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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 pipes—

are given in Table 88, page 254.

Whitworth's Standard Screw-Threads for Watch and Instru

ment Makers are given in Table 90, page 256.

Whitworth's standard Sizes for Nuts and Bolt Heads are given

in Table 108, page 285.

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Table 68.—CHANGE WHEELs for Screw-cutting. LEADING Screw, 4 THREADs to the INch.

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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 72.-CHANGE WHEELs for CUTTING Pitches IN MILLIMETREs, For LATHEs with LEADING ScREws of +, + AND # INch Pitch.

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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 *pproximately 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

*qual 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.

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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,

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LEADING Screw 4 INCH LEADING Screw : Inch LEADING Screw INch Pitch of Pitch. Pitch. Pitch.

Screw to be Cut.

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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. Q

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