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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.
  • 2 circles and slot does not sound like much of a datum structure. I am guessing that there is more to it than that.

    Iterative works great, unless your part is egregiously out of tolerance.

    What feature(s) make the primary? What feature(s) make the secondary? What feature(s) is the tertiary?
  • Try Interitive alignment: 2 Auto circles with 3 surface points and 1 auto circle 3 surface points on [1 Radius of slot].
  • 2 circles and slot does not sound like much of a datum structure. I am guessing that there is more to it than that.

    Iterative works great, unless your part is egregiously out of tolerance.

    What feature(s) make the primary? What feature(s) make the secondary? What feature(s) is the tertiary?

    As I said, the blueprint shows 2 circles and a slot with their coordinates for the alignment. Also, I used many time 3 circles as datum for the alignment, 3 circles to level the plane, 2 to rotate an axis and 1 as the origin.

    In the blueprint there is:

    1st circle A1, B1 and it's coordinates XYZ.
    2nd circle A2, B2, C1 and it's coordinates XYZ.
    Slot A3 and it's coordinates XYZ.

    And yes, part is egregiously out of tolerance, I never had to measure such a deformed part like this one.
  • As I said, the blueprint shows 2 circles and a slot with their coordinates for the alignment. Also, I used many time 3 circles as datum for the alignment, 3 circles to level the plane, 2 to rotate an axis and 1 as the origin.

    In the blueprint there is:

    1st circle A1, B1 and it's coordinates XYZ.
    2nd circle A2, B2, C1 and it's coordinates XYZ.
    Slot A3 and it's coordinates XYZ.

    And yes, part is egregiously out of tolerance, I never had to measure such a deformed part like this one.

    Ok, in your Auto-features, on the "Contact Find Hole properties" tab, check "Read Position". This will make the CMM stop and ask you to move the probe to the center of the hole. Later, when the parts are better, you can deselect it.
  • 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.
  • If the part is significantly off, it really doesn't matter what type of alignment you use. In either way the probe will hit the part instead of measuring the hole if it's really off. The only way to measure the features that are off by that much, is to manually using a joystic program those features( aka measured features) and report the values. Once you know how much the features are off, the. You can get them fixed. Once they are fixed, only then you can removed the measured festures and create auto features feom the cad.
  • As Wolfman say's find out if the actual part is in error and which feature is incorrect.
  • 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.
  • 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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