e 209. The lines for
the full diameter of the thread being drawn, the pitch is stepped off by
arcs, as 1, 2, 3, etc.; and from these, arcs, as 4, 5, 6, etc., are
marked for the width of the flat places at the tops of the threads.
Then one side of the thread is marked off by lines, as 7, which meet the
arcs 1, 2, 3, etc., as at _a_, _c_, etc. Similar lines, as 8 and 9, are
marked for the other side of the thread, these lines, 7, 8 and 9,
projecting until they cross each other. Line 10 is then drawn, making a
flat place at the bottom of the thread equal in width to that at the
top. Line 12 is then drawn square across the bolt, starting from the
bottom of the thread, and line 13 is drawn starting from the corner _f_
on one side of the thread and meeting line 12 on the other side of the
thread, which gives the angle for the tops of the thread. The depth of
the thread may then be marked on the other side of the bolt by the arcs
_d_ and _e_, and the line 14. The tops of all the threads may then be
drawn in, as by lines 15, 16, 17 and 18, and by lines, as 19, etc., the
thread sides may be drawn on the other side of the bolt. All that
remains is to join the bottoms of the threads by lines across the bolt,
and the pencil lines will be complete, ready to ink in. If the thread is
to be shown curved instead of drawn straight across, the curve may be
obtained by the construction in Figure 208, which is similar to that in
Figure 207, except that while the pitch is divided off into 16
divisions, the whole of these 16 divisions are not used to get the
curves, some of them being used twice over; thus for the bottom the
eight divisions from _b_ to _i_ are used, while for the tops the eight
from _g_ to _o_ are used. Hence _g_, _h_ and _i_ are used for getting
both curves, the divisions from _a_ to _b_ and from _o_ to _p_ being
taken up by the flat top and bottom of the thread. It will be noted that
in Figure 207, the top of the thread is drawn first, while in Figure
208 the bottom is drawn first, and that in the latter (for the U.S.
standard) the pitch is marked from centre to centre of the flats of the
thread.

[Illustration: Fig. 210.]

To draw a square thread the pencil lines are marked in the order shown
in Figure 210, in which 1 represents the centre line and 2, 3, 4 and 5,
the diameter and depth of the thread. The pitch of the thread is marked
off by arcs, as 6, 7, etc., or by laying a rule directly on the centre
line and marking