"Are your indicators and probes telling you the truth?"

"Are your indicators and probes telling you the truth?"

Often times a customer will simply mount his or her indicator/probe into comparator stand, set zero and start taking measurements.

However, an improperly mounted gage can cause many issues like: Extra or uneven wear on bearing surfaces and gears - particularly if many measuring cycles are taking place daily.  Accuracy and repeatability problems are also a possibility when this occurs, which no Metrologist likes to hear.

Over clamping the indicator or probe in a stand This can be a sneaky one because you may not notice a huge performance reduction, but excessive clamping force compresses the mounting shank to the spindle of the gage.  Ultimately this will cause binding and then...repeatability problems.

When clamping an indicator in a stand, just use common sense and ensure that the spindle travel is smooth and the gage repeats!  There are also applications that require the indicator or probe to be mounted in a fixture.  Naturally, the same attention to clamping force or “set screw” force should be observed.  In the case of a dial or electronic indicator, mounting with a lug back will eliminate this problem but is not the preferred method when accuracy is critical.

 

Another common error is “cosine error” caused by the indicator or probe sitting crooked in the stand.  Instead of the probe moving vertically it actually moves at an angle forming the hypotenuse of a right triangle. 

Using trigonometry it very easy to calculate this error.



The error will probably not have a major effect with a .001” reading dial indicator in a stand because hopefully your stand’s error is less than 3 degrees and the indicator itself does not have high enough resolution for you to see the error.  However, when you are using a high accuracy probe with resolution down to .000010” even a small geometry problem with the stand could affect your measurements.  To combat this problem, many digital displays have calibration sequences that will eliminate the error mathematically.  This is done with a minimum of 2 (or many more) data points taken while measuring the known sizes of gage blocks or similar. 

Another place where you could run into major cosine error is when you are using a setup with a flexible arm or similar, because you are setting the position by hand and it is very tough to get good alignment.  These types of setups are mainly used in automotive runout or other relatively low accuracy applications where measuring performance is not as critical.


The last problem we run into (at least the last one I can think of right now) with customer’s probes and indicators is excessive lateral force on the spindle.  This is caused by spindle/contact point to be touching a moving surface.  This too can cause uneven wear and repeatability problems.  Some probes actually have a lateral force specification that should not be ignored!  To alleviate lateral force issues, a roller ball or low friction contact point can be used.

We will be adding more tips to this blog as they come along.  If you have any ideas please let us know.

Gaging.com - Measure Smarter

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