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Probe Calibration Parameters

Spikalus
I have searched the forum extensively on this topic and found a lot of good information but I am still not completely satisfied.

I use TP20's with a wide variety of tip configurations at multiple index angles. I have gathered that different probe configurations may require different calibration parameter settings (i.e. Number of Hits, Touch Speed, Number of Levels, etc) in order to get the best calibration. I have been told this is especially true for miniature probe tips and tips on the long end of what a module is rated for.

The one piece of advice that I am struggling with the most is in regards to the Number of Hits to use. I have heard lots of opinions that span from 5-25 hits and from multiple sources that the number of hits needs to be odd. I have also heard that the number of hits needs to be prime. I can not rationalize why this matters and I have in fact only seen evidence to the contrary. I accept that there may not be a "good" answer to this but I was hoping someone out there may have some information that would give me the confidence to go one way or the other when deciding how many hits to use.

If anyone has any advice on how to pick the other parameters I would love to hear it as well.
  • Aaand I'll just respond with, "Hey, How 'bout those Cubs!"
  • I have searched the forum extensively on this topic and found a lot of good information but I am still not completely satisfied.

    I use TP20's with a wide variety of tip configurations at multiple index angles. I have gathered that different probe configurations may require different calibration parameter settings (i.e. Number of Hits, Touch Speed, Number of Levels, etc) in order to get the best calibration. I have been told this is especially true for miniature probe tips and tips on the long end of what a module is rated for.

    The one piece of advice that I am struggling with the most is in regards to the Number of Hits to use. I have heard lots of opinions that span from 5-25 hits and from multiple sources that the number of hits needs to be odd. I have also heard that the number of hits needs to be prime. I can not rationalize why this matters and I have in fact only seen evidence to the contrary. I accept that there may not be a "good" answer to this but I was hoping someone out there may have some information that would give me the confidence to go one way or the other when deciding how many hits to use.

    If anyone has any advice on how to pick the other parameters I would love to hear it as well.

    There is no definitive answer. It really all depends on the type of material you're measuring, the CMM, the controller, the environment, your speed, it just goes on and on. Trial and error mixed with some good old professional common sense will get you where you need to go here.

    How about them Patriots? =)
  • I would suggest you collect your own empirical data and then season your calibrations to taste.
    Not really confident that 5 hits is the 'opinion' you want to adhere to.
  • The 'analytical' way to determine required number of points is to remeasure with more points, and as long as the form error increases you need even more points...

    For example, if a circle is two-lobed (oval) you may completely miss the form deviation with four equidistant measuring points if you place them 'unlucky', independent of the actual form error!

    For a three-lobed circle (very common) you may completely miss the form deviation with six measuring points. With seven, you're mathematically guaranteed to find at least 79% of the actual form error (Assuming the surface is perfect, only error is form).

    In a way, it's a pity that we have the autofeatures in PC-DMIS, as they always generate equidistant mp:s, while it is statistically better to divide the measured surface into equidistant areas and place an mp randomly inside each area.


  • With a TP20 I've done experimenting and found it's best to do a high number of hits that ARE a multiple of 3. I used to believe it was to be avoided because of the tri-lobed nature of the TP20, but when all of my tests on an XX gage indicated the opposite, it made sense. By measuring in multiples of three, you are equally displacing the inherent form error of the TP20 and causing it to average to no (OK, virtually no) error.

    As for number of hits, ask yourself what linear spacing you'd be comfortable with on a straight line. 24 hits on a 10mm diameter is a hit every 1.3mm, on a 200mm diam it's every 26mm.
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