2011

Table Of Contents
Datum Reference Frames
The tolerance values in the feature control frame are followed by up to three
optional datum reference letters and their modifying symbols.
A datum is a theoretically exact point, axis, or plane from which you make
measurements and verify dimensions. Usually, two or three mutually
perpendicular planes perform this task best. These are jointly called the datum
reference frame.
The following illustration shows a datum reference frame verifying the
dimensions of the part.
Quick Reference
Commands
LEADER
Creates a line that connects annotation to a feature.
TOLERANCE
Creates geometric tolerances contained in a feature control frame.
Projected Tolerance Zones
Projected tolerances are used to make the tolerance more specific.
Projected tolerances are specified in addition to positional tolerances to make
the tolerance more specific. For example, projected tolerances control the
perpendicularity tolerance zone of an embedded part.
The symbol for projected tolerance ( ) is preceded by a height value, which
specifies the minimum projected tolerance zone. The projected tolerance zone
840 | Chapter 29 Dimensions and Tolerances