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Master Probe

Scott Brown
Good Afternoon

I have read a number of posts regarding probe calibration and using a Master probe to relate multiple probes. I have a few questions that I did not find the answers in the posts.
Question #1 - If you are using the Master probe for measurement does it affect the relationship of the other probes if you re-calibrate A0/B0 of the Master Probe after answering "N0" to the calibration sphere move. For example - Start of shift the Master probe is calibrated at A0 / B0 and you answer "Yes" the tooling sphere has moved. Now you open a program that uses the master probe for measurement. You want to perform a calibration on the angles that are used in the program. Perform a Marked Used which includes A0/B0. You answer "No" the tool has not moved and perform calibration. Is the relationship of the other probes and angles lost???

Question #2 - Is it necessary to calibrate Master Probe every time the machine is homed??

PC-DMIS 2014.1

Thanks for your help
Scott


  • Question #2 - Is it necessary to calibrate Master Probe every time the machine is homed??


    I think machines are homed after the controller is restarted no? I don't have a big set up so I normally calibrate after the machine is restarted and homed. That is what I instruct the operators to do.

    I'm still thinking about your first question. I was always told try not to use the master probe for measurements. I would think no, the relationship between the other probes and angles is not lost as long as you answer "no the tool has not moved".
  • #1
    What are you expecting to gain by re-calibrating the master probe as a 'normal' probe? The only effect I can imagine is that you add one level of uncertainty to the relation between master probe and the other probes, making everything worse. First you create a relation between master, datum and other tips, then you change the master - why should that make anything 'better'?

    Probe calibration is not some magic that makes the probe (or machine) 'better' - it is the way to have a consistently known relation between everything that is used during the measurement, and that means that any change somewhere in the middle of the calibration chain breaks the chain at that point. Depending on your tolerance needs, the break may be insignificant, but it is still a break.

    It might be a good idea not to use the physical master probe too much for daily work, to minimize the risk of damaging it. And always reset it to THEO before locating the datum sphere!

    #2
    As for the question about homing, it might differ between different CMM:s. If the CMM claims 'absolute scales' it shouldn't be necessary; if it's a homing CMM it depends on the quality of the 'home detectors'. It's easy to do a little experimentation and find out what goes for your particular CMM(s), or you take the easy way out and treat all CMM:s the same and say "yes, it is necessary". Depending on your tolerance needs, the deviation may be insignificant, but there is probably still a deviation.
  • #1 There is no logical reason to qualify A0B0 of your master again. I do not know if qualifying again would effect the relationship...but since there is no reason to do this then why across that bridge?

    #2 I do not trust PC DMIS. At all. Andersl is correct regarding CMM's with absolute scales...but still...bearing in mind the instability of this software & its habit of doing what it wants from version to version.... You can do what you'd like but I would personally qualify every time I do a controller re-start. Rather be safe then sorry.
  • My master probe is exactly just that. It never gets used for any measurement.
  • There are a lot of posts about "Master probe"...
    Some of us have a real "master probe", which is used only to do a "zero" (like AndersI said) with resetting to theo the offsets and repositioning the sphere (answer "yes")
    Another way to consider the "Master probe" is only to say "yes, the sphere had moved", then all the offsets are calculating from this tip. In this case, recalibrating this tip and answering "no" will only give an offset to this tip from its previous calibration.
    Re-calibrate after a homing takes the same way. If the zero on the scale is not repeatable, you can say that the sphere physically moved compared to the scale. If you re-calibrate only the tip answering "No", you just add to the offsets the "zero defects", so the measured part should be measured in a reference close to the reference before homing (because the zero offset has the same effect on the part and on the shere Slight smile ). So it has no effect on others calibrated tips...

    About both points, it's only my opinion... But I constructed it with a lot of tests on ring gages, gauge blocks and different parts...
  • You reset the master probe to THEO and calibrate it with "yes, the sphere has moved"; the THEO offsets of the master tip are used to calculate the position of the sphere. You then calibrate all other probes. An unbroken chain of relations.

    Now, if you calibrate the master tip answering "no", some deviation (machine uncertainty, different temperature, vibrations from a truck, etc.) will give the master probe MEAS offsets that differ from THEO (more or less), which will affect the relation between master and everything else until everything else is calibrated. It is a broken calibration chain, although with (mostly) very small deviations. But larger errors may lurk in the background... Read on!

    Next time you calibrate the master with "yes, the sphere has moved" these deviations will be in use when calculating the position of the sphere, consequently included as an error in the calculated position of the sphere. This error will be included in the offsets of every probe that is now calibrated.

    If you use different probes when you answer "Yes, the sphere has moved", each time you add a small offset to the sphere position - statistically these will cancel out, and the sphere will still be almost correctly located, but it is (relatively) easy to devise a sequence of calibrations that will consistently move the sphere more and more away from its actual location, adding more and more offset to the calibrated probes, until you finally can't calibrate a probe which is reset to THEO because it misses the sphere (or collides with it).

    - OT -

    For the historically interested, the physical procedure on a Johansson CMM with our older softwares was different. The sphere locating process always began with measuring a calibrated hole in the table to 'locate/calibrate the probe' (this is echoed in PC-DMIS by resetting the probe to THEO), and then locating the sphere.
  • You reset the master probe to THEO and calibrate it with "yes, the sphere has moved"; the THEO offsets of the master tip are used to calculate the position of the sphere. You then calibrate all other probes. An unbroken chain of relations.

    Now, if you calibrate the master tip answering "no", some deviation (machine uncertainty, different temperature, vibrations from a truck, etc.) will give the master probe MEAS offsets that differ from THEO (more or less), which will affect the relation between master and everything else until everything else is calibrated. It is a broken calibration chain, although with (mostly) very small deviations. But larger errors may lurk in the background... Read on!

    Next time you calibrate the master with "yes, the sphere has moved" these deviations will be in use when calculating the position of the sphere, consequently included as an error in the calculated position of the sphere. This error will be included in the offsets of every probe that is now calibrated.

    If you use different probes when you answer "Yes, the sphere has moved", each time you add a small offset to the sphere position - statistically these will cancel out, and the sphere will still be almost correctly located, but it is (relatively) easy to devise a sequence of calibrations that will consistently move the sphere more and more away from its actual location, adding more and more offset to the calibrated probes, until you finally can't calibrate a probe which is reset to THEO because it misses the sphere (or collides with it).

    - OT -

    For the historically interested, the physical procedure on a Johansson CMM with our older softwares was different. The sphere locating process always began with measuring a calibrated hole in the table to 'locate/calibrate the probe' (this is echoed in PC-DMIS by resetting the probe to THEO), and then locating the sphere.

    What he said.....

    The MASTER PROBE A0B0 should show "perfect" in all ways except diameter, if it doesn't, it is time to reset tips and re-cal the entire rack.
  • I keep seeing Master Probe, never heard of that name, I was told years ago that you had to have a common probe each time the sphere position changed, I use my #1 Probe T1A0B0 to align to each sphere position, If you go into my probe sets for each tool tool #1 has 4 sets one each for the sphere position and then some of my other sets (star probes) has multiple sets due to which sphere I use. I calibrate 12 probes once weekly and use 2 star probes with at least 40 angle and so far never had a problem. (knocking on wood)
  • Good Morning

    Thank-you for your responses, I now have a better understanding of the Master Probe concept. With regards to the resetting the tips, this is done on A0/BO of the master probe only??Do you reset the tips for the Master probe for machine homes??

    Thanks
    Scott
  • Good Morning

    Thank-you for your responses, I now have a better understanding of the Master Probe concept. With regards to the resetting the tips, this is done on A0/BO of the master probe only??Do you reset the tips for the Master probe for machine homes??

    Thanks
    Scott

    In an "ideal Pcdmis world", the master probe is nothing more than an A0B0 file. Resetting it takes care of the "total calibration" routine. For the other probes will depend on who you talk to. Some say they should also be reset, BUT, if you have less than perfect hardware or a less than perfect alignment of the head, this can cause the other probes to fail to calibrate. If your head is (and always was) 3 degrees out of square, resetting a LONG probe can make it not run. They have added (over the years) extra things to the calibration routine, like DCC+DCC, but that doubles the calibration time (or close enough). Pcdmis used the CURRENT values of the calibration as the 'target' values for the next, helping to ensure that a non-perfect probe will run on future calibrations.
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