Chapter 2 Dimensioning and Tolerancing Symbology 37
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Notes in a CAD model may indicate that all
CAD data is basic unless otherwise indicated.
In this case, a hole with a size tolerance applied
would have a size dimension and size tolerance as
shown in the CAD model. Its size would not be
considered basic. The hole would have a basic loca-
tion and orientation.
Past Practices
Drawings and CAD models may be created
under contractual requirements from a customer.
A contract may require compliance with a specifi c
previous edition of the applicable standards, espe-
cially if the contract was created prior to publica-
tion of the current standard. This is common for
long-term contracts, such as for aircraft. It is also
common for parts that are in production and were
designed when a previous version of the standards
was in place. Many engines and transmissions in
cars are in production over a long period of time,
so the documentation for the parts may have
been created in the past. As a result, the creation
of drawings and CAD models under continuing
contracts, and the ability to understand existing
drawings, may require knowledge of previously
used methods. Past practices related to some of
the dimensioning and tolerancing symbols were
noted in the preceding paragraphs of this chapter.
The following section provides additional infor-
mation about past practices.
Past Symbols
The symbology used for tolerance specifi ca-
tion has gone through changes as the dimension-
ing and tolerancing practices have advanced to the
current standard. Knowledge of past practices is
needed for those situations in which an old draw-
ing must be used or when a continuing contract
still references an old version of the dimensioning
and tolerancing standard.
Previous practice (prior to the 1982 standard)
required the use of a circle divided into quadrants
for the datum target symbol. See Figure 2-29. The
datum letter was placed in the upper-left quadrant,
and the number was placed in the lower-right
quadrant.
Prior to 1982, the current total runout symbol,
with two arrowheads, was not used in the United
States. See Figure 2-30. The previously used total
runout symbol looked like the symbol for circular
runout, but the word TOTAL was shown beneath
the feature control frame.
A separate symbol existed for symmetry toler-
ances prior to the 1982 standard. The 1982 version
of the standard eliminated the symmetry symbol,
and the position symbol was defi ned to be correct
for specifi cation of symmetrical location require-
ments. The symmetry symbol should not be used
in documentation that must meet the 1982 stan-
dard requirements. The 1994 standard reinstated
the symmetry symbol and clarifi ed its defi nition.
A regardless-of-feature-size modifi er was
included in the standard prior to 1994. Effective in
1994 and continuing to be applicable today, toler-
ances are assumed to apply at RFS unless other-
wise specifi ed. Because tolerances are assumed to
apply at RFS, the symbol is no longer needed. The
1994 standard included an alternate practice that
allowed the application of the RFS symbol on posi-
tion tolerances. The current standard does not sup-
port use of an alternate practice.
The datum feature symbol prior to 1994 was
a rectangle. The rectangle included a datum iden-
tifying letter with a dash at each side of the letter.
Past Feature Control Frame Format
Feature control frames can look different
depending on the standard that was in effect
when the drawing was completed. One difference
in appearance is caused by previously permitted
options on the location of datum feature references
1973 Practice
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Figure 2-29. Prior to 1982, the datum target symbol
was divided into quadrants.
Past Practices
Allowed
1994
RFS
Symmetry
1973 Control 1982
Total
runout
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Figure 2-30. Some tolerance symbols have changed
as a result of advancements in the dimensioning and
tolerancing standard.