But as the
rule has to be applied along line 1 to mark line 7 for the length of the
bolt, it is just as easy to mark the head thickness at the same time.
The line 8 showing the length of the thread may be marked at the same
time as the other lengths are marked, and the outlines 9, 10, 11, 12, 13
may be drawn in the order named. We have now to mark the arcs at the top
of the flats of the head to show the chamfer, and to explain how these
arcs are obtained we have in Figure 153 an enlarged view of the head. It
is evident that the smallest diameter of the chamfer is represented by
the circle A, and therefore the length of the line B must equal A. It is
also evident that the outer edge of the chamfer will meet the corners at
an equal depth (from the face of the nut), as represented by the line C
C, and it is obvious that the curves that represent the outline of the
chamfer on each side of the head or nut will approach the face of the
head or nut at an equal distance, as denoted by the line D D. It follows
that the curve must in each case be such as will, at each of its ends,
meet the line C, and at its centre meet the line D D, the centres of the
respective curves being marked in the figure by X.

[Illustration: Fig. 153.]

It is sufficiently accurate, therefore, for all practical purposes to
set the pencil on the centre-line at the point A in Figure 152 and mark
the curve 14, and to then set the compasses by trial to mark the other
two curves of the chamfer, so that they shall be an equal distance with
arc 14 from line 9, and join lines 10 and 13 at the same distance from
line 9 that 14 joins lines 3 and 4, so that as in Figure 153 all three
of the arcs would touch a line as C, and another line as D.

[Illustration: Fig. 154.]

The United States standard sizes for forged or unfinished bolts and nuts
are given in the following table, Figure 154 showing the dimensions
referred to in the table.

UNITED STATES STANDARD DIMENSIONS OF BOLTS AND NUTS.

KEY:
A: Nominal. D.
B: Effective.[*]
C: Standard Number of threads per inch.

------------------------+----------------------------+---------------------
BOLT. | BOLT HEAD AND NUT. |
------------------------+-----------------+----------+----------+----------
Diameter. | | Long diameter, | Short | |
-------+------| | I, or diameter | diameter | |
| | | across corners. | of |