Table 67.-CHANGE WHEELS FOR SCREW-CUTTING. LEADING SCREW, 3 THREADS TO THE INCH. Whitworth's Standard Screw-Threads for Engineers' Taps.— The change wheels for cutting these threads are given in Table 83, page 251; and the proportions of screws and bolts in Table 89, page 255. Whitworth's Standard Gas Screw-Threads, for gas piping.-The change wheels for cutting these threads are given in Table 86, page 253. Whitworth's Standard Screw-Threads for Hydraulic Pipes, and gas and water pipes-and the correct thickness of metal for these pipesare given in Table 88, page 254. Whitworth's Standard Screw-Threads for Watch and Instrument Makers are given in Table 90, page 256. Whitworth's Standard Sizes for Nuts and Bolt Heads are given in Table 108, page 285. Table 68.-CHANGE WHEELS FOR SCREW-CUTTING. LEADING SCREW, The above table will suit a lathe with a leading screw of inch pitch by dividing the mandrel wheel by 2. Cutting Right-hand and Left-hand Screws.-In cutting a righthand thread, the tool in a lathe travels from right-hand to left-hand, and in cutting a left-hand thread, the tool travels from left-hand to right-hand. Double and Treble Threads.-The distance between the centres of the threads of a screw is only one-half the actual pitch in a double-thread screw, and one-third the pitch in a treble-thread screw. To cut double or treble threads, find the wheels to cut a screw of the required pitch with a single thread, and multiply the number of teeth in the lathe spindle-wheel by the number of threads to be cut—that is, by 2 for a double-thread, or by 3 for a treble-thread-the product will be the number of teeth in the lathe spindle-wheel; the other wheels to complete the set will be the same as for a single thread. In cutting a double-thread screw, a single thread is first cut, a mark is then placed on a tooth of the lathe spindle-wheel and on the space it occupies in the first driven wheel, the change wheels are thrown out of gear and the lathe spindle is turned round, and the wheels are re-placed in gear at one-half the number of teeth of the wheel beyond the marked tooth; the lathe is then ready for cutting the second thread. The wheels for cutting three or more threads can be found in a similar way. Table 69.-CHANGE WHEELS FOR SCREW-CUTTING. LEADING SCREW, The above Table will suit a lathe with a leading screw of inch pitch by dividing the first driving-wheel by 2. Weight of Screws.-The weight of a screw with a single thread is approximately equal to that of a solid bar, whose diameter is equal to the diameter of the screw minus the depth of thread. Thus, the weight of a single-thread screw, of 3 inches diameter, with a thread inch deep, would equal that of a solid bar―of the same material—of 2 inches diameter. The Strength of Screws and Bolts is given at pages 283 and 284. The proportion of V and square threads are given at page 256. Table 70.-CHANGE WHEELS For Cutting WHITWORTH'S SCREW Threads FOR GAS, WATER, AND HYDRAULIC IRON PIPING. LEADING SCREW, 2 THREADS PER INCH. NOTE.-All larger sizes of piping have 11 threads per inch. Table 71.-CHANGE WHEELS FOR CUTTING SCREWS FROM INCH TO 4 INCH PITCH. LEADING SCREWS,, AND INCH PITCH. 3 LEADING SCREW IN. PITCH. Leading Screw IN. PITCH. LEADING SCREW IN. PITCH. Pitch of Thread to be Cut. Table 72.-CHANGE WHEELS FOR CUTTING PITCHES IN MILLIMETRES, FOR LATHES WITH LEADING SCREWS OF, AND INCH PITCH. 17 23456 NON SE D 35 45 50 100 35 45 75 100 63 35 160 63 30 50 80 63 63 40 50 80 63 40 63 50 50 80 63 50 63 60 50 80 63 60 80 63 70 80 63 80 75 ས་་་་N 30 75 80 63 75 80 63 40 100 80 75 80 63 50 100 80 75 80 63 60 100 80 75 80 63 70 100 80 80 63 9 80 63 90 75 80 63 90 100 80 80 63 100 80 63 80 63 100 100 80 75 80 63 IIO 100 80 50 40 63 65 75 40 60 63 63 75 40 63 70 80 50 63 75 40 63 16 63 80 40 63 85 40 48 60 50 63 95 40 50 63 95 60 50 63 45 50 50 85 80 50 90 80 50 95 80 50 20 63 100 40 50 63 100 60 50 63 100 80 50 21 63 105 40 22 63 40 50 63 60 30 50 63 60 25 70 90 40 40 70 26 63 65 25 28 63 70 20 50 63 70 30 50 63 70 30 63 90 40 30 63 45 63 90 60 30 25 63 60 45 25 63 60 85 40 25 63 85 60 25 63 85 70 40 20 63 70 60 20 63 70 90 50 30 63 38 63 95 40 25 63 40 63 80 40 20 63 105 40 25 63 105 44 63 110 40 25 63 IIO 60 25 63 105 63 90 40 46 63 115 40 48 63 90 30 50 20 63 90 90 60 75 ទន9 99 99 99 99 9 9 9 9 9 9 Millimetre pitches are the best for small screws. Where very great accuracy is required, a wheel with 127 teeth should be substituted for the 63 wheel in the above table, and the remainder of the set of wheels altered accordingly. |