12 and 13 in
Figure 210.

In a top or end view of a nut the thread depth is usually shown by a
simple circle, as in Figure 214.

[Illustration: Fig. 214.]

To draw a spiral spring, draw the centre line A, and lines B, C, Figure
215, distant apart the diameter the spring is to be less the diameter of
the wire of which it is to be made. On the centre line A mark two lines
_a b_, _c d_, representing the pitch of the spring. Divide the distance
between _a_ and _b_ into four equal divisions, as by lines 1, 2, 3,
letting line 3 meet line B. Line _e_ meeting the centre line at line
_a_, and the line B at its intersection with line 3, is the angle of the
coil on one side of the spring; hence it may be marked in at all the
locations, as at _e f_, etc. These lines give at their intersections
with the lines C and B the centres for the half circles _g_, which being
drawn, the sides _h_, _i_, _j_, _k_, etc., of the spring, may all be
marked in. By the lines _m_, _n_, _o_, _p_, the other sides of the
spring may be marked in.

[Illustration: Fig. 215.]

The end of the spring is usually marked straight across, as at L. If it
is required to draw the coils curved instead of straight across, a
template must be made, the curve being obtained as already described for
threads. It may be pointed out, however, that to obtain as accurate a
division as possible of the lines that divide the pitch, the pitch may
be divided upon a diagonal line, as F, Figure 216, which will greatly
facilitate the operation.

[Illustration: Fig. 216.]

Before going into projections it may be as well to give some examples
for practice.

[Illustration: Fig. 219. (Page 169.)]

CHAPTER IX.

_EXAMPLES FOR PRACTICE._

Figure 217 represents a simple example for practice, which the student
may draw the size of the engraving, or he may draw it twice the size. It
is a locomotive spring, composed of leaves or plates, held together by a
central band.

[Illustration: Fig. 217.]

Figure 218 is an example of a stuffing box and gland, supposed to stand
vertical, hence the gland has an oil cup or receptacle.

In Figure 219 are working drawings of a coupling rod, with the
dimensions and directions marked in.

It may be remarked, however, that the drawings of a workshop are, where
large quantities of the same kind of work is done, varied in character
to suit some special departments--that is to say, special extra drawings
are made for these departments. I