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Machine Trades Print Reading
TABULAR DIMENSIONS
Tabular dimensioning is a common format for
dimensioning parts or assemblies being produced
multiple times that have the same shape but
different sizes. TABULAR DIMENSIONING uses
letters or numbers referenced to a table instead of
dimension lines. A table on the drawing supplies
the corresponding values for the lettered parts, as
shown in Fig. 4-9.
Drawings that contain a large amount of
repetitious features, such as holes, also benefit
from tabular dimensioning. Letters or numbers label
the features, and XYZ coordinates specify their
locations. Additional information such as description
and quantity is included as needed. The elimination
of excessive extension and dimension lines makes
reading dimensions easy. See Fig. 4-10.
RECTANGULAR COORDINATE DIMENSIONS
Rectangular coordinate dimensioning is another
method that eliminates excess dimension and
extension lines. RECTANGULAR COORDINATE
DIMENSIONING uses distances of two or three
intersecting planes referenced from a baseline or a
datum. A DATUM is an exact point of origin used for
dimensioning or locating a feature. A datum can be
a line, a surface, or intersecting planes.
A feature such as a hole is located horizontally
and vertically from a datum to the feature’s center
point. Centerlines mark the center point’s coordinate
directions from the datum. This system helps clarify
and avoid difficulty in interpreting the drawing. Refer
to Fig. 4-11. Arrowless dimensioning, or datum
dimensioning, are other names for coordinate
dimensioning.
Fig. 4-8. A dimension placed between parentheses indicates a
reference dimension.
Fig. 4-10. Tabular dimensioning is a beneficial way to
dimension repetitious features that would otherwise require
many lines. Letters code the holes and a chart correlates them
to dimensions and other relevant information.
Fig. 4-9. Tabular dimensioning uses letters or numbers instead
of numerical values for dimensions. A chart correlates the
different measurement values to the coded parts.
REFERENCE
DIMENSIONS