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Problem with CAD/RPS Alignment

Davcom
Hello, I'm fairly new to CMM programming with CAD models, I just started a month ago and I haven't had any major problems until now. I have to say that I never had a proper formation on CMM programming at all, I learned everything I know by my own.

My problem is that I have to measure a part which it's datums are given in car body coordinates (2 circles and a slot) and I've never aligned a part like this before. I tried doing a 3-2-1 iterative alignment and then a best fit as I always did, but I can't get it to work because the deviations of the real part from the CAD are so big that whenever I try to measure a hole the probe bumps into the part like 3 or 4 mm away from the hole and as much as I try to do a better best fit alignment it doesn't seem to work. so after plenty of time of trial and error I gave up and asked for help to my co-workers, but they use different CMM machines and software (Metrolog XG), so they don't know how to use PC-DMIS. They suggested me to perform a RPS alignment to try to get around this problem, but I have no clue on how to do it in PC-DMIS. Anyone who knows how can I get this to work?

Any help I could get would be highly appreciated.

Please, be aware that i'm a newbie in CMM programming, so try to be as clear as you can.
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  • Are the normal vectors of the three features close to the same direction? If they are, you should be able use the three features to make an iterative alignment. Depending on the version of PC-DMIS, you might get an error message saying that you don't have enough features to make the alignment. Ignor the message and keep going. You can also create read points at the center of each of the measured features (holes) and use those points to make a rough alignment. Just make sure that your features have the CAD theos and vectors correct.
    It might make it easier for you to visualize if you are able the set your part on the CMM so that the X Y and Z axis of your part is in line with the X Y and Z axis of the CMM but that is not necessary. The way we normally align to a part that is not on a fixture is to do a manual rough alignment so that the CMM can find the part features and then do a final alignment under dcc control. The iterative alignment tolerances are set to a large number such as 50mm for the rough alignment to help prevent alignment error messages and then use a tighter tolerance for the final alignment.
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  • Are the normal vectors of the three features close to the same direction? If they are, you should be able use the three features to make an iterative alignment. Depending on the version of PC-DMIS, you might get an error message saying that you don't have enough features to make the alignment. Ignor the message and keep going. You can also create read points at the center of each of the measured features (holes) and use those points to make a rough alignment. Just make sure that your features have the CAD theos and vectors correct.
    It might make it easier for you to visualize if you are able the set your part on the CMM so that the X Y and Z axis of your part is in line with the X Y and Z axis of the CMM but that is not necessary. The way we normally align to a part that is not on a fixture is to do a manual rough alignment so that the CMM can find the part features and then do a final alignment under dcc control. The iterative alignment tolerances are set to a large number such as 50mm for the rough alignment to help prevent alignment error messages and then use a tighter tolerance for the final alignment.
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