Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 182

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 it coming off, turn back the thread of the cramp with a wrench that fits the square, withdrawing the screw with it. It is a keen sense of touch, not eyesight, which determines the degree of pressure of the cramp screw, as the broken part is turned back far enough to enable a small hand vice to grip it, and turn it right out. It is believed that this device was first suggested by a workman named Crundle.

Fig. 476.—Using Screwdriver.