In one, the motion of the measurement is directly in line with the indicating device. In the other, motion of the sensitive anvil is translated around to offer different viewing combinations.
Each design has its own characteristics, and selecting the right gage for the application often comes down to the little details, including accuracy, ease of use, the gage’s adjustable range and actual measuring range. Here are a few snap-gage characteristics to keep in mind:
Many snap gages have an adjustable range (the range that the gage can be set to measure) of typically 0.5 to 2 inches. Most parts made in manufacturing are less than a few inches, so selecting a gage that can cover the widest range for your application is important.
How the measuring-range adjustment is made is also important. The most basic design allows the fixed, or reference, anvil to either slide or be screwed up or down in the frame, then locked in place with a clamping arrangement. With these gages, the sensitive anvil remains fixed. Other gages will move the sensitive contact and indicator assembly along a slide and leave the reference anvil fixed. What’s key to the performance of any snap gage is how well the reference and sensitive anvil stay parallel to each other. With the sliding or rotating style, more geometry is involved, and the chances of anvils getting slightly out of parallel are greater. Units that move the whole sensitive assembly employ an aligned channel that locks the slide in place and keeps it naturally aligned.
The parallelism of the anvils greatly influences gage performance and is easy to check. Select a precision ball that is equal to about the middle range of the gage and set the gage to read “zero” on the ball. Now, move the ball front to back and side to side observing the indicating device. Remember the high and low points. This is the anvil’s parallel capability. You can even do this with balls at both extremes of the snap adjustable range, and you can easily get a good feeling for how well the gage will perform over its full range.
The backstop is critical to positioning the part for repetitive measurements, especially when the part is cylindrical. If it is large and not square to the reference anvil, the part will not come to the same location. Also, if the anvils are not parallel, then the size will not be measured correctly.
A retracting mechanism for the sensitive contact can be important for thin parts or parts that are of a softer material, which can easily be marred. Lighter gaging springs can also be employed to help prevent part marking.
Wide anvils will ensure that the gage seats itself firmly and squarely on the part. If the anvils are narrow or blade styled to check narrow grooves, you may have to hold the gage as square and steady as you can. However, using anvils that are much wider than the parts can result in uneven anvil wear and degradation of accuracy.
Knowing the details about your snap gage and how to determine its best performance can increase the accuracy and repeatability of your measurements and increase the life of your gage.