S-W-I-P-E For Gaging Accuracy

Last month we looked at a new Ingress Protection, or IP, standard for evaluating how well a gage will handle the various conditions it will encounter on the shop floor. We described how a caliper or micrometer with a rating of IP-67 is impervious to dust and dirt, and is also both water- and coolant-proof. However, we also pointed out that just because the gage is impervious to environmental conditions, does not mean you can ignore environmental conditions when taking measurements.

Indeed, the gage or instrument is only "one-fifth" of the total system for taking precision measurements. In all, there are five elements of a measuring system, which can be conveniently remembered with the acronym, SWIPE--Standard, Workpiece, Instrument, People and Environment. Whenever there is an apparent gaging problem, examining these five elements is a good approach to troubleshooting. Let's look at them more closely.

"S" represents the standard used when the system is setup or checked for error, such as the master in comparative gages or the leadscrew in a micrometer. Remember, master disks and rings should be handled as carefully as gage blocks, because nicks and scratches can significantly contribute to error.

"W" is the workpiece being measured. Variations in geometry and surface finish of the measured part directly affect a measuring system's repeatability. These part variations are difficult to detect, yet can sometimes manifest themselves as apparent errors in the measuring system. For example, when measuring a centerless ground part with a two-jet air ring, a three-point, out-of-round condition will not show up because you are only seeing average size.

"I" is the instrument itself. Select a gage based on the tolerance of the parts to be measured, the type of environment and the skill level of the operators. Also remember what your customers will use to measure the parts. Say, for example, you are checking bores with an air gage, but your customer inspects them with a mechanical gage. Since the surface is not a mirror finish, your air gage will give the average of the peaks and valleys, while the customer's mechanical gage may say the bores are too small because it only sees the peaks. Neither measurement is "wrong," but you could end up blaming each other's instruments.

"P" is for people. Modern gages are increasingly easy to use, but failure to adequately train operating personnel will still ensure poor performance. Even the operation of the simplest of gages, such as air gaging, requires some operator training for adequate results. Most importantly, the machine operator must assume responsibility for maintaining the instruments. Checking for looseness, parallelism, nicks and scratches, dirt, rust and so on, is absolutely necessary to ensure system performance. We all know it, but we can sometimes forget when in a hurry.

"E" represents the environment. As we've said before in this column, thermal factors such as radiant energy, conductive heating, drafts and room temperature differentials can significantly impact gage system performance. And again, while dirt may no longer affect your gage, it still can affect the measurement--making dirt the number one enemy of gaging.

In short, the problem that has you pulling your hair out and cursing your instruments could be as simple as your shop being a little warmer or a little dustier than your customer's. So before blaming the gage (or the customer), take a SWIPE at all the factors influencing accuracy.