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Thread: True Position Calculation Question

  1. #1
    Associate Engineer
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    True Position Calculation Question

    Hello folks

    Firstly, thanks to everyone who takes the time out on this forums to help out. Really appreciate.

    Ok bit of a newbie and requesting some help on a true position calculation

    Attached a sample format, am very clear about the dimension Y (which would help me with delta Y)

    For the X my belief is, since the datum C is specified on axis, it would be "theoritical centre of the width W" and the basic X dimensions would be D2/2 (for inner holes) and D1/2 for outer holes.

    1. Would this be correct? and

    2. Practically how would I be able to measure this? Say on a CMM...how would the datum be determined as 0 point for X?

    3. Am sure we cannot but throwing it out there to check it (to get a "physical reference" some people have been recommending to subtract D2 from W and then divide it by 2 to get the centre of the outside holes (i think that is not correct but do not have a reference for the correct theory as well). Closest I can think of is Fig 4-33 of ASME Y14.5-2009. There is no note on the dwg which states holes are equidistant from the edges so a highly doubtful

    If somebody could guide me here, would appreciate. As always thanks for your time and reading this - Appreciate
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  2. #2
    Administrator Kelly Bramble's Avatar
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    Feb 2011
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    Lets work on your question to ensure we are all on the same page..


    The image you attached is very incomplete so I will need to estimate much to provide you with an answer.


    For the X my belief is, since the datum C is specified on axis, it would be "theoritical centre of the width W" and the basic X dimensions would be D2/2 (for inner holes) and D1/2 for outer holes.

    According to the drawing, Datum C which I think you are saying is X is not an axis it a center plane derived from the left and right side surfaces (dimension D) specified as Datum C.


    To establish Datum C with a CMM one would instruct the CMM that they are creating a datum center plane and then touch the CMM stylus on many surface areas/points high and low on both the left and right sides of the feature of size D. The CMM will then determine a center plane or zero for measurements left and right.


    When establishing a zero(s) from a specified datum(s) we always use the surface(s) associated with the Datum specification.
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