188 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
MMB, not its MMC size. The maximum material
boundary for a hole is equal to the virtual con-
dition resulting from the combined effects of the
maximum material size and any applied geomet-
rical tolerance that is related to the higher prece-
dence datum.
The maximum material boundary for sec-
ondary datum feature B on the given part is .988″
diameter. This is determined by subtracting the
.002″
perpendicularity tolerance from the .990″
maximum material size limit of the hole. The toler-
ance is subtracted because an out-of-perpendicular
hole would appear to be smaller when fi tting over
a perpendicular pin.
A condition similar to the previous descrip-
tion for the maximum boundary of a hole also
exists for external features such as shafts. The
MMB for a shaft is caused by the effect of the
maximum material size and any applicable toler-
ance that tends to make the shaft appear to have
an increased diameter. To determine the MMB for
a shaft, any applicable tolerance is applied to the
MMC size of the shaft. The MMB for a datum fea-
ture of size is the same as the virtual condition of a
toleranced feature of size as described in detail in
following chapters.
It is sometimes necessary to reference one
plane and two features of size in order to estab-
lish the needed datum reference frame. See
Figure 6-44. Four holes are shown with a position
tolerance that is related to datums A primary, B
secondary at MMB, and C tertiary at MMB. Pri-
mary datum feature A is a fl at surface. Secondary
datum feature B is a hole. Datum feature C is the
width of the slot.
Datum simulators are shown in the fi gure to
illustrate how the datum reference frame may be
established for this part. Datum A is simulated by
a fl at surface. Secondary datum axis B is simulated
by a fi xed-diameter pin. The pin diameter must be
equal in size to the MMB of the hole, because the
hole is referenced as a secondary datum at MMB.
Typically, a secondary datum hole used in this
manner will have a size and a perpendicularity
tolerance applied. The MMC and the perpendicu-
larity tolerance must both be used to calculate the
applicable MMB. The datum B simulator pin must
be perpendicular to datum simulator A.
Tertiary datum C is simulated by a fl at key.
The key must have a width equal to the MMB of
the slot. Typically, a tertiary datum slot of this
type will include a size tolerance and a position
tolerance relative to the primary and secondary
datums. The MMC and position tolerances must
both be used to calculate the applicable MMB. The
datum C simulator must be positioned relative to
the datum A and datum B simulators and perpen-
dicular to the datum A simulator.
All three datum feature references are
required to completely stabilize the given part.
Datum A stops (constrains) translation of the part
in one direction and also prevents (constrains)
rotation around two coordinate axes. Datum axis
B locates two planes that intersect at the axis and
prevents (constrains) translation along two of the
coordinate axes. These planes are perpendicular to
datum A, but are free to rotate on datum axis B.
Primary datum
surface A makes
three point contact
Datum axis B
Two perp planes
passing through
datum axis B
Datum plane A
MMB specified
Fixed diameter simulator sized
at the MMB (virtual condition)
of the datum feature
Pin diameter = .988
Goodheart-Willcox Publisher
Figure 6-43. A secondary or tertiary datum feature
reference showing an MMB modifi er requires the
datum to be simulated by a tool that has a size equal
to the applicable maximum material boundary of the
datum feature.