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Slot True Position with Datum Conditions

BCB12416
So I have looked at every bidirectional TP thread on - I am programming for a customer with 2011 MR1, so width is out of the question (and has been flaky at best in 2014.1).


With only a few years experience, I was never taught constructing generic features. I think I have a grasp on creating generic features for the two slots, my trouble is creating generic features for the datums called out and applying MMC or LMC with xactmeasure when dimension true position. Program seems to be running fine without the conditions, but obviously want to get the bonus and match print.

Any help would be greatly appreciated.
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  • As far as material condition goes, without width feature, I think you're screwed, especially for A. You can construct a feature, but nothing that would orient in the same manner as a width feature. The problem is that the size of a width feature is parallel to the direction of the vector of the mid-plane, whereas a circle/slot/cylinder, the size is perpendicular to the vector. A sphere could work for purpose of size being oriented properly, but you can't level to a single sphere.
  • Basically, you do it the exact same way as for slots. Datum A might be hard to do "correct" as it should be made as a width with (M) applied. Datum B is the same but easier as the (M) is left out, meaning we can use a symmetry plane or line as datum. We can use the width as a generic circle diameter to calculate bonus for datum A, but if we assign a datum to this generic circle, we lose the datum freedom control since datum A should be a line or a plane. Hopefully someone can chime in with other construction ideas.

    Maybe it would be wiser to ditch the (M) completely on datum A and use the datum without (M) which would mean correct datum usage minus the bonus=tougher tolerance?
  • in reply to vpt.se
    I'd use it without the MMC modifier and then put in a note in my insp rpt to state that bonus tolerance is not being computed by the software but based upon "X" measurement, there may be some that can be assessed to this reading
  • Vpt.se & Dan_M's suggestion of leaving the Circle-M off the A datum is also what I recommend.

    The problem with MMC-boundary fitting on the primary datum (which, by the way, Xactmeasure can handle perfectly if the datum is a cylinder) is that it allows for 4 degrees of freedom of fitting. This creates results that simply cannot be replicated with mechanical measurements without spending $30k on a complex fixture.

    As Vpt.se pointed out, the less fitting means it's harder to pass which may be difficult for manufacturing. I have customers who routinely drop all the Circle-Ms to make it harder on themselves, and the result is their passing-inspection parts always have a safety margin and are never near the true limit of failing.
  • I think he is talking about FCF 15 and 17 there is no Datum A at MMB in those FCF's
  • Thank you for the speedy responses, guys.

    In regards to Obelix, I am referring to all FCF's. If it would be difficult (or simply easier to calculate out of Dmis and apply bonus yourself), to get MMC on datums as generics, I would think it would be equally hard to apply LMC - unless I am mistaken.

    I posted looking to see if there was an easier work around for width, or at least have something to go back to my team/customer with and was rewarded. I am still open to any and all ideas/discussion if there is anything further.
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