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420 SCREWS.
TABLE No. 58.—Dimensions of Whitworth Screws.
Diameter of Number of Diameter of Number of Diameter of Number of
Screw in Threads per
J
Screw in Threads per Screw in Threads per
Inches. Inch. Inches. :
Inch. Inches. Inch.
X 40 IX 1
7 sy
2 3X
T
3
6
24 1H 6 3X 3
X 20 1J* 6 4 3
A 18 1H 5 4X 27/
s
H 16 IX 5 4K 27/
8
A 14 i# 4^ 4X w
% 12 2 4^ 5 2U
% 11 2% 4 5X 25/8
% 10 2# • I 4 5^ 2/8
n 9 2X 3# 5X 2/
2
i 8 3 s/
2 6 2/
z
i% 7 3X 3X
Diameter of Tap Drill.
The diameter of the drill with which to drill for a tap is, if
we want full thread in the nut, equal to the diameter of the
screw at the bottom of the thread, and is, therefore, obtained by
the same formulas. However, in practical work it is always ad-
visable to use a drill a little larger than the diameter of the
screw at the bottom of thread, because in threading wrought iron
or steel the thread will swell out more or less, and a few thou-
sandths must be allowed in the size when drilling the hole. In
drilling holes for tapping cast-iron, a little larger drill is used,
because it is unnecessary in a cast-iron nut to have exactly full
thread. Table No. 59 gives sizes of drill for both wrought and
cast-iron, which give good practical results for United States
standard screws.
Table No. 59 gives sizes of hexagon bolts and nuts. The
size of the hexagon is equal to \/
2 times the diameter of bolt +
>£-inch ; the thickness of head is equal to half the hexagon.
The thickness of nut is equal to the diameter of the bolt.
When heads and nuts are finished they are TVinch smaller.
The table is calculated by the following formulas
:
d = D 1.299
C = 1.155 A E=±A
f= — X
B=1AUA
F—D
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