Figure 2-8.-Parallel straightedge.
Turn the T square over and redraw the line with
the same edge. If the blade is warped, the lines
will not coincide.
If the blade swings when the head is held
firmly against the edge of the drawing board, the
blade may be loose where it is joined to the head,
or the edge of the T square head may be warped.
You can usually tighten a loose blade by adjusting
the screws that connect it to the head, but if it
is out of square, warped, or in bad condition, you
should select a new T square.
Many draftsmen prefer to use a PARALLEL
STRAIGHTEDGE (fig. 2-8) rather than a T
square. The primary purpose of the parallel
straightedge is the same as the T square.
The parallel straightedge is a laminated maple
blade with transparent plastic edges similar to
those on the T square. The parallel straightedge
uses a system of cords and pulleys so that it is
supported at both ends by a cord tacked to the
drawing board. You can move the straightedge
up or down the board with pressure at any point
along its length and maintain parallel motion
automatically. It comes complete with cord,
tacks, cord tension adjuster, and mounting
instructions. Some straightedges, like the one
shown in figure 2-8, are equipped with a cord lock
on one end of the blade. The straightedge is locked
into place by turning the cord lock clockwise. This
permits use of the straightedge on an inclined
board. It also prevents accidental movement when
you are inking or using mechanical lettering
devices. The advantages of the parallel straight-
edge become particularly significant when you are
working on large drawings. While the T square
works well for small work, it becomes unwieldy
and inaccurate when you are working on the far
right-hand side of large drawings.
When drawing long, straight lines, you should
use a STEEL STRAIGHTEDGE (fig. 2-9)
because its heavy weight helps keep the
straightedge exactly in position. The steel
Figure 2-9.-Steel straightedge.