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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1869 edition. Excerpt: ... Dies finished with such a hob cannot cut much at the outer edges, because the large curve of the die-gap causes the bolt to bear at the bottom of the gap when the screwing begins, and through this contact being effected most of the blunt character of such dies arises. It is to secure the steady bearing while cutting that dies made with master-taps are used, because little or no care is necessary to use them, through the firm grip which is given immediately the dies are screwed tight to the bolt. Probably it is better to sacrifice a little of the firm bearing quality, and to secure a power for cutting, because for nearly all the screws that are made with dies, an accurate thread is not required; superior threads are always expected from lathe-screwing, not from dies; therefore dies made withlsmagl hobs are preferable for most of the general screw-makin g when dies are the instruments emp o e. 13 pair of dies finished with a master-tap are also much more liable to stretch the screw of a bolt while being screwed, than a pair screwed with a small hob, through the greater pressure and friction resulting from the greater bearing surface during the entire time of screwing; and this stretching is often sufficient to render a screw which is only two inches long, unfit for its nut, box, lever, rod, or other piece for which the screw is made. The stretching places the summits of the thread further apart than they should be, and the screw must therefore be reduced below its proper diameter to get it into its place. A pair of master-tap dies are also much weakened by the comparative large gaps for shavings, such gaps being necessary to cause the dies to cut at the bottoms of their screw-gaps. Master-tap dies are represented in Figs. 607 and 608, by...