The screw-driver is too familiar to need description, but in buying one see that the end is shaped like either of those shown in Fig. 663 and not as shown in Fig. 664. Cheap screw-drivers are often made in the latter way. If ground with a short bevel (Fig. 664) it will bear only on the top of the slot in the screw and will be all the time slipping out, on the principle of the inclined plane, while if the sides are parallel or concaved slightly the end will remain at the bottom of the nick of the screw. This is also a help in extracting screws, as it saves the need of pressing against the screw so hard to keep the screw-driver from slipping out of the slot. It is well to have screw-drivers of different sizes, as it is difficult and often impossible to use a screw-driver with an edge much too large or too small.

Remember that a long screw-driver is always preferable to a short one, except where lack of space makes a short one necessary. The reason for this is in the fact that in using the screwdriver you do not, as a practical matter, keep it exactly in the line of the screw, but keep wobbling it round more or less, which gives a leverage in the form of a crank-like action as you turn the handle. The longer the screw-driver the larger the circle or wobbling curve you describe with your hand and the greater the leverage (Fig. 665).

A screw-driver to be turned by the bit-brace is very useful for driving screws rapidly and with force, on account of the greater leverage gained by using the brace instead of the common handle. This is particularly useful where they need to be driven in very hard or when tight screws have to be loosened. It also saves much time when many screws are to be used. It is not advisable to buy automatic screw-drivers. They work satisfactorily for light work, but are not suitable for such wrenching and straining as your screw-drivers are liable to be subjected to. You want screw-drivers to which you can apply all your strength. See Screws.

Screws, - There are many kinds of screws. You will use the common wood-screws for most of your work. These are either flat-headed or round-headed, and of steel (either bright or blue or bronzed or nickled) or of brass. When others are required they will be mentioned. It is doubtful economy to buy second-hand or waste screws, but a pound or two of " mixed " screws, which you can get at any hardware store, will be very useful when you want some odd screw for some special purpose. Many of the screws in the " mixed " lots, which are sold very cheap, are defective, but you can often find among them just the peculiar screw you need, and so save time and money.

Fig. 663.

Fig. 664.

Fig. 665.

Nails are often used where it would be better to use screws, which will, as a rule, hold the pieces more securely. When work becomes loose, screws can be tightened, while nails usually have to be redriven.

To make a screw drive easily, rub the point on a piece of common soap. Oil is objectionable for nice work on account of the spot made by it. If screws are to be used in places where they may rust, it is a good plan to warm them slightly and then dip them in melted tallow or lard. They can also be inserted and removed more easily for this treatment. Try to keep the screwdriver from slipping from the slot of the screw (see Screw-driver). In boring holes for screws, considerable discretion must be used. The hole in the outer piece (the one nearer the head of the screw) should be large enough to allow the screw to slip through freely - that is, you should not screw it into both pieces, but only the inner one, the screw acting somewhat in the nature of a clamp to bind the outer piece to the inner by pinching it tight between the screw-head and the inner piece (Fig. 666). How much of a hole to bore in the piece into which the point of the screw enters depends on circumstances. The stouter the screw the less hole required. The softer and larger the piece and the farther from the edge the less hole required. If the piece is small or liable to split, the hole must be carefully made - the more carefully in proportion to the slenderness of the screw, as a slim screw is liable to twist off in hard wood unless a sufficient hole is provided. Brass screws are very apt to do this, and much care must be used, particularly with slender ones in hard wood. If the hole is a bit too large, they will not hold. If a trifle too small, they will twist off, which is very annoying, especially in such cases as hinge-screws, for instance, where the place for the screws cannot well be changed. The hole should be somewhat smaller than the diameter of the screw. In good-sized pieces of soft wood there is frequently no need of any hole.

Fig. 666.

In rough work, especially in soft wood, the screws may be pounded part way with the hammer, driving them home with the screw-driver. Some theoretical workman will be quite sure to tell you never to do such a thing as that, so be sure to understand what is meant. Theoretically there may be some loss of holding power by that process, but practically the screws will hold just as well for the cases in which you are advised to do that way. Judgment must be used about all such things and theories are only of value when used by the light of common-sense. For example, if you are screwing the top on a mahogany table or framing a nice boat never think of using a hammer to start your screws, but if you are putting cleats on an old shed door or screwing up a packing-case do not spend an hour and a lot of strength driving screws all the way with a screw-driver when you can do the work in half an hour by driving the screws three quarters of the way in with the hammer. Good practical workmen are just as certain to use the hammer in such cases as they are careful not to use it for nice work or where the full holding power of the screw is needed.

Flat-headed screws almost always should be countersunk (see Countersink), for neatness if for no other reason, and in hard wood you should cut the depression for the head of the screw with the regular countersink made for the purpose. This should be done for nice work in soft wood where a good surface is required, but for common work in soft wood there is no need, as a rule, for the head of the screw will sink itself easily until flush with the surface.

If a screw hole requires to be moved a little, but not far enough so that a new hole can be bored without the bit slipping into the old hole, plug the old hole with a wooden pin dipped in glue, and when dry bore the new hole where required.

See Screw-driver.