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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?

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  • 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.
  • 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?
Reply
  • 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?
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