GDT : Material Condition Symbol implications

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    I am not very clear about the effect of Material Condition Symbol when applied to Secondary or Tertiary Datum in Feature Control Frame in locating a hole/hole pattern.
     Couple of thought were running through my mind.

    1.  why or why not secondary / tertiary datum have Material Condition symbol MMC or LMC while defining the position tolerance for a hole?

    2.  What is the implications of applying Material condition MMC/LMC to secondary datum while defining the position tolerance for a hole/ hole pattern. 
    Information on this would be of great help.


    Van Loon

    Maybe I dont understand the question correctly.
    But I dont think it has different meanings for primary/secondary….
    If you use the Material Condition symbol, your tol. changes
    according to what material condition your feature is in.


    Paul Jackson

    When (M) is added to the datum feature in the feature control frame… it means that the measurement origin that is established by that datum feature is permitted to “shift” commensurate with its departure from MMC… up to its size boundary of LMC.
    That permissible shift cannot be applied independently to various feature controls that use it for reference… but rather the shift can only be applied in a single magnitude and direction to all features that identically reference it simultaneously.
    If a part is placed on a gauge with its datum “features-of-size” aligning and locating the part over “virtual condition” fixed size pins-holes-slots-rails-whatever… the residual wiggle or rotation reflects the permissible shift available to verify the location of other features relative to that datum reference. One cannot shift the part left to verify one feature then extract its checking pin and shift the part right to check another feature… all features identically related to that datum reference must be verified simultaneously from any candidate shift.
    Comprehensive hard attribute gauges accomplish this simply but it is extremely difficult to do this analytically in a layout inspection or with a CMM consequently this variable portion of tolerance is either ignored in variables measurement or it is improperly applied to individual features independently via device programs that enable it. There are some software programs that can perform the analysis if all of the relative specifications are evaluated simultaneously.

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