s drawn. Point C is obtained by drawing the
dotted line across where the outline curve meets the stem. Suppose that
the corner where the round stem meets the square under the head was a
sharp one instead of a curve, then the traditional cross would require
to be put on the square, as in Figure 199; or the cross will be
necessary if the corner be a round one, if the stem is reduced in
diameter, as in Figure 200.

[Illustration: Fig. 199.]

[Illustration: Fig. 200.]

[Illustration: Fig. 201.]

Figure 201 represents a centre punch, giving an example, in which the
flat sides gradually run out upon a circle, the edges forming curves, as
at A, B, etc. The length of these curves is determined as follows: They
must begin where the taper of the punch joins the parallel, or at C, C,
and they must end on that part of the taper stem where the diameter is
equal to the diameter across the flats of the octagon. All that is to be
done then is to find the diameter across the flats on the end view, and
mark it on the taper stem, as at D, D, which will show where the flats
terminate on the taper stem. And the curved lines, as A, B, may be drawn
in by a curve that must meet at the line C, and also in a rounded point
at line D.

CHAPTER VIII.

_SCREW THREADS AND SPIRALS._

[Illustration: Fig. 202.]

[Illustration: Fig. 203.]

The screw thread for small bolts is represented by thick and thin lines,
such as was shown in Figure 152, but in larger sizes; the angles of the
thread also are drawn in, as in Figure 202, and the method of doing this
is shown in Figure 203. The centre line 1 and lines 2 and 3 for the full
diameter of the thread being drawn, set the compasses to the required
pitch of the thread, and stepping along line 2, mark the arcs 4, 5, 6,
etc., for the full length the thread is to be marked. With the triangle
resting against the $T$-square, the lines 7, 8, 9, etc. (for the full
length of the thread), are drawn from the points 4, 5, 6, on line 2.
These give one side of the thread. Reversing the drawing triangle,
angles 10, 11, etc., are then drawn, which will complete the outline of
the thread at the top of the bolt. We may now mark the depth of the
thread by drawing line 12, and with the compasses set on the centre line
transfer this depth to the other side of the bolt, as denoted by the
arcs 13 and 14. Touching arc 14 we mark line 15 for the thread depth on
that side. We have now to get the slant of the threa