Métrologie
Métrologie
The Case of the Sticking Test Indicator
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The Case of the Sticking Test Indicator
George Schuetz, Mahr Federal Inc.


  There are few worse feelings for a manufacturer than having a product returned because the customer determines it is defective. Not to say that things are always perfect, but the vast majority of products manufactured do meet their design requirements. When a large number of the same product is returned by one user, it tends to raise eyebrows.

  The Case of the Sticking Test Indicator started with a few Test Indicators the customer returned as defective: they said they were "sticking" during calibration. Their report—which was very detailed—noted that as the calibrating device was moved through the measuring range, it was observed that the indicator hand was not moving smoothly but seemed to jump. It appeared that something was causing it to stick until more calibrator travel forced it along.

  Once the indicators were returned, they were given a complete check in the calibration room. They appeared to work perfectly. Travel was smooth and calibration was spot on. Deemed 'per spec,' they were returned to the customer with full certification documenting their performance.

  "Not so fast" said the customer. "They still stick, and we can prove it," and along came a video that showed quite clearly that the indicators had a symptom of sticking. It showed the gage calibrator and a close-up of the indicator face and the hand jumping as the calibrator was slowly moved through the indicator's range. There was definitely a problem somewhere! We had the customer send the indicators back again—and again they passed the calibration tests here at our lab.

  This is one of those cases where the evidence is right in front of you, but you can't see it. The video really provided the evidence that we needed. One of our Senior Quality Engineers looked at the video and said "That set-up seems a little shaky. Let's make a set-up exactly like the customer's and see what happens." Figure 1 shows the set-up which was duplicated in the calibration room. It consisted of a test stand clamped to a calibration gage. The post and arms, similar to the arrangement as shown, were pretty much extended to their full range.

  When the calibration runs were performed on this set-up, guess what? The indicators appeared to be sticking; but were they sticking, or was something else going on?

  Here's what was really happening:

• The test indicators have a gaging force.
• The post and arms on the test stand have some "flex."
• As the calibrator was brought into contact with the test indicator, the force that was needed to flex the post and arm was less than the gaging force needed to move the test indicator.
• So as the calibrator moved against the contact point, the post and arm would flex, making it appear that the indicator hand was not moving while the calibrator was.
• Then, as the flexing post and arm overcame the gaging force of the indicator, the indicator hand would jump to catch up.
• This process would be repeated continuously as the calibrator was moved slowly through the indicator's range: a continuous stopping and catching up of the indicator hand.

  The problem was found: not a bad indicator but a bad set-up. To further prove the point, a block was inserted between the arm holding the indicator and the calibrator to eliminate any arm movement, and the "sticking" disappeared.

  A new video was made of this and sent to the customer demonstrating the flaw in the set-up and offering suggestions for better fixturing. Figure 2 shows this set-up with a test indicator holder that is very rigid and as short as possible, providing no chance of flexing.

  Case solved. Sometimes a gage or indicating device speaks to us, telling us there is something wrong. Finding out what is wrong begins with looking at the complete gaging process.

Figure 1. 
 

  This set-up of the customer's calibration stand was duplicated in the calibration room. It consisted of a test stand clamped to a calibration gage. The post and arms, similar to the arrangement as shown, were pretty much extended to their full range.

Figure 2.
 

  This set-up with a test indicator holder is very rigid and as short as possible, providing no chance of flexing.