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Thread: Advanced GD&T Question Composite True Position LMC

  1. #1
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    Bang Head Advanced GD&T Question Composite True Position LMC



    This is the FCF is question. I would love to hear how this FCF is interpreted by multiple people. I am almost certain I understand it but I would like to be sure.


    For info:
    J is the surface these holes are drilled into.
    G & H are larger diameter holes also drilled into this surface.
    Nominal Diameter is .250
    Measurements are in mm

    Thanks Everyone! I am also new to engineers edge.
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  2. #2
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    Welcome aboard!


    It simply means that each of the 128 holes shall be positioned within Ø0.05, with respect to the J, G, H datum reference frame, if made at 0.263. But as a hole's size is made smaller, it will be allowed to deviate more off center (bonus tolerance).

    Then the lower part of the FCF says that each of the 128 holes shall be positioned within Ø0.025 if made to a size of 0.263, but the only datum here is J. So this controls the perpendicularity of each hole, but it also locates the holes to one another -- don't forget this aspect.
    If a hole is made to a size smaller than 0.263, then a bonus tolerance is allowed here, just as in the upper part of the FCF.

    I suspect you pose the question because it's LMC, rather than the more common MMC use of composite tolerancing. Using LMC here is legal, but I can't say whether it's wise; that totally depends on the function. I will say that this doesn't allow you to use functional gaging, which is allowed when the MMC modifier is shown.

  3. #3
    Technical Fellow Kelly_Bramble's Avatar
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    Quote Originally Posted by Belanger View Post
    I suspect you pose the question because it's LMC, rather than the more common MMC use of composite tolerancing. Using LMC here is legal, but I can't say whether it's wise; that totally depends on the function.
    I'm having the same thoughts..

    So afotion, what is the function or application of these hole features?
    Last edited by Kelly_Bramble; 05-31-2015 at 09:15 AM.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  4. #4
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    Quote Originally Posted by Belanger View Post
    Welcome aboard!


    It simply means that each of the 128 holes shall be positioned within Ø0.05, with respect to the J, G, H datum reference frame, if made at 0.263. But as a hole's size is made smaller, it will be allowed to deviate more off center (bonus tolerance).

    Then the lower part of the FCF says that each of the 128 holes shall be positioned within Ø0.025 if made to a size of 0.263, but the only datum here is J. So this controls the perpendicularity of each hole, but it also locates the holes to one another -- don't forget this aspect.
    If a hole is made to a size smaller than 0.263, then a bonus tolerance is allowed here, just as in the upper part of the FCF.

    I suspect you pose the question because it's LMC, rather than the more common MMC use of composite tolerancing. Using LMC here is legal, but I can't say whether it's wise; that totally depends on the function. I will say that this doesn't allow you to use functional gaging, which is allowed when the MMC modifier is shown.
    Thank you so much for the help everyone!

    The function is simple for these holes. Ink is simply jetted through these holes and basically supposed to make lines parallel to each other and within a certain distance apart at a consistent width. It is for a print head.

    I was also very curious as to how the Bonus Tolerance for this FCF is calculated.

  5. #5
    Technical Fellow Kelly_Bramble's Avatar
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    Quote Originally Posted by afotion View Post
    I was also very curious as to how the Bonus Tolerance for this FCF is calculated.
    For the upper horizontal frame:

    When the holes are as-manufactured at LMC dia. 0.263, the allowable position tolerance is dia. 0.050. As the as-manufactured hole size decreases an increase in the allowable position equal to that amount of size divergence is summed to the starting position tolerance specification. So, drill the hole at dia. 0.262 - position tolerance allowed is 0.051. drill the hole at dia. 0.261 - position tolerance allowed is 0.052 and so on until MMC size.


    For the lower horizontal frame:

    When the holes are as-manufactured at LMC dia. 0.263, the allowable position tolerance is dia. 0.025. As the as-manufactured hole size decreases an increase in the allowable position equal to that amount of size divergence is summed to the starting position tolerance specification. So, drill the hole at dia. 0.262 - position tolerance allowed is 0.026. drill the hole at dia. 0.261 - position tolerance allowed is 0.027 and so on until MMC size.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  6. #6
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    Good Job

    Quote Originally Posted by Kelly Bramble View Post
    For the upper horizontal frame:

    When the holes are as-manufactured at LMC dia. 0.263, the allowable position tolerance is dia. 0.050. As the as-manufactured hole size decreases an increase in the allowable position equal to that amount of size divergence is summed to the starting position tolerance specification. So, drill the hole at dia. 0.262 - position tolerance allowed is 0.051. drill the hole at dia. 0.261 - position tolerance allowed is 0.052 and so on until MMC size.


    For the lower horizontal frame:

    When the holes are as-manufactured at LMC dia. 0.263, the allowable position tolerance is dia. 0.025. As the as-manufactured hole size decreases an increase in the allowable position equal to that amount of size divergence is summed to the starting position tolerance specification. So, drill the hole at dia. 0.262 - position tolerance allowed is 0.026. drill the hole at dia. 0.261 - position tolerance allowed is 0.027 and so on until MMC size.
    Kelly,

    Thank You so much!

    Perfect, easy to understand, and well written explanation!

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