182 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
That virtual condition in this example is an MMB
of 1.007″ diameter.
The applicable MMB size is affected by the
order of precedence of the datum feature reference as
well as the tolerances applied to the datum feature.
If an orientation tolerance is applied to a secondary
datum feature of size, then the MMC and orienta-
tion tolerance combine to create another MMB. See
Figure 6-37. The secondary datum feature has a size
tolerance and an orientation tolerance. The MMC
size and orientation tolerance combine to create an
applicable MMB of .881″ diameter.
If a datum feature of size includes a size, form,
orientation, and positional tolerance, the applicable
MMB must be determined using the tolerance
that is applied relative to higher order precedence
datums. The applicable boundary is always deter-
mined in the context of the datum feature refer-
ence and its order of precedence. See Figure 6-38.
In the fi gure, there is a position tolerance
applied on four holes and that tolerance references
a tertiary datum feature C at MMB. The size of the
tertiary datum simulator must be determined. The
tertiary datum feature has a size tolerance, per-
pendicularity tolerance, and position tolerance.
The tertiary datum feature has a position toler-
ance that references a higher precedence datum.
Therefore, the MMC and location tolerance com-
bine to create a maximum material boundary. The
correct MMB for the tertiary datum simulator is
calculated using the MMC size plus the position
tolerance. The applicable MMB is equal to .882″
diameter and that is the size for a simulator based
on the tolerances shown in the fi gure.
Primary datum reference
Size tolerance applicable
MMB = MMC
1.005
Primary datum reference
Size and form tolerance applicable
MMB = MMC + Straightness
1.007
Goodheart-Willcox Publisher
Figure 6-35. A primary datum reference at MMB
requires the simulator to be equal to the maximum
material size if Rule #1 has not been superseded by
note or form tolerance.
Goodheart-Willcox Publisher
Figure 6-36. A primary datum reference at MMB
requires the simulator to be equal to the maximum
material boundary when a form tolerance is applied in
a manner that supersedes Rule #1.
Secondary datum reference
Size and orientation tolerance applicable
MMB = MMC + Orientation
.881
Goodheart-Willcox Publisher
Figure 6-37. A secondary datum reference at MMB
results in a simulator sized to the virtual condition of the
feature relative to any higher order precedence datums.
Pro Tip
Applicable MMB
Showing the translation modifi er on the ter-
tiary datum reference in Figure 6-38 would
indicate the tertiary datum simulator is free to
translate. That removes the position tolerance
from the calculation of the applicable MMB,
because the simulator is free to translate to the
location of the datum feature. In this case, only
the orientation tolerance is added to the MMC
size when calculating the applicable MMB.