Fig. 471.—Secret Nailing.
Fig. 472.—Flat-head Wood Screw.
Fig. 473.—Round-head Wood Screw.
Fig. 474.—Cup Wood Screw.
SHAPES OF WOOD SCREWS.
Screws, which, like nails, consist of two parts—head and shank—are made in almost endless variety, but the three following examples are sufficient for the present purpose. Fig. 472 is the most common, being made from iron, steel, or brass, according to the purpose for which it is to be used. Fig. 473 shows a round-headed screw. These are usually japanned, being used principally in fixing bolts, locks, etc. Fig. 474 is a variety known as a cup screw, because it has a cup into which the head of the screw fits; this cup is let into the work flush with the surface, the screw being turned down through it. It will be noticed that the thread extends for only about two-thirds the length of the screw, this being all that is really requisite. Further threading would only weaken the lateral strength of the screw. The sizes in which wood screws are made are almost endless, but usually they range from 1/4 in. to 6 in.
Fig. 475.—Wood Bored ready for Screw.
DRIVING IN SCREWS.
For starting the screw, a hole should be bored through the top piece sufficiently large to allow the screw to pass freely, as A (Fig. 475). This may be done with a gimlet, the top of the hole being enlarged with a gouge to receive the head of the screw, so that it may be a trifle below the surface. This is shown at B B (Fig. 475). The two pieces of wood should now be placed together, and a bradawl inserted in the bottom piece (as G), to help the screw in its course. Drive home the screw as follows: Take the screwdriver in the right hand, and guide its end into the cut on the head of the screw with the left hand. The handle should now be turned from left to right (or “clockwise”), using both hands in the process, until the screw is driven as tightly down as possible. The method of holding the screwdriver is shown by Fig. 476, this, however, not showing the left hand, though this invariably must be employed. The practice of putting screws in the mouth just before using them is bad; it causes the screws to rust and to be difficult to withdraw. Before inserting screws in woodwork, dip their points in grease. This assists both insertion and withdrawal, and keeps the screws free from rust. A small box to hold grease is a useful adjunct of the bench.
WITHDRAWING DIFFICULT SCREWS.
For one particular job, at any rate, the long screwdriver is more powerful than the short one, and that is the starting of stiff screws. Let D (Fig. 477) represent the screw, and D E the centre line of screw produced. Take a screwdriver, say of half the length of D E, and, for the sake of simplification, ignore the width of handle, and let the driver be represented by F. The leverage of the driver in starting the screw will depend upon the amount it may be safely canted from the vertical line. In Fig. 477 this is represented by F C. Now, the amount of safe canting will be the same for a long or short driver if the points are similar; so a driver of twice the length of F is taken and represented by P. It will be seen at a glance that the leverage is now twice that of F; for whereas in the first instance it is represented by F C, it is now represented by P E. So the greater power, whenever present, of a long driver depends on the canting of the tool from the screw centre line. Often an old screw is in so tight that it cannot be loosened by the screwdriver. The best plan then is to get a piece of bar iron, flat at the end, make it red hot, and place it on the head of the rusty screw; remove the iron in two or three minutes, and then the screw can be drawn with the screwdriver as if it had only been recently inserted. The expansion of the screw by the heat breaks the rust contact previously existing between it and the wood. If these methods fail, the wood has to be bored out with a shell bit, the screw coming away with the core.
REMOVING BROKEN-IN SCKEWS.
Many years ago screws were so badly made that coach-makers preferred to put countersunk clout nails in the hinges of carriage doors, as on extra strain the screws were apt to break off at the end of the threaded part; or if the head impinged a little more on one edge of the countersink than the other, the countersink of the hinge being iron recessed conically and unyieldingly to the canted head, not at right angles to the countersunk face. The attempt to use a screw in hard wood had a like effect, but to a less degree. If the cone of the countersink were fainter in its angle than the cone of the screw head, the chances of a screw breaking would be lessened, as the neck of the cone of the screw head would take the bearing on one side of the hinge, or wood countersink. The screw (see Fig. 478) is bent out of straight at its weak part, and at every half-turn round it is bent back when the opposite side of the screw is turned to B, which, in effect, is the same as holding the thread part tightly, while the stem part A is crooked first one way then the other, the strain being augmented also by the tightening of the thread part producing a torsion strain on the weak part of the screw. A broken screw in the hinge of a heavy house door or of a carriage door was, and is, a serious matter. One plan, in the case of a carriage door, was to bore a larger hole from the other side of the pillar opposite the point of the screw, and punch the broken part through the hole; then plug up with a wood pin and glue, which needed a few hours to dry hard before it could be bored into for another screw. Another plan was to drive a clout nail in to fill the hole, beside the broken screw. Sometimes the head would be the trouble. If a blunt screwdriver were driven into the narrow slot, half the head would fly off; even impinging on the countersunk hole, more one side than the other, would cause the head to break half-side off. If the screw could not be got out, the remedy then was to leave it, and put in putty to hide the mischief; but if not quite tight it had to be got out somehow. To get out of a hole a tight screw without a head was not an easy task, and would give much trouble at the present day, with all the many handy tools now available for any troublesome jobs. It is understood easily then why countersunk clout nails were used in preference to screws in post-chaise and stage-coach work in many shops, and often in door hinges on oak door posts. A method of withdrawing a headless screw from a hole is to cut with a fine chisel the stem of the screw to the shape of a triangle; then fit on it a triangular steel pipe stem, like a short length of a padlock key, and make this square on the projecting part for an inch, and with an iron cramp force this key tightly on the broken screw; while holding it thus tightly, which prevents