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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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  • Just a word of warning that I don't think anyone has picked up on in the previous posts. It is not recommended to use a slot as part of an iterative alignment (search the help for iterative alignment rules) due to the maths behind how a slot is calculated - it basically allows the center point to slide up and down the long axis of the slot and therefor you may have problems getting the alignment to resolve. The easiest way around this is to create a cast point from the slot and then reference the cast point instead of the slot in your iterative alignment. Having said that, you will still have the problem that if your part is a long way from nominal the probe could still crash as WolfMan said. If you are using auto-features try exploring the find hole functionality. Find hole is basically an automated search routine within most of the auto-features (its the last tab on the bottom row of icons). There is also a tick box called READ POSITION. If this is selected then PC-Dmis will prompt you to manually position the probe at the center of the feature before it attempts to measure it.

    Thanks for the advice, i'll try that!
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  • Just a word of warning that I don't think anyone has picked up on in the previous posts. It is not recommended to use a slot as part of an iterative alignment (search the help for iterative alignment rules) due to the maths behind how a slot is calculated - it basically allows the center point to slide up and down the long axis of the slot and therefor you may have problems getting the alignment to resolve. The easiest way around this is to create a cast point from the slot and then reference the cast point instead of the slot in your iterative alignment. Having said that, you will still have the problem that if your part is a long way from nominal the probe could still crash as WolfMan said. If you are using auto-features try exploring the find hole functionality. Find hole is basically an automated search routine within most of the auto-features (its the last tab on the bottom row of icons). There is also a tick box called READ POSITION. If this is selected then PC-Dmis will prompt you to manually position the probe at the center of the feature before it attempts to measure it.

    Thanks for the advice, i'll try that!
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