This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
These are intended for cutting screw threads in circular work, such as on the outside of pipes or rods, and in the holes cut in solid work, for the purpose of making screwed joints. Figs. 52-63 show a double-handed screw stock with 4 pairs of dies, and 4 each of taper and plug taps; Fig. 64 is a clock screw plate; Fig. 65, a double-handed screw plate with taps; Fig. 66, Whitworth's screw stock.
Fig. 67 illustrates the centre gauge for grinding and setting screw tools, and the various ways of using it. At a is shown the manner of gauging the angle to which a lathe centre should be turned; at b the angle to which a screw thread cutting tool should be ground; at c the correctness of the angle of a screw thread already cut. At d, the shaft with a screw thread is supposed to be held in the centres of a lathe, and by applying the gauge, as at d or e, the thread tool can be set at right angles to the shaft and then fastened in place by the screw in tool post, thereby avoiding imperfect or leaning threads. At f g the manner of setting the tool for cutting inside threads is illustrated. The angle used in this gauge is 55°. The 4 divisions upon the gauge of 14, 20, 24, and 32 parts to the inch are very useful in measuring the number of threads to the inch of taps and screws. The cost of the gauge is only 2s. 3d.