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Thread: Position Callout using basic PCD and basic Angle

  1. #1
    Associate Engineer
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    Mar 2016
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    Position Callout using basic PCD and basic Angle

    Hi. I am unsure how to interpret the following and wonder if anyone can help.

    I have 6 equispaced holes on a PCD with each hole having a .25 position call out.

    When tolerancing this on a CMM it will not allow me to use the basic PCD dimension. I have to use the radius. So to compensate for this I have been halving the position tolerance. My thinking behind this is if you have a basic PCD of 100mm abd the holes have a position tolerance of .25, that means the basic PCD can technically be a min max of 100.25 and 99.75.... So using the radius you will need to halve the tolerance to still achieve this 'band'.

    Can anyone point me in the right direction as to what to do in this situation.

    Thanks.

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
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    If the tolerance is called out as diameter .25 the radius is .25/2...

    It seems unusual to that a modern (any for that matter) CMM cannot properly handle diametric tolerance boundaries on hole features.

    Be aware that if the tolerance is diametric the as-manufactured hole feature can be offset from the true position (basic dimension location) in any direction not just inward and outward from the reference datums.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
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    Hi Kelly. Thanks for the reply.

    I have attached a sample of the drawing I am working to. Maybe my description wasn't great.

    The CMM software can output true position but when it comes to this i dont see how to tolerance it, previously the basic dimension would have been a radial dimension making it much easier.

    In my example I was using the extreme values which would give the min max PCD.

    Hopefully this makes a little more sense.
    Attached Images Attached Images

  4. #4
    Technical Fellow Kelly_Bramble's Avatar
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    This seems to be a CMM programming question..

    The specification simply states that the derived axis of the as-built hole feature shall not deviate greater than dia. .25 from the basic dimension location - -I think you know that..

    I would contact the CMM software or equipment folks and ask them as most modern CMM's should be able to handle these basic dimension specifications.

    Just for grins... see:

    Measuring Bolt Circle Geometry Equations and Calculators
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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