True position of a datum slot

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  • True position of a datum slot

    Good Morning! I have this part (shown below) that calls out a slot as the tertiary datum and I am having trouble trying to get the true position to show the MMC to .0002 for the callout for -D-. I am measuring it by taking 1 hit on each side of the slot and making a midpoint between the two. I am also not able to add the MMC modifier to either of the .100 TP or Perpendicular callout or the .125 TP callout. Is there a certain way to do this?


  • #2
    Is your slot stepped? I dont understand how there's [TP][0.0002(M)][A][B(M)] to the un-dimensioned extension line AND the composite [TP][0.000(M)][A][B(M)][C(M)]
    also neither of those true position have a diameter callout in them, so the tolerance zone of position is linear/square, not round.

    Re: how to measure the left callout [TP][0.0002(M)][A][B(M)]
    you need to measure the slot walls as planes, then construct a midplane of the two walls of the slot. as crazy as this sounds, if that slot is not perpendicular to A, or rotated so it is dead-nuts concentric with B axis by 0.0002" (at MMC) relative to [A] and [B (at MMB)], it would fail.

    This is effectively redundant to the right top TP FCF, with exception that somehow the right FCF is controlling it tighter. That's why i suspect there are two features which are in that slot... there's no logic behind the redundant callouts otherwise.


    • VVPTD
      VVPTD commented
      Editing a comment
      Hello and thank you for the reply! The slot does not have any other features in it, it is a through all slot. The TP of the extension line is calling out -D- as the plane on the midpoint of the slot that may be .0002 @ MMC/ A / B @ MMC in respect to the basic dimension. It is confusing and only acts as a "bonus tolerance" to make sure that the slot is within tolerance. With that being said, there are some other features on the part that call out TP in respect to D @ MMC. Is that possible at all, since -D- is not a measured feature?

    • louisd
      louisd commented
      Editing a comment
      push back on your customer or engineer asking them which FCF is intended to control your feature of size (slot width).

  • #3
    My best guess what somebody was thinking drawing this: they picked center of the slot only as D. Evaluate position of the center of the slot only to left callout [TP][0.0002(M)][A][B(M)]. For the right callout you need to evaluate entire center line of the slot up to intersection points with the arcs so basically what louisd said. It just other TP in respect to D @ MMC don't make sense since D have no size. You will have to use measured width as D.


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