# The Key Geometric Parameters Of The Thread

To design and manufacture the thread, we should have some knowledge of the thread, especial the geometric parameters of the thread. Then we can select the suitable thread machining processes like turning, milling, whirling and tapping etc to finish the task. There are nine key parameters for us to get the whole picture of the thread type.

1)Major diameter

Major diameter is the largest diameter of the thread. Outside or external thread diameter means the diameter of a screw and includes a raised helix height around the thread. The major diameter of the screw shaft is the distance between two opposite crests, while the major diameter of the nut is the distance between two opposite roots.

2) Minor diameter

The minor diameter is the smallest diameter of the thread. The minor diameter of the screw shaft is the distance between two opposite roots, while the minor diameter of the nut is the distance between two opposite crests.

3）Pitch diameter (Effective Diameter)

Effective or pitch diameter is the diameter of an imaginary co-axial cylinder that can intercept the surface of the thread, which lies concentrically and approximately halfway between the major and minor diameters. Pitch diameter can cross so that the intercept on a cylinder generator at the point where pitch diameter meets the opposite flanks of the thread groove. The thread groove is equal to half of the nominal pitch of the screw thread.

You can use a thread pitch gauge tool to check and identify the thread diameter and size.

4) Pitch

The Pitch of the thread, or we can say it as thread pitch, is the a distance from a point on the same screw thread to a corresponding threshold.

The thread pitch will be on the next thread that can measure parallel to the axis. It should be between corresponding points on an adjacent surface in the same axial plane.

The lead and pitch are the same on a single-threaded screw, while the lead is two times the pitch on a double-threaded screw. The lead is three times the pitch on a triple-headed screw.

The number of starts refers to the number of independent threads running around the length of the thread. The lead of a screw is determined by multiplying the number of independent threads by the pitch. A single-start lead screw has a lead that is equivalent to its pitch. a single-threaded screw has one thread start. a double-threaded screw has two thread start.

7)Helix Angle

The helix angle is the angle formed between the helix of the thread and the line perpendicular to the axis of rotation. Generally, a lead screw with a higher helix angle has lesser frictional losses and therefore has higher efficiency.