The thread of the screw begins in a fine point so that it may penetrate the wood easily where no hole 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.