huangyhg

contribution mmc on datum
would anyone know of a good book that explains how to calculate the contruibution of the mmc on second and third datum boxes?
i have a true position where datum a is the flatt surface and datum b and datum c have the m behind them.
i understand that these would establish a functional gauge to check the part. but what if i wasn't using a gauge. how would i know the out of true position to allow?
does anyone know of a good text or reference which explains how to calculate this (without using a graphics terminal to calculate it)?
robert
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hi imnotfrommars
it might help if you said what datums b & c are?
regards
desertfox
datum a is a surface, datum b and datum c are diameters that are both perpendicular to the specified true position.
the shift permitted is the actual diameter of feature of size(f.o.s.) less the mmc condition of f.o.s. if datum b is 10 +/- 0.5 the mmc=9.5. if the hole actually measures 10, then the part would be permitted to shift on the datum 10-9.5=0.5 (diametrically). if there is a perpendicularity callout as well i.e. 0.5 diametrical, that would be added to the shift as well. actual hole diameter - mmc + perpendicularity = diametrical shift allowance.
the extra allowable tolerance is the difference between the actual size of the datum feature and its mmc size or virtual condition size depending upon if the feature was designated as a datum or qualified as a datum.
it wish life was this easy but it isn't.
any time the datum is referenced at mmc, it is conducive to a checking fixture but there are certain situations where one can just accumulate the tolerances.
if one had 2 coaxial diameters with one positional to the second one at mmc and also referenced the datum at mmc, we would accrue the difference between the feature size and it virtual condition size and the add the difference between the datum feature and it mmc or virtual condition size. easy stuff to do.
now we will look at 4 holes positional at mmc referencing datum b & c (holes) at mmc. one cannot just figure out the differences and then split it among the 4 holes. it just does't work out. this is conducive to a checking fixture and then you will be able to accrue the tolerances.
over the 20 years that i have been training in gd&t, all cmm operators have told me that they disregard the mmc on the datum holes and go rfs. there was only 1 person who said it could be done and i wasn't about to argue with her since she was as tall as i am and could probably arm wrestle me into the ground.
the checking fixture supersedes the cmm since the checking fixture uses all the tolerances given while the cmm only uses the positional tolerances at mmc.
i will have at least 1 person in here saying it can be done but would love to see the logic.
dave d.
thank you for the assistance with this. i am finding it very difficult to assess and stack up the contribution of the second and third datums when referenced at mmc. what works in practice does not always work in theory.
it seems to me that the asme spec is weak in this area.
if anyone does know of a text or method for calulating this especially when perpendicular datums are involved, i would appreciate hearing.
robert
datum shift or mobility allowances are most easily addressed with hard gages but they certainly can be addressed analytically with all due respect to simultaneous requirements when the translation and rotation of the drf is applied in one unique magnitude and direction to all simultaneous requirements and that shift/rotation is limited to the estimated actual difference between the datum feature’s actual mating size and their virtual condition size.
the analysis becomes more difficult as the number of related features or patterns of features linked by the simultaneous requirement increases but there is commercial software available to perform the analysis. valisys and checkmate are software brands that require all related features to be probed prior to iterating an optimally common datum shift/rotation to the conglomerate pattern.
if you are handy with spreadsheets and/or programming you can shift and rotate measurement data one candidate move at a time while limiting the magnitude of those shifts/rotations so that the datum features do not exceed their actual boundaries of freedom for mobility.
attached is a presentation illustrating common practices in inspecting features that are permitted datum shift from disregarding the allowance to fully applying its estimated liberty.
paul
pauljackson,
thank you. this is excellent.
robert
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