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Thread: True Position callout

  1. #1
    Associate Engineer
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    True Position callout

    I am wondering about a perticular True Position callout. The callout is TP .025 I understand this type of callout if there is bores below. for the features to be "inline" with eachother. in this perticular aplication, it is a set of 5 OD's on a polor radius equaly spaced.


  2. #2
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by joshmxz View Post
    I am wondering about a perticular True Position callout. The callout is TP .025 I understand this type of callout if there is bores below. for the features to be "inline" with eachother. in this perticular aplication, it is a set of 5 OD's on a polor radius equaly spaced.

    Ok, you have three separate positional requirements for a pattern of five features with size (FOS).
    MMC of the ODís (outside diameters) should be 51.1

    The top feature control frame (FCF) specifies that the pattern of features have a true position of diametric tolerance boundary of 2.5 (mm) at MMC relative to datumís A, B at MMB and Z.

    Where:
    MMC is Maximum Material Condition
    MMB is Maximum Material Boundary (size) of the datum feature.

    This is a separate requirement and measurement.

    --------------------------------------------------------------------------

    The middle feature control frame (FCF) specifies that the five FOS's have a true position requirement of diametric tolerance boundary of 0.13 (mm) at RFS relative to datumís A, B at MMB.

    Where:
    RFS is Regardless of Feature size

    This is a separate requirement and measurement.

    --------------------------------------------------------------

    The bottom FCF specifies that the five FOS have a true position requirement of diametric tolerance boundary of 0.1 (mm) at RFS relative to EACH OTHER - no datum reference.

    This is a separate requirement or measurement.

    The affectivity is that you have five tolerance boundaries, diameter 0.1 located relative to each other at the specified basic (or Theoretically Exact) dimensions and the as-manufactured derived axis of the five OD's must fall within these tolerance boundaries simultaneously.

  3. #3
    Technical Fellow
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    OMG!! Even after reading Kelly's explanation I am completely bamboozled. A good thing I am retired and don't need to try and understand all this new fangled stuff!!

  4. #4
    Lead Engineer RWOLFEJR's Avatar
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    I'm with you on that Dave. Although the whole GD&T thing makes good global sense over the written word... When I get prints loaded with all that... uh, stuff ... It takes me a while to figure out what they're after. Usually have to pick up my book...

    Fortunately we don't run into much of it for most of what we manufacture where I'm at. Pretty basic stuff for the most part and we always convert customer prints over to our own style of prints for our floor. This way each operation always knows where to look on the prints for the information they need.

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