m.

CHAPTER VII.

_EXAMPLES IN BOLTS, NUTS, AND POLYGONS._

[Illustration: Fig. 149.]

[Illustration: Fig. 150.]

[Illustration: Fig. 151.]

Let it be required to draw a machine screw, and it is not necessary, and
therefore not usual in small screws to draw the full outline of the
thread, but to represent it by thick and thin lines running diagonally
across the bolt, as in Figure 149, the thick ones representing the
bottom, and the thin ones the top of the thread. The pencil lines would
be drawn in the order shown in Figure 150. Line 1 is the centre line,
and line 2 a line to represent the lower side of the head; from the
intersection of these two lines as a centre (as at A) short arcs 3 and
6, showing the diameter of the thread, are marked, and the arcs 5 and 6,
representing the depth of the thread, are marked. The arc 7,
representing the head, is then marked. The vertical lines 8, 9, 10, and
11 are then marked, and the outline of the screw is complete. The thick
lines representing the bottom of the thread are next marked in, as in
Figure 151, extending from line 9 to line 10. Midway between these lines
fine ones are made for the tops of the thread. All the lines being
pencilled in, they may be inked in with the drawing instruments, taking
care that they do not overrun one another. When the pencil lines are
rubbed out, the sketch will appear as in Figure 149.

[Illustration: Fig. 152.]

For a bolt with a hexagon head the lines would be drawn in the order
shown in Figure 152. At a right-angle to centre line 1, line two is
drawn. The pencil-compasses are then set to half the diameter of the
bolt, and from point A arcs 3 and 6 are pencilled, thus showing the
width of the front flat of the head, as well as the diameter of the
stem. From the point where these arcs meet line 2, and with the same
radius, arcs 5 and 6 are marked, showing the widths of the other two
flats of the head. The thickness of the head and the length of the bolt
head may then be marked either by placing a rule on line 1 and marking
the short lines (such as line 7) a cross line 1, or the pencil-compasses
may be set to the rule and the lengths marked from point A. In the
United States standard for bolt heads and nuts the thickness of the head
is made equal to the diameter of the bolt. With the compasses set for
the arcs 3 and 4, we may in two steps, from A along the centre line,
mark off the thickness of the head without using the rule.