Certain definitions in regard to the screw should be carefully noted. A screw may be either right-handed or left-handed. Right-handed means that, when turning it into a nut or threaded hole the screw must be turned in the same direction as the hands of a clock. When the thread inclines or slopes so that the under side is nearer the right hand, it is right-handed.

The thread shown in Fig. 113 is a single thread. Figure 114 shows a double thread. If three threads are wound around the cylinder it would have a triple thread. Four or five threads are sometimes wound around the cylinder, but this type is not often found in shop practice.

The distance from the bottom of one groove to the bottom of the next is called the pitch, as P in Fig. 114. The pitch is always the distance from one thread to the next, no matter whether it is single, double, or triple thread. The distance that a screw enters a nut or hole for one complete turn is called the lead. For a single thread the lead is equal to the pitch, for a double thread the lead is twice the pitch, and for a triple thread the lead is three times the pitch.

(a) Round Head

(a) Round Head.

(b) Flat Head

(b) Flat Head.

(c) Fillister Head

(c) Fillister Head.

Fig. 112. - Bolts.

Fig. 113.   Single Thread Screw.

Fig. 113. - Single-Thread Screw.

The point of a thread is the projecting end. The diameter of a thread is the distance measured over it, and is the same as the diameter of the bolt before the thread is cut. The perpendicular distance from the top of the thread, to the bottom of the groove is called the depth or height; twice this distance is called double-depth. The root is the bottom of the groove. The diameter at the root is the outside diameter minus the double depth. This is called the root diameter.