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Questions about DCC accuracy after initial manual alignment setup.

steelweaver120
Hello everyone!

So, I've been lurking for some time on these forums, and finally feel I am experienced enough to post. I'm relatively new to programming and using our CMM in day-to-day use, but I've been familiar with them since about 2012. I work in aerospace, and work is getting to the point where I need to have more output from our CMM (I work in a small shop and am the only QC person here). I've been told by QC guys that I know from a few local businesses that it is NOT acceptable to use DCC to inspect parts after a Manual alignment is ran. Basically, I have a small fixture where the X and Y of the part does not move much if at all. I run the manual alignment on the first part for the day/ if I am just setting this part number up, let the program run the DCC and then the rest of the program. After I get the results, I record/save the .PDF file for our records, and then I clear execution list, slap another part on the table and bolt it down, and run the program from where the DCC starts. Essentially, the QC guys I know say that the repeatability on this is atrocious, and not good calibration practices. I've been doing some snooping around on this forum and it seems that quite a few experienced users here think the opposite: that as long as the fixture is good and your probes are calibrated after every change that things should be smooth sailing. I keep my touch and move speeds the same as cal too. I'm super new to all of this, and I'm not *exactly* sure about all the values either, and even though I've looked through this forum, don't really know the difference between LF/SF/MF/EF type probes or the differences between touch speed and all that. I try to read up on as much of this stuff as I can, but changing some of those small parameters doesn't seem to change much...but then again I'm a rookie here, so I probably wouldn't know the difference. If anyone could point me towards a guide where it might help me understand the small nuances it would be greatly appreciated. This is especially important because we have many parts that have Total Positioning tolerances of as tight as .010, so repeatability is my biggest concern. (well, that and getting off as many parts as humanely possible from our CMM)

Also, I looked around for an answer for this next question...but how do I change parameters for to automatically calibrate a single tip in the program? I finally figured out how to change probe tips in a program, the user has to manually change it (we don't have an auto changer), and then the program can calibrate the newly inserted tip. It took me FOREVER to figure out that I had to draw the sphere because info was kinda scarce/confusing. When I insert the Calibrate Single tip command, it works great..but how do I change the settings for the auto-cal? Changing any values in the very first probe loadout in the program does nothing to change the values for the auto-cal. The auto-cal always takes 13 hits from my sphere, and then stops without going up which I find odd. I just add in +6 inches for an incremental move after that just to be on the safe side.

Thank you for any information that y'all can provide. This place is awesome, thanks for having me!

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  • If I understand the first part correctly, I think I agree with your friends. A manual alignment simply tells the CMM where the part is. If your fixture is super repeatable, then you could align your fixture, save that as an external alignment and recall it prior to DCC measurement. If the fixture has wiggle room then I feel you at least need to copy your manual alignment, paste in in DCC, and correct any non-perfect theo's and let DCC mode align to the part every time it's changed. Dimensioning a part measured in DCC mode relative to a manual alignment is not good practice IMO.
    As for the second question, I'm not sure I understand and not sure I could answer if I did. I don't use Auto-cals
  • in reply to Hi-Tech Redneck
    Thank you! Okay...so, my "fixture" is really a bolt with a smooth rim on the bottom on the left for X, and 2 bolts for Y. (Our CMM has a fixturing plate with holes drilled in a square grid 1" apart from each other in every direction). I gently slide the part against the X bolt and 2 Y bolts, and then clamp it down in a spot where the probe will not hit it. The part does not lift or slide, even with me applying quite a bit of force. I really don't think the part is moving at all, and the places where the parts touch are fairly smooth/machined. When I did tests using the same part and CAD model, I would run Manual AND DCC on the first part. Unclamp it, then put it back in, reclamp, and then run it on DCC only. The values would be slightly different, but the part would still measure great. I also tried moving the same part and doing a full manual and DCC alignment on it. The numbers also slightly changed even using the same part and an initial manual alignment. I'm just curious as to what is going on under the hood I suppose...

    So, the second question is just related to the "insert calibration commands". You have the option of doing an auto-cal, which calibrates all the angles of the current probe tip right then and there. Or, you can just calibrate a single probe tip (the current probe and angle). The single tip is what I am doing. If you don't auto-cal, what method of calibration do you use mid-program to recalibrate the tip if you change one? Any tips and tricks are much appreciated!!
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  • in reply to Hi-Tech Redneck
    Thank you! Okay...so, my "fixture" is really a bolt with a smooth rim on the bottom on the left for X, and 2 bolts for Y. (Our CMM has a fixturing plate with holes drilled in a square grid 1" apart from each other in every direction). I gently slide the part against the X bolt and 2 Y bolts, and then clamp it down in a spot where the probe will not hit it. The part does not lift or slide, even with me applying quite a bit of force. I really don't think the part is moving at all, and the places where the parts touch are fairly smooth/machined. When I did tests using the same part and CAD model, I would run Manual AND DCC on the first part. Unclamp it, then put it back in, reclamp, and then run it on DCC only. The values would be slightly different, but the part would still measure great. I also tried moving the same part and doing a full manual and DCC alignment on it. The numbers also slightly changed even using the same part and an initial manual alignment. I'm just curious as to what is going on under the hood I suppose...

    So, the second question is just related to the "insert calibration commands". You have the option of doing an auto-cal, which calibrates all the angles of the current probe tip right then and there. Or, you can just calibrate a single probe tip (the current probe and angle). The single tip is what I am doing. If you don't auto-cal, what method of calibration do you use mid-program to recalibrate the tip if you change one? Any tips and tricks are much appreciated!!
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