PRIN. OF DRAFTING AND SHOP DRAWINGS - OD1641 - LESSON 1/TASK 4
(2) Dimensioning Systems.
Dimensions may be positioned on a drawing by using
either the unidirectional or the aligned system. The unidirectional system is the
preferred system. In the unidirectional system, all dimensions are placed so that
they can be read from the bottom of the drawing, that is, with their guidelines
horizontal. In the aligned system, dimensions are placed so that they may be read
from either the bottom or the right side of the drawing, that is, with their
guidelines parallel to the surface that they are defining. Figure 69 illustrates
the difference between the two systems.
FIGURE 69.
COMPARISON BETWEEN THE
UNIDIRECTIONAL AND ALIGNED
DIMENSIONING SYSTEMS.
(3) Dimensioning Holes.
Figure 70 (on the following page) illustrates several
ways to dimension holes. Holes are usually dimensioned to their diameters because
most drills, punches, and boring machines are set up in terms of diameters. Arcs
are usually dimensioned according to radii.
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