Most companies
still use ± linear and angular general tolerances found
in the titleblock of a drawing. They do not know why
they use it, they just do. "That's the way we've always
done it." This general note causes much confusion
because it does not relate tolerances to a datum
reference frame, allows tolerance accumulation and wedge
shaped tolerance zones. To clear up the drawing's
meaning, many companies are changing to a general
profile of a surface tolerance with datum referencing.
This requires that the dimensions describing surfaces be
basic. Since this will usually be most of the
dimensions, it is becoming a common practice to add the
note "UNTOLERANCED DIMENSIONS ARE BASIC". This note
eliminates a lot of boxes on the drawing. Then apply
tolerances directly to those dimensions that should not
be basic such as hole diameters and slot widths. Since
the general profile tolerance controls location of the
entire surface of each feature, the only reason to put
any geometric tolerances on the field of the drawing is
for refinements of orientation, size and
form.
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