Datum features can give you a shift-not a bonus - 1994 (#18)
(In accordance with Y14.5M-1994 standard)
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When datum features are modified at MMC or LMC, the datum feature and the features being controlled may be able to shift relative to the datum axis or centerplane. On the drawing shown below, datum feature B, in the position callout of the four hole pattern, is modified at MMC. This means the datum feature simulator for datum feature B would have a theoretical design size of Ø19.6.
Since the actual datum feature could be produced as large as 20.2, the datum feature could shift as much as 0.6 total. This means the four-hole pattern may shift out of position in one direction while the datum feature shifts in the other direction as shown. A very common error made at inspection is to use this allowed datum shift as a bonus on the features being controlled. This would allow the features to be out of position to each other more as the datum feature departs from MMC. This practice does not agree with the drawing or a hard gage, should one be produced.
