Chapter 6 Datums and Datum Feature References 193
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Compound datum features may also be used
to establish a datum plane. See Figure 6-51. The
given part has two slots, one cut in each end of the
part. Each slot is identifi ed as a datum feature. The
profi le tolerance on the curved surfaces references
compound datum feature B-C as a secondary
datum. The two slots are picked up simultane-
ously to establish the center plane through them.
Compound datum features may be made of
two or more fl at surfaces. Each surface is identifi ed
as a datum feature and referenced by showing all
of the datum feature letters separated by dashes.
If the surfaces lie in the same plane, there is more
than one way to establish that a single datum
is to be established from them. The compound
datum feature method may be used, it is possible
to use the continuous feature symbol to link them
together, and a profi le tolerance may be applied to
the surfaces and a datum feature symbol applied
to the profi le tolerance.
A Surface and Two Holes
Flat panels and many other part types often
include two holes that locate the part in an assem-
bly, and it is common to use two holes to locate
a part during fabrication. See Figure 6-52. These
holes may be identifi ed as datum features, and tol-
erances referenced to them.
The given part has six holes. They are all toler-
anced relative to datum A. The holes act as a single
pattern. Two of the holes are identifi ed as datum
features B and C. The perimeter of the part has a
profi le tolerance that is referenced to datums A
primary, B secondary at MMB, and C tertiary at
MMB. This tolerance completely controls the loca-
tion, orientation, and size of the perimeter with
respect to the specifi ed datums. In effect, the out-
line of the part is controlled to the positions of the
holes.
The primary and secondary datums in the
given example locate all planes in the datum refer-
ence frame, but rotation of the second and third
planes is not constrained until the tertiary datum
is defi ned. This was previously explained with
respect to
Figure 6-44.
Figure 6-52 shows that a datum reference
frame may be completely defi ned by one surface
and two holes. The datum features used are physi-
cal features on the part. It is important that the
datum reference frame is created by referencing
features on the part. Datum feature symbols are
not placed on the drawing centerlines between or
through the holes.
Center plane established
by datum feature A
Center plane established by
compound datum features B and C
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Figure 6-51. Rectangular features of size may be used
as compound datum features to establish a datum
plane.
Datum A simulator
(locates one plane
and constrains one
translational and
two rotational
degrees of freedom)
Datum axis B simulator (constrains two
translational degrees of freedom)
Datum axis C simulator
(constrains rotation of the
planes on datum axis B)
Goodheart-Willcox Publisher
Figure 6-52. A surface and two holes may be used to
create and orient a datum reference frame.