Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 181

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.