table. Arrange your lighting to come from the
front-left, if you are right-handed; from the front-
right, if you are left-handed. This minimizes
shadows cast by drawing instruments and your
hands.
Never place your drafting board so that you
will be subject to the glare of direct sunlight.
North windows are best for admitting daylight in
the Northern Hemisphere. Conservation of vision
is of the utmost importance. You must make every
possible effort to eliminate eyestrain.
T SQUARES
The T square gets its name from its shape. It
consists of a long, straight strip, called the blade,
which is mounted at right angles on a short strip,
called the head. The head is mounted under the
blade so that it will fit against the edge of the
drawing board while the blade rests on the
surface. T squares vary in size, ranging from 15
in. to 72 in. in length, with the 36-in. length
being the most common.
The T square shown in figure 2-7 is typical of
the ones used by an EA. The head is made of
hardwood and the blade, usually of maple with
a natural or mahogany finish. The edges of the
blade are normally transparent plastic strips glued
into grooves on both edges of the blade, as shown
in the cross section in figure 2-7. This allows the
edge of the T square to ride above the drawing
as the blade is moved up or down the board. This
arrangement is a great advantage when you are
drawing with ink. Since the tip of the ruling pen
does not come in contact with the blade, but is
below it, ink cannot be drawn under the blade to
blot the drawing.
The T square is used for drawing horizontal
lines only. Always draw lines along the upper edge
of the blade. The T square also serves as a base
for the triangle when vertical and inclined lines
are drawn. Some T squares are designed with
adjustable heads to allow angular adjustments of
the blade.
Handle your T square carefully. If dropped,
it may be knocked out of true and become useless.
Additionally, to prevent warping, hang the T
square by the hole in the end of the blade or lay
it on a flat surface so that the blade rests flat.
Before beginning a new job, you should test
the top edge of your T square for warp or nicks
by drawing a sharp line along the top of the blade.
29.275D
Figure 2-7.-Drafting board with T square and drafting paper in place.
2-8
