38 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
within the feature control frame. Another differ-
ence is that rules regarding the usage of material
condition modifi ers have changed.
Beginning with the 1982 standard, datum fea-
ture references are shown following the tolerance
value. In accordance with the 1973 standard, datum
feature references were permitted in either of two
locations. They could be shown between the toler-
ance symbol and the tolerance value, or they could
be shown in the same manner as defi ned for pres-
ent practices. See Figure 2-31. Prior to 1973, datum
feature references were required to be located
between the tolerance symbol and tolerance value.
Prior to 1982, the diameter symbol was not
required and the diameter abbreviation (DIA) was
used. DIA followed the tolerance value.
A signifi cant difference in the feature con-
trol frame appearance involves material condition
modifi ers. Prior to 1982, position tolerances were
assumed to apply at the maximum material condi-
tion (MMC) if no modifi ers were shown in the fea-
ture control frame. Assumptions were not permitted
for positional tolerances in 1982. Beginning with the
1994 standard, the RFS modifi er was assumed for
all tolerances and datum feature references.
Past Basic Dimensions
Old drawings might include the word BASIC
or the abbreviation BSC adjacent to some dimen-
sions. These notations were permissible under
early issues of the dimensioning and tolerancing
standard. As part of the effort to replace words
with symbology, the notations are no longer used.
Past practices used the abbreviation REF to
indicate a dimension was for reference purposes.
Current practice is to enclose the dimension value
in parentheses.
Chapter Summary
✓ Symbology is now used for dimensioning
and tolerancing of CAD models and
drawings in place of the previously used
notations and abbreviations. Symbology
for general dimensioning and tolerance
application is standardized and well defi ned.
✓ Dimensioning and tolerancing symbols,
when correctly applied, provide better clarity
of part requirements than can be achieved
through the utilization of notes.
✓ There are standardized rules for applying
dimensioning and tolerancing symbols. These
rules must be followed properly if the full
advantage of symbology is to be achieved.
1982 Practices
1973 Practices
1994 Practices
RFS modifier assumed but allowed to
be shown for position tolerances
Material condition modifiers on datum references
Modifier required for
position tolerances
RFS modifier assumed except
on position tolerances
Optional location for datum references
MMC assumed for
position tolerances only
RFS modifier assumed except
on position tolerances
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Figure 2-31. Previous standards included different feature control frame format requirements.