Chapter 6 Datums and Datum Feature References 189
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Datum feature C stops (constrains) the last rota-
tional degree of freedom of the two planes that
intersect at datum axis B.
Multiple Feature Control Frames Related
to the Same Datum Reference Frame
Multiple groups of features, controlled by mul-
tiple single-segment position or profi le tolerance
specifi cations, act as a single pattern when the fol-
lowing conditions are met. The effect of creating a
single pattern is often referred to as creating a simul-
taneous requirement or as the rule of simultaneity.
1. The feature control frames reference the same
datums.
2. The order of precedence for the datums is
identical.
3. Each datum feature reference has the same
material condition modifi er.
Features may be any size or shape and the single
pattern requirement applies if the above condi-
tions are met.
Figure 6-45 shows two drawings that show
two means of controlling the positional relation-
ships between the same features. In the fi rst draw-
ing, the slot is controlled relative to datum feature
A primary, and datum feature B secondary at
MMB. The slot width is identifi ed as datum feature
C. The four hole pattern is controlled by a position
tolerance referenced to datum feature A primary,
datum feature B secondary at MMB, and datum
feature C tertiary at MMB. The fi rst two datum
feature references in the feature control frames are
identical.
However, the feature control frame showing
the position tolerance on the four holes includes a
reference to tertiary datum feature C at MMB. The
intent of this tertiary datum reference is to estab-
lish a rotational position of the holes relative to the
slot. Establishing a rotational position is typically
referred to as clocking but technically is a means
of constraining one of the rotational degrees of
freedom. The reference to tertiary datum feature
C is not wrong, but it is not necessary. In fact, refer-
encing datum C at MMB permits some additional
variation between the slot and the holes that are
position toleranced with datum C referenced ter-
tiary at MMB.
Based on the rule of simultaneity, a rota-
tional relationship between the holes and the slot
may be achieved without referencing the slot as
datum C. The second drawing shows the same
part. Datum C has been omitted from this draw-
ing. Although datum C has been omitted, the rela-
tionship between the holes and slot is controlled.
Because the position tolerances applied to the
slot and to the four holes have the same datum fea-
ture references, in the same order of precedence,
and with the same material condition modifi ers, the
slot and the holes act as a single pattern. Because
they act as a single pattern, the rotational position of
the holes relative to the slot is controlled.
The principles of groups acting as a single
pattern are covered in more detail in the chapters
Datum axis B locates two perp planes
Datum plane A
Pin diameter equal to MMB
(virtual condition) of the hole
Tab width equal to MMB
(virtual condition) of the slot
Datum C stops rotation
of planes on axis B
Goodheart-Willcox Publisher
Figure 6-44. A primary datum surface and secondary
datum feature of size may be used to establish the datum
reference frame with a tertiary datum feature of size used
to set the orientation of the datum reference frame.