Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 183

During the last sixty or seventy years screws have been progressively improved, the chief improvements relating to the tougher iron used in the acute chisel edge of the thread, and, as has been mentioned, the screw tapering to a sharp point, with the thread right up to the tip. The American improved screw has a stem smaller in diameter than the thread part, so that the old evil of having to make a big hole in one piece, to prevent it becoming “stem-bound” and not drawing, is averted, though a smaller hole is used (Fig. 479). It is almost impossible for the head of this screw to break half off, as the slot does not extend to the edge of the head, but is as a mortise in it, as the diagram shows. The cutting thread is exceptionally sharp. These screws are made of a mild steel of intense toughness, swaged and rolled cold. The cutting is done laterally, not longitudinally, the latter making a feeble thread. A big attempt, made about ten years ago, to introduce these screws into Great Britain failed, but under circumstances that threw no reflection upon the capabilities of the screw, which mechanically shows great talent in its invention.

Fig. 477.—Advantage of Long over Short Screwdriver in withdrawing Difficult Screw.