A screw is a rod of metal, either short or long, having a helical ridge around the outside, the ridge being solid with the remainder of the screw. The ridge of a screw is named the thread, and the helical space which adjoins the thread is the thread-groove. The distance across the thread-groove, measured from the centres of two contiguous summits of the thread, is the pitch or step of the thread; and the distance from any summit to the cylindrical part of the screw is the depth of the thread. That portion of the thread which is the termination of it, and also the commencement of the cylindrical part of the screw, is the bottom of the thread, named also the thread-junction. To make a screw is to form a piece of metal into a screw by cutting or otherwise making a thread-groove into the piece to be screwed. The simplest mode of screw-making which is generally adopted for small work consists in screwing a hard steel screwed plate on to the piece to be made into a screw; a plate of this sort is shown by Fig. 366. Screwed plates are therefore screw formers for rods, wire, small bolts, or any other work not exceeding half an inch in thickness; but for larger work screwed plates are not suitable, except for rough, unimportant work. Small pieces, not exceeding a quarter of an inch in thickness, can be screwed by only one screwing on of the plate, but larger work requires two or three screwings, and in such cases three holes in the plate are used, each of a different size, the smallest one being that which is the diameter of the intended screw. By reference to the Figure 366 it may be noticed that each screwed hole is provided with three gaps, to admit the shavings which are formed during the process of screwing. One screwed plate has frequently sixteen or eighteen holes of different sizes to suit various screws. The taps which are used in connexion with small plates are rotated with an ordinary file handle ; for this purpose each tap is provided with a taper tang, resembling a file tang, instead of a square head for an ordinary tap spanner. A small tap in a handle is denoted by Fig. 365.