Fig. 350.
For gas, steam pipe, and other connections made by means of screw threads, and which require to be without leak when under pressure, the tap shown in [Fig. 350] is employed. It is made taper and full threaded from end to end, so that the fittings may be entered easily into their places and screwed home sufficiently to form a tight joint.
The standard degree of taper for steam-pipe taps is 3⁄4 inch per foot of length, the taper being the same in the dies as on the taps. The threading tools for the pipes or casings for petroleum oil wells are given a taper of 3⁄8 inch per foot, because it was not found practicable to tap such large fittings with a quick taper, because of the excessive strain upon the threading tools. Ordinary pipe couplings are, however, tapped straight and stretch to fit when screwed home on the pipe. Oil-well pipe couplings are tapped taper from both ends, and there is just enough difference in the taper on the pipe and that in the socket to show a bearing mark at the end only when the pipe and socket are tested with red marking.
PITCHES OF TAP THREADS IN USE IN THE UNITED STATES.
| Diameter. | Length. | No. of Threads to Inch. | Diameter. | Length. | No. of Threads to Inch. | ||||||||||
| 1⁄4 | 2 | 3⁄4 | 16, | 18 | & | 20 | 3⁄4 | 5 | 13⁄16 | 10, | 11 | & | 12 | ||
| 5⁄16 | 2 | 7⁄8 | 16 | & | 18 | 13⁄16 | 6 | 10 | |||||||
| 3⁄8 | 3 | 1⁄2 | 14 | & | 16 | 7⁄8 | 6 | 1⁄8 | 9 | & | 10 | ||||
| 7⁄16 | 3 | 13⁄16 | 14 | & | 16 | 15⁄16 | 6 | 3⁄8 | 9 | ||||||
| 1⁄2 | 4 | 5⁄16 | 12, | 13 | & | 14 | 1 | 6 | 13⁄16 | 8 | |||||
| 9⁄16 | 4 | 3⁄4 | 12 | & | 14 | 1 | 1⁄8 | 7 | 1⁄4 | 7 | & | 8 | |||
| 5⁄8 | 5 | 1⁄8 | 10, | 11 | & | 12 | 1 | 1⁄4 | 8 | 7 | & | 8 | |||
| 11⁄16 | 5 | 3⁄8 | 11 | & | 12 | ||||||||||
Fig. 351.
[Fig. 351] represents the form of tap employed by blacksmiths for rough work, and for the axles of wagon wheels. These taps are given a taper of 1⁄2 inch per foot of length, and are made with right and left-hand threads, so that the direction of rotation on both sides of a wagon wheel shall be in a direction to screw up the nuts and not to unscrew the nut, as would be the case if both ends of the axle were provided with right-hand threads.
Taps that are used in a machine are sometimes so constructed that upon having tapped the holes to the required depth, the pieces containing the tap teeth recede from the walls of the hole, so that the tap may be instantly withdrawn from the hole instead of requiring to be rotated backwards. This is an advantage, not only on account of the time saved, but also because the cutting edges of the teeth are saved from the abrasion and its consequent wear which occur in rotating a tap backwards.