Basics of thread

Threads varies in many ways. In the following we explain the basics of thread. You can also check out our FAQ section for thread.

GENDER

Every matched pair of threads, external and internal, can be described as male and female.  For example, a screw has male thread, while the matching hole has female thread.

HANDEDNESS

The helix of a thread twists in one of two possible directions. Most threads are oriented so that the threaded item when seen from a point of view on the axis through the center of the helix, moves away from the viewer when it is turned in a clockwise direction and moves towards the viewer when it is turned counter-clockwise.

By convention right-handedness is the default handedness for screw threads. Most threaded parts and fasteners therefore have right-handed threads.

DESIGN

The type of thread can be identified by the following characteristics: parallel and tapered design (see images right).

PITCH/TPI:

The pitch is the distance from the crest of one thread to the next in milimeters (mm). 
TPI (threads per inch) is used by inch thread.

DIAMETER

The major diameter is determined by the thread tips.
The minor diameter is determined by the groove of the thread.
The pitch diameter is the distance between the tips of two opposite flanks or the length of the centreline of the profile.

ANGLE

The flank angle is the angle between the flank of a screw thread and the perpendicular line to the axis of the screw. Tapered threads have a taper angle. This is the angle between the taper and the center axis of the pipe.

CREST/ROOT

The outermost part of the thread is called 'crest', the innermost part of the thread is called 'root'.