e has been bored as is often the
case in soft wood. The thread extends about two-thirds the length of
the screw. Any longer thread would only weaken the screw where it most
needs strength, near the head, and it does not need friction with the
piece thru which it passes.

The size of screws is indicated by their length in inches, and by
the diameter of the wire from which they are made, using the standard
screw-gage, Fig. 220, p. 117. They vary in size from No. 0 (less than
1/16") to No. 30 (more than 7/16") in diameter, and in length from
1/4" to 6".

[Illustration: Fig. 229. a. Flat-head Wood-screw. b. Round-head
Wood-screw. c. Fillister-head Wood-screw. d. Oval-countersunk-head
Wood-screw. e. Drive-screw. f. Square-head (lag- or coach-) Screw.]

The following is a good general rule for the use of screws: make the
hole in the piece thru which the screw passes, large enough for the
screw to slip thru easily. Countersink this hole enough to allow the
head to sink flush with the surface. Make the hole in the piece into
which the screw goes small enough for the thread of the screw to catch
tight. Then all the strength exerted in driving, goes toward drawing
the pieces together, not in overcoming friction. The hole must be deep
enough, especially in hard wood and for brass screws, to prevent the
possibility of twisting off and breaking the screw. Soap is often
useful as a lubricant to facilitate the driving of screws. Where it is
desirable that the heads do not show, a hole may first be bored with
an auger-bit large enough to receive the head and deep enough to
insert a plug of wood, which is cut out with a plug-cutter, Fig. 131,
p. 84, and glued in place. If pains are taken to match the grain, the
scar thus formed is inconspicuous.

In rough work, the screw may be driven into place with a hammer
thru most of its length, and then a few final turns be given with a
screwdriver, but this breaks the fibers of the wood and weakens their
hold. In "drive-screws," Fig. 229, e, the slot is not cut all the way
across the head, in order that the blows of the hammer may not close
the slot.

The advantages of screws are, that they are very strong and that
the work can easily be taken apart. If they loosen they can be
retightened. The disadvantages are, that they are expensive, that they
take time to insert, that they show very plainly, and that they do not
hold well in end grain.

BOLTS

Bolts with nuts are useful where grea