|Re: True Position with MMC Explanation|
|Re: True Position with MMC -- Chad B.||Post Reply||Top of thread||Forum|
Posted by: Dave |
: I know how to calculate true position, with no regard to feature size. I wan't to know how to calculate true position when MMC is applied to the feature and datums.True position is always the exact location of the feature with respect to the referenced datums. It will either be inferred as being on the centerline, axis, or centerplane or given explicitly with a basic dimension. Calculating whether the feature is within the tolerance zone allowed relative to that true position is what you are after. In this it doen't matter too much if the MMC reference is for the feature or one or more datum references: In either case there is a virtual condition boundary that must fit within (internal) or around (external) the feature the reference refers to. If you said "huh?" to the above - here is a simplification. For the datum references - whatever variation you allow for the datums features, you allow for the gage that will represent that datum. Make the reference hole so that at its minimum it will only accept a .250 pin, then the gage can have - at true position - a .250 pin. Allow the part feature to accept a .375pin at true position, you use a .375 pin. But, you say, what of the tolerance on the hole location and its size tolerance? These were combined to find out what the part will accept. The .375 was in fact a .505 hole +.100/-.005 with a .125 positional tolerance. The .250 pin went into a .500+.1/-.01 hole with a .250 perpendicularity tolerance relative to the same primary datum as the related feature. How these are evaluated for conformance on a real part without the use of gages can depend on clever geometric construction or the use of good CMM software.Sometimes poorly generated schemes are more difficult to analyze than logic puzzles are. It takes skill on both the generation and the evaluation ends of things.Dave.
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