Fig. 664.
Screw-Driver.—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 screw-driver 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).
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.
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 screw-driver from slipping from the slot of the screw (see Screw-driver).
