The “C” in Gage and Other Components

Gages and measuring instruments come in all different shapes and sizes.

Columns From: 3/21/2011 Modern Machine Shop,

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Gages and measuring instruments come in all different shapes and sizes. But whether it's a snap gage, a micrometer or a bench stand, you will find a capital "C' in almost all of them.


Whether it’s a snap gage, a micrometer or a bench stand, you will find a capital “C” in almost all of them. Along with this “C,” you will find other common components that may not be capitals, but they are nonetheless essential to a gage’s function.
Before we look for the hidden “C,” let’s learn about the common parts. Most dimensional gages have some type of reference. That is the part of the gage that provides a hard stop, and it is the starting point from which the dimensional measurement is taken. On a micrometer, snap gage or even a bench gage, this reference is usually called a fixed anvil because it tends to be flat. Sometimes it’s simply a fixed reference contact, jaw or point, as seen on a measuring tool such as a dial caliper. In any case, it’s the reference point from which the measurement is started, and every contact gaging system has to have one.
Directly opposite and in-line with the reference anvil is the sensitive part of the tool. Somewhere within this part of the instrument is the mechanism that does the actual comparative or direct sensing of the sensitive contact’s position. While the contact that actually touches the part is not sensitive, it is linked directly to this sensing mechanism. Take the case of a bench stand with a dial indicator: The contact—usually having a radius—is the actual tip of the indicator.

For a micrometer, it’s the sensitive anvil, and with a dial caliper, it’s the sensitive jaw. But you get the message with all this sensitivity that somehow the point is really “feeling” the measurement.
While this sensitive point may be what is actually touching the part under observation, the sensing unit, or scale, may be tied directly or indirectly to that point. In the case of most measuring instruments—calipers and micrometers, for instance—the sensitive jaw has the measuring scale built into it. The same might be true with the bench stand and its comparative indicator. However, snap gages, ID/OD gages and others usually have some transfer mechanism, either in-line or offset, to get the sensitive movement to the readout device.
When gages are the measuring system of choice because of speed and increased performance requirements, another reference may be added. A backstop may be employed with these gages. The backstop is a way to ensure that the part always fits into the gage in the same position. This helps eliminate both operator and gage variability errors, and it improves gage repeatability. By having the part go to the same depth every time, parallelism errors in the anvils are also virtually eliminated.
Now, the fixed reference anvil, sensitive contact and backstop just can’t live by themselves. They must be held together by that thing called the gage itself, and this is where our big “C” comes in. For a snap gage, it’s the frame. For a bench stand, it might be a few parts, such as the base, post and arm. For a caliper, it is the rails that hold the reference and sensitive jaws.
In all these examples, this structure is really the backbone of the gage. It is what holds these three parts together in a rigid manner, and it is usually formed in the shape of a “C.”
In some cases it may be hard to see, but there really is a “C” in every gage. It may be obvious or it may be hidden beneath covers, but look deep enough, and you will find it. The more mass in this “C” structure, the better the stability and the stiffer the moment of movement. The frame is the base on which the measurement is taken. You may have the most accurate sensor available, but if it’s mounted to a frame that flexes and twists, it just won’t pay off.
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