Wood-screws, i.e., metal screws with thin, deep, sharp-edged tap-worms, are used for screw-joints. Screws which are gimlet-pointed penetrate the wood more easily than others.

The wood-screws used in screw-joints are of different kinds, with half-round, curtailed conical, or square heads. The two first only are used in wood-slojd. In both, the head of the nail is furnished with a slit for receiving the screwdriver.

Jointing with wood screws.

Fig. 87. Wood-Screws.

A with half-round, B with conical head.

When A (Fig. 87) is used, the head of the screw remains above the surface of the wood. In joints made with B, the head is made to lie level with the surface, for which purpose the hole bored for its reception is afterwards counter-sunk. Wood-screws are made in many-lengths from about 1/4 inch to 3 inches, and of varying thickness. They are very generally-used, and are especially useful for articles which require sometimes to be taken apart and put together again.

In consequence of their peculiar form, screws give a much stronger and firmer joint than nails, which hold the pieces together simply by friction. A screw cannot be drawn out without unscrewing, unless the wood around it is cut away. The hole bored for the reception of the screw should be as deep as the length of the unwormed portion.

Strength of a screw joint