Metrology
Metrology
Two Cases Where Gage Improvements Can Go Wrong
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Two Cases Where Gage Improvements Can Go Wrong
George Schuetz, Mahr Federal Inc.


  The call is a common one: tolerances are tight and better gages are needed to improve the measurement process. But what may seem like a simple thing can sometimes lead to unforeseen issues.

  Take the idea of improving gage repeatability by getting more resolution from the readout device. With today's indicators, performance has approached that seen in bench amplifiers at a fraction of the price. So why not just go out and get a digital indicator with a higher resolution and place it on the gage? It's not always so simple. Sometimes the gage design just does not allow for this kind of upgrade. All it may do is let you see errors that were invisible before.

  Take the case of a portable height gage that had been used for years to check the height of small buttons inserted into the surface of the part. The buttons had a smooth rounded head, and the gage consisted basically of a portable height gage that was moved around the part to check the height of the buttons relative to the surface. As with most height gages this gage employed a flat anvil that would be placed over the button to check its height.

  A simple gaging concept for improving the resolution of the readout from 50 µin. per digit to 10 µin. per digit should have been easy. That is, until the operators lost confidence in the gage because it was not as repeatable as it used to be. What used to be one flick of the digit was now 8 or 9 counts—and this seemed terrible to the user.

  Two things were happening here. First, the operator could now see something that was hidden before. The one count of +/- 50 µin. was now ten counts to the operator. Not a change in magnitude but rather a change in the ability to actually see inherent inaccuracies in the gaging. In this case the flat anvils were not designed with a tight enough parallel specification to the base of the height gage for the higher resolution. As seen in Figure 1, any out of flatness or parallelism of the anvil can now be read by the indicator and seen by the user. What once was invisible is now visible.

  In another example, an inside diameter had been measured for years with an internal tri-contact bore gage. This had always produced great success and lots of good parts, but then the tolerance tightened. Now, gages with a measuring capability of 0.00015 in. had to measure parts with tolerances of +/-0.0003 in. That was clearly not going to work.

  To measure these types of tolerances on the shop floor, a fixed plug gage is generally the best, either mechanical or air. These gages are best because they are high performance and virtually eliminate operator influence.

  But again things did not work out as planned. Soon there were no good parts. The operators did not trust the results of the gage. In fact, they could not get repeatable readings with the gage at all. When they rotated the plug or moved in or out to explore the bore, a wide range of readings were seen. The gages just did not work as expected.

  In this case lots of things were going on in the parts that were just not being seen by either the previous digital indicators or the gages themselves. To resolve gaging problems such as this, one has to start by looking at all the components of the measuring process. We started by taking a close look at the parts to see what the diameters looked like, knowing that when the tolerances get very tight, form and surface finish can take up a large portion of the tolerance.

  A quick run on a form machine clearly showed a four point out of round condition that was, in fact, greater than the part tolerance. And because there was an even number of lobes, the two point fixed plug gage was ideal for showing the MIN and MAX values of the lobing. Therefore the gages did not have a repeatability problem, but were actually displaying what they saw: the part out of roundness. No matter where the gages were placed in the part a different number was seen, and with the high resolution the readings were just a jumble of numbers to the users.

  So why did they never see bad parts with the tri-bore gages? This all comes back to the idea that an odd number of gage contacts works best when there are an odd number of lobes in the part. Because this part had an even number of lobes, the tri-bore gage was always reading an average of the points that the contacts were measuring. Actually, no variation was seen because no matter where the gage was placed, it always read the same results: good.

  But even though the operators did not like the results of the new gage, it was the first time that someone actually understood what the part shape was really like—and revealed a process problem that had not previously been evident.

  So in both cases, steps towards improved gaging did not work out as planned. However, as they both uncovered either gage or part issues, that in itself could be termed a success.

                                


  In this case the flat anvils were not designed with a tight enough parallel specification to the base of the height gage for the higher resolution.

                                 

 


  A quick run on a form machine clearly showed a four point out of round condition that was, in fact, greater than the part tolerance. And because there was an even number of lobes, the two point fixed plug gage was ideal for showing the MIN and MAX values of the lobing.