External and internal threads

. uses of screw threads

. differentiate between external and internal threads

. elements of screw threads

What is screw threads?

A screw thread is a ridge of uniform section formed helically on the surface of a cylindercal body.

An external screw thread is formed on the outer surface of a cylindercal part. Example bolts, screws, studs, threaded spindle etc.

An internal screw thread is formed on the inner surface of a hollow cylindercal part.example nuts, threaded lids etc.

External threads and internal threads are assembled together for different engineering uses.

Uses of screw threads

Screw threads are used

As fasteners to hold together and dismantle components when needed.

To transmit motion on machines from one unit to another

To make accurate measurements

To apply pressure

To make adjustment

Parts of a screw thread

Crest

The top surface joining the two sides of a thread

Root

The bottom surface joining the two sides of adjacent threads

Flank

The surface joining the crest and the root

Thread angle

The included angle between the flanks of adjacent threads.

Depth

The perpendicular distance between the roots and crest of the thread

Major diameter

In the case of external threads it is the diameter of the blank on which the threads are cut and in the case of internal threads it is the largest diameter after the threads are cut that are known as teh major diameter.

This is the diameter by which the sizes of screws are stated.

Minor diameter

For external threads, the minor diameter is the smallest diameter after cutting the full thread. In the case of internal threads, it is the diameter of the hole drilled for forming the thread which is the minor diameter.

Pitch diameter

The diameter of the thread at which the thread thickness is equal to one half of the pitch.

Pitch

It is the distance from a point on one thread to a corresponding point on the adjacent thread measured parallel to the axis.

Lead

Lead is the distance a threaded component moves along the matching component during one complete revolution. For single start thread the  lead is equal to the pitch.

Helix angle

The Angle of  inclination of the thread to the imaginary perpendicular line.

Hand

The direction in which the thread is turned to advance. A right hand thread is turned clockwise to advance, while a left hand thread is turned anticlockwise.