Datum Features can Give You a Shift - not a Bonus.
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 simulator for datum feature B would
have a theoretical design size of 19.6.
|
|
|
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. |
|
|
|
|
|
| |
