A screw thread may be conceived as formed on a cylinder by winding thereon a piece of triangular wire, as shown in Fig. 183. N is a nut, threaded inside to turn on the screw.

If the wire is wound as shown on the end B, the thread is right handed, if wound in the reverse direction, as on the end D, the thread is left handed. A right-handed thread is one on which a nut is screwed by turning the nut in the direction of the hands of a watch when the bolt end is pointed toward the operator.

The distance between two adjacent ridges of the thread, as CC, is the pitch of the screw.

If two triangular wires (of the same size) are wound side by side on the cylinder the thread is a double thread. The pitch remains the same, but a nut turned one turn on the double thread will advance twice the pitch. The distance which the nut advances in one turn is called the lead of a screw, and it will be seen that in this case the lead is twice the pitch.

The form of a thread is the profile it shows in a section made by a plane passing through the axis of the cylinder on which the threads are cut. The form most used is the V thread, and other forms used for special purposes are the square thread, the Acme or worm thread, and the buttress or trapezoidal thread. These forms are shown in Fig. 184.

Buttress Thread.

Acme or Worm Thread.

Fig. 184. - Forms of Threads.