mmc from datum -c- that is offset

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  • mmc from datum -c- that is offset

    Hello everyone I have a part with 24 slots where first slot is datum -C- which is 11.2500 from center line.Then it's .250 from that line like on the sketch.I read on the forum about using with as datum feature but I don't think it would apply in this case.How can i use MMC from datum -C-
    Profile .020 AB is applied to datum -C-
    My plan was to level to top Plane -A-
    origin on -B-
    take a point on-C-
    rotate From B to C
    then rotate back to get .250 but after that I'm not sure.
    Have you come across that type of scenario?


    Last edited by GREGF; 08-08-2018, 10:27 AM.

  • #2
    Please post the print.

    I'm having a real hard time understanding the geometry of the part.
    SF7107(PCD), SF454(PCD), 152614(Quindos), 9159(Quindos), 7107(Quindos), B&S Manual, M&M Gear Checker

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    • #3
      a
      Last edited by GREGF; 08-08-2018, 10:27 AM.

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      • #4
        there are notches that will be done later and those are to A_ B mmc and C mmc

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        • #5
          pic removed
          Last edited by GREGF; 08-08-2018, 10:27 AM.

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          • #6
            I don't see that C is a feature of size, so I don't see how MMB could be applicable.
            "This is my word... and as such is beyond contestation."

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            • GREGF
              GREGF commented
              Editing a comment
              I agree with that I will talk to our engineer to get his opinion on that

          • #7
            I've seen many datum systems similar to this before.

            The issue which isn't clear from your sketch is just what the **** is the C/L? - Designers always dimension from C/L's but it's not really correct, a C/L is not a datum.

            Regardless if you want to arrive with your coordinate system square to the drawing (i.e. square to the C/L) then the following will work.

            Level to A, Origin on B. For C, take either a surface point or measure a very small plane then construct a Cast point (to avoid cosine error).

            Then construct an offset line from B and C with the 0.25 offset applied to C (check you've offset the right way.)

            You can then align from A, B and the Offset line, then rotate back the 11.25°.


            Edit - As Vinni said, no MMC / MMB possible.
            Automettech - Automated Metrology Technology

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            • GREGF
              GREGF commented
              Editing a comment
              Thank You I will try that. I'm trying to figure that out before I get the part

          • #8
            [ATTACH=JSON]n413938[/ATTACH]
            sorry i should have added that

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            • #9
              pic removed
              Last edited by GREGF; 08-08-2018, 10:29 AM.

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              • #10
                Communication breakdown here!

                I understood what the C/L was on your sketch. What I was getting at is what does it relate to on the actual part! (That's a rhetorical question by the way, my point is that C/L's are massively ambiguous things).
                Automettech - Automated Metrology Technology

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                • louisd
                  louisd commented
                  Editing a comment
                  It is an implied orientation relative to the drawing's orthogonal view. Engineers don't often see past the drawing and model. There is a very specific method to finding the CL: Close one eye, put your thumb between your open eye and the part and use the midpoint of your thumb to create the datum the CL. voila!

                • GREGF
                  GREGF commented
                  Editing a comment
                  good one ha ha

              • #11
                The Basic 11.25 is simply a rotational offset to the alignment, not a big deal you guys.

                I would recommend reporting the position of each slot to ABC, not ABC(M).

                The circle-M on the C, which isn't present in the sketch anyway, would allow the datum-scheme-coordinate-system to rotate around the B centerpoint by the amount that C slot's width is greater than it's smallest-allowed size, in order to best-fit the other slots' positions into place.

                Leaving the circle-M off the C does make it harder for the parts to pass, but it also makes the report easier to understand and it makes it easier for manufacturing to make adjustments to the process as the goalposts are not wiggling around. Additionally, the unused datum-shift creates a nice margin of error or safety buffer so that parts reporting as marginally passing are actually fine.

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                • #12
                  Thanks for all the answers I'll use ABC without modifier then

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                  • #13
                    And now here is the Plan B for when the powers-that-be force you to put the modifier on the C:

                    1) Measure 3 lines in the Datum C slot: both sides and the back wall.

                    2) Construct a mid-line between the sides.

                    3) Construct a Pierce Point where the mid-line pierces the back wall line. Name it SLOTCENTER. Make sure it's Theos are print/CAD correct!

                    4) In Command Mode, toggle the NO to a YES on each of those constructed features to lock their Theos.

                    5) Create a Distance dimension that measures the width of the slot: first side to second side, perp to feature. Toggle the output to NONE. Name it DISTWIDTH

                    6) Create a Generic Circle, name it DATUM_C_SLOT_WIDTH. Make sure you toggle the default "Radius" setting to "Diameter".

                    7) In Command Mode, manually edit the NOM X, Y and Z fields of this circle to be print/CAD correct, same as SLOTCENTER.

                    8) Manually edit the MEAS fields to be SLOTCENTER.X, SLOTCENTER.Y, SLOTCENTER.Z

                    9) Now the format for the Diameter is different than the Noms and Actuals, it goes NOM, ACTUAL. So you will enter the nominal width inthe first field, then DISTWIDTH.MEAS in the second field.

                    10) Final touch up: go back and add +0 to each Nominal value to lock them in place.

                    11) This Generic circle is now your Datum C feature with a nominal size and an actual size, nominal location and actual location.

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                    • GREGF
                      GREGF commented
                      Editing a comment
                      Thank You very much for that I will try to use that when I get the part

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