A tap consists of an external screw of the required size, formed of steel and more or less tapered, part of the thread being cut away by longitudinal grooves in order to present a series of cutting edges. By screwing into a nut in the manner of an ordinary bolt this tap forms the thread required.
Plug-taps are usually made in sets of three. The first, called the entering tap or taper tap, generally tapers regularly throughout its length; the second, or intermediate tap, sometimes tapers, but is usually cylindrical with two or three tapering threads at the end; the third, called the plug-tap or bottoming-tap, is always parallel, with the full thread carried to the end.
Fig. 615.
Fig. 616.
Fig. [615] shows a crow. This is used to hook underneath a pipe and to support and feed a ratchet drill in cutting a hole. The sliding head is fastened by a double ended gib key which secures it in any desired position. A swivel bench vise is shown in Fig. [616]. This tool has cast steel jaws with a wrought iron slide and is attached to the bench with a screw so that it may be turned in any position.
A pipe vise is shown in Fig. [617]. This is mounted on a journal bearing so that it may be clamped in any position from a horizontal to a perpendicular.
