Metrology
Metrology
The "C" in Gage and Other Components
EN-US

The "C" in Gage and Other Components
George Schuetz, Mahr Federal Inc.


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. Along with this "C" you will also find other common components that may not be capitals, but are nonetheless essential to a gage's function.

But before we look for the hidden "C", let's learn about the common parts. Most dimensional gages have some type of reference. It's the part of the gage that provides a hard stop and 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 since it tends to be flat. Sometimes it's just simply a fixed reference contact, jaw or point, as seen on a measuring tool such as a dial caliper. But 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 position of the sensitive contact. 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 the 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 to eliminate both operator and gage variability errors, and therefore 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. With a snap gage, it's the frame. A bench stand may have a few parts such as the base, post, and arm, while a caliper will have the rails holding the reference and sensitive jaws.

But 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.

So next time you're playing Scrabble and spell "GAGE" with a "C", tell them it's OK, it really is there.

                          

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.