d across the bolt.
It is obvious that in passing once around the bolt the thread advances
to the amount of the pitch as from _a_ to _b_; hence, in passing half
way around, it will advance from _a_ to _c_; we therefore draw line 16
at a right-angle to the centre line, and a line that touches the top of
the threads at _a_, where it meets line 2, and also meets line 16, where
it touches line 3, is the angle or slope for the tops of the threads,
which may be drawn across by lines, as 18, 19, 20, etc. From these lines
the sides of the thread may be drawn at the bottom of the bolt, marking
first the angle on one side, as by lines 21, 22, 23, etc., and then the
angles on the other, as by lines 24, 25, etc.

[Illustration: Fig. 204.]

There now remain the bottoms of the thread to draw, and this is done by
drawing lines from the bottom of the thread on one side of the bolt to
the bottom on the other, as shown in the cut by a dotted line; hence, we
may set a square blade to that angle, and mark in these lines, as 26,
27, 28, etc., and the thread is pencilled in complete.

If the student will follow out this example upon paper, it will appear
to him that after the thread had been marked out on one side of the
bolt, the angle of the thread might be obtained, as shown by lines 16
and 17, and that the bottoms of the thread as well as the tops might be
carried across the bolt to the other side. Figure 204 represents a case
in which this has been done, and it will be observed that the lines
denoting the bottom of the thread do not meet the bottoms of the thread,
which occurs for the reason that the angle for the bottom is not the
same as that for the top of the thread.

[Illustration: Fig. 205.]

[Illustration: Fig. 206.]

In inking in the thread, it enhances the appearance to give the bottom
of the thread and the right-hand side of the same, heavy shade lines,
as in Figure 202, a plan that is usually adopted for threads of large
diameter and coarse pitch.

A double thread, such as in Figure 205, is drawn in the same way, except
that the slant of the thread is doubled, and the square is to be set for
the thread-pitch A, A, both for the tops and bottoms of the thread.

[Illustration: Fig 207.]

A round top and bottom thread, as the Whitworth thread, is drawn by
single lines, as in Figure 206. A left-hand thread, Figure 207, is
obviously drawn by the same process as a right-hand one, except that the
slant of the thread is give