FCF Options with MMC on a Datum

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • FCF Options with MMC on a Datum

    ok, so I'm really new at this. Our CMM programmer left, and I got a crash course from him in PC-DMIS before he left. I have a little down time, so I'm going through some programs to train myself and get deeper into PC-DMIS.

    Anyway, I can't seem to figure out how he got the FCF to have a MMC on the secondary datum. Can anyone please help me out? I attached the FCF he made, is structure within XactMeasure, as well as the drawing call out.

    Any guidance would be appreciated.
    Attached Files

  • #2
    Strictly speaking that callout is invalid because it references the cone as a datum at MMB. Cones are not considered features of size so applying the MMB would be illegal. It looks like your predecessor got around this by creating an intersection circle (CIR20) between the cone and top face to represent the Ø0.279/0.289 and referenced that as datum R at MMB instead. That approach works in this case because datum R is the secondary datum and is only being used as the origin. It would also work as a tertiary datum but for a primary it would be better to construct a generic cylinder that uses the circle's D value and the cone's X,Y,Z,I,J,K values. That would ensure that it used the cone's axis to control the appropriate degrees of freedom (two rotation and two translation), since a circle is only a 2D feature it's vector would be inherited from the top face and would therefore control things differently.
    Last edited by neil.challinor; 04-14-2021, 08:24 AM.
    Neil Challinor
    PC-DMIS Product Owner

    T: +44 870 446 2667 (Hexagon UK office)
    E: [email protected]

    Comment


    • #3
      I think he was just trying to copy what our customer did on a legacy drawing. The customer is notorious for having little datums set up, even if they do use position callouts. The times they do, the datums are set up correctly anyway. The majority of the drawing is in one of the screen shots.

      I was just going to change it over to a standard position FCF, as seen in one of the screen shots. Would this be the correct thing to do?

      Normally I would give more information, but the drawing is ITAR controlled, so I can only show so much of it.
      Attached Files

      Comment


      • #4
        Hey don't feel bad about being new, as you are displaying some of this print, It looks like the Engineer who made the print is new also. He or she can't even display a FCF correctly, nor do they even understand the purpose of a countersink, if anything the countersink has one, maybe 2 purposes, one to guide a "screw" into the hole, 2nd would be a tapered screw that they want to lay flat or just below the surface evenly, for this to happen nice and evenly your countersink would be called back to the hole to make sure it aligns as close to perfect as possible, to call the hole to the countersink, Your telling me the countersinks position is more important than the hole position. Clearly this Imageneer don't know what they are doing.
        (In Memory of my Loving wife, "Ronda" who I lost March 7, 2016. I love you baby.)
        They say "Nobody's Perfect." I must be Nobody.

        Comment


        • Mike Ruff
          Mike Ruff commented
          Editing a comment
          I would say that the countersink position is more important than the hole position. When fastened correctly, the screw should make full contact with the countersink and no contact with the hole

      • #5
        Mike Ruff commented
        Today, 09:07 AM
        I would say that the countersink position is more important than the hole position. When fastened correctly, the screw should make full contact with the countersink and no contact with the hole

        So it doesn't matter that the screw doesn't align with the mating part, as long as the screw head lies securely? I'm sorry but I would disagree, You want the hole to work with the mating part, you want the countersink to work with the hole. The Datums are your mating part, The whole purpose we make programs is to create the mating part. The mating part will never touch that countersink, but I will agree that it will help secure the 2 together. You can eliminate the countersink all together, but you can't eliminate the hole.
        Last edited by KIRBSTER269; 04-14-2021, 10:30 AM.
        (In Memory of my Loving wife, "Ronda" who I lost March 7, 2016. I love you baby.)
        They say "Nobody's Perfect." I must be Nobody.

        Comment


        • #6
          As this is one of my triggers and you are new and seeking information:

          I want to emphasize what Neil eluded to, which is that the M in a circle symbol has two very different meanings depending on whether it is applied to the feature tolerance itself or to a datum feature in the DRF of an FCF.

          In the first case, when applied to a feature tolerance it is MMC, Maximum Material Condition. This allows the amount of allowed tolerance to increase as the actual feature deviates from it's MMC state.

          The second case, when applied to a datum feature it is MMB, Maximum Material Boundary. Here the symbol allows datum shift, that is as the datum feature deviates from it's MMB state, the DRF is allowed to shift. The amount of shift gets really complicated quickly as it is a combination of difference in size as well as the geometry of the DRF and the order of precedence within the DRF. This is some of the most advanced and hardest to understand GD&T.

          ASME Y14.5-1994 called both MMC but defined what it means when applied to each case in the same manner described above. Because so many folks were confused and treating MMB as if it were MMC, with ASME Y14.5-2009 they started calling it MMB when applied to a datum feature to help distinguish the two.

          IMNSHFO MMB is great info if you desire to build a hard functional gage to check the FCF. CMM software has long struggled to handle MMB properly in all cases. Legacy dimensions in Pc-Dmis do not. Xact measure does better but does not handle all cases correctly. The new GD&T Dimensions tool strives to do it all correctly, but it is so new . . . caveat emptor.

          It is also important to point out that the Simultaneous Requirement is one of the biggest challenges in checking parts that have MMB in multiple FCFs.

          The safest bet for CMM application is if the FCF is created and evaluated as if there were no MMB. If the feature passes at RMB, Regardless of Material Boundary, the default, then it is good and will not fail if MMB is considered.

          HTH & ymmv (Hope This Helps & your mileage may vary)



          sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

          Comment


          • #7
            Originally posted by KIRBSTER269 View Post
            Mike Ruff commented
            Today, 09:07 AM
            I would say that the countersink position is more important than the hole position. When fastened correctly, the screw should make full contact with the countersink and no contact with the hole

            So it doesn't matter that the screw doesn't align with the mating part, as long as the screw head lies securely? I'm sorry but I would disagree, You want the hole to work with the mating part, you want the countersink to work with the hole. The Datums are your mating part, The whole purpose we make programs is to create the mating part. The mating part will never touch that countersink, but I will agree that it will help secure the 2 together. You can eliminate the countersink all together, but you can't eliminate the hole.
            Say you have a lid with countersinks on thru holes, and a true position of Ø.010 to ABC attached to said holes, with primary datum on the face these holes are on, secondary and tertiary are the CLs of the outer profile of the lid in X and Y. Now imagine that the thru holes are exactly where they should be but the countersinks are shifted +.005 in Y and perfect in X. When you go to mount this lid on its housing, the lid will be shifted .005 off-center of the housing because the countersinks are locating the lid (making contact with the screws on the countersinks), not the thru holes (as they are not making contact with the screws).

            Comment


            • VinniUSMC
              VinniUSMC commented
              Editing a comment
              Yes, if that is how it mates. Seeing the OP's print, I would guess it is similar. If the countersinks are all off in different directions, then only the first 2 screws will have any effect.

              Something I saw a lot in aerospace was covers that get attached, and where the screw goes is a floating nut (on the backside of the mating part), so the panel fits on the form, and the screws/countersinks don't locate anything.

              So, it depends entirely on the application, but I think in this case, your assumption is probably correct.

          • #8
            Originally posted by Mike Ruff View Post

            Say you have a lid with countersinks on thru holes, and a true position of Ø.010 to ABC attached to said holes, with primary datum on the face these holes are on, secondary and tertiary are the CLs of the outer profile of the lid in X and Y. Now imagine that the thru holes are exactly where they should be but the countersinks are shifted +.005 in Y and perfect in X. When you go to mount this lid on its housing, the lid will be shifted .005 off-center of the housing because the countersinks are locating the lid (making contact with the screws on the countersinks), not the thru holes (as they are not making contact with the screws).
            Mike Ruff I know what your talking about, we call this the ol shotty door hinge effect. I stand by statement, "poor engineering." If you make things at IKEA then yeah guess it's ok. You guys make door hinges? This is what happens when you choose this method. "Well it doesn't look good, but at least it holds it down nice and tight" <<<<This is what a hillbilly says, and I'm a hillbilly.

            257xNxthrow_the_hinges.png.pagespeed.ic.4IMZkhidkp.png

            Last edited by KIRBSTER269; 04-15-2021, 09:07 AM.
            (In Memory of my Loving wife, "Ronda" who I lost March 7, 2016. I love you baby.)
            They say "Nobody's Perfect." I must be Nobody.

            Comment


            • KIRBSTER269
              KIRBSTER269 commented
              Editing a comment
              VinniUSMC in fact I'm working on a very deep countersink feature right now as we speak. 4 level 90° ±0.5° my result 98.3015°

            • VinniUSMC
              VinniUSMC commented
              Editing a comment
              KIRBSTER269 My front door is not that color. You must have found the guy I went to school with whose name was almost the same as mine.

              That doesn't change the simple physics of how a countersink works. Sometimes it works the way you envision, sometimes not. So, your one-size-fits-all approach isn't always right. Sounds like someone used the wrong countersink bit. Good thing you're checking the countersink.

            • KIRBSTER269
              KIRBSTER269 commented
              Editing a comment
              VinniUSMC yeah they were using 100° had them pull out the tool and even though the writing has not visible, put it on the comparator, sure enough

          Related Topics

          Collapse

          Working...
          X