Accuracy is Spelled S-W-I-P-E
ACCURACY IS SPELLED S-W-I-P-E
Accuracy, When a gauging system is not performing as expected, we often hear the same dialogue. The operator, who has only his gauge to go by, says, “Don’t tell me the parts are no good- they measure okay on my gauge.” The inspector replies, “Well, the parts don’t fit, so if your gauge says they are okay, your gauge is wrong.”
This is the natural reaction. People are quick to blame the instrument because it is easy to quantify. We can grab it, take it to the lab and test it. However, this approach will often fail to find the problem or find only part of it, because the instrument is only one-fifth of the total measuring system.
The five elements of a measuring system listed in an acronym: SWIPE. Rather than immediately blame the instrument when there is a problem, a better approach is to examine all five elements.
S represents the standard used when the system is set up or checked for error, such as the master in comparative gauges of the lead-screw in a micrometer. Remember, master disks and rings handles as carefully as gauge blocks, because nicks and scratches can be a significant contributor to error.
W is the work-piece measured? Variations in geometry and surface finish of the measured part directly affect a system’s repeat-ability. These variations are difficult to detect, yet can sometimes manifest themselves as an apparent error in the measuring system. For example, when measuring a centre-less 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. Remember that as part tolerances get tighter and tighter, form and surface finish errors will eat up a larger part of the tolerance span.
I stands for the instrument itself. Select a gauge based on the tolerance of the parts to measure, the type of environment and the skill level of the operators. Remember with what your customers will be measuring the parts. Say for example, you are checking bores with an air gauge but your customer inspects them with a mechanical gauge. Since the surface is not a mirror finish your air gauge is giving you the average of the peaks and valleys, while the customer’s mechanical gauge is saying 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. Failure adequately to train operating personnel will ensure poor performance. Even the operation of the simplest of gauges, such as air gauging, requires some operator training for adequate results. Most important, the machine operator must assume responsibility for maintaining the instruments. Checking for looseness, parallelism, nicks and scratches, dirt, rust, etc., is necessary to ensure system performance.
E represents the environment. Unbelievably, with very tight tolerances, environmental errors are probably the biggest sources of gauging errors. Thermal factors such as radiant energy, conductive heating, drafts and room temperature differentials can significantly affect gauge system performance. Again, dirt is the number one enemy of gauging. The problem that gets 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 customers are.
Before blaming your gauge, take a S.W.I.P.E. at it and consider all the factors influencing its accuracy.
George Schuetz, Mahr Federal Inc.