Metalwork Screw Threads

Screw Threads A screw thread is a specially shaped ridge (inclined plane) around a cylinder.

Screw Threads Definition “A ridge that moves around a cylinder while at the same time it moves along the cylinder”.

Screw Threads Definition

Screw Threads The correct name for this form is a helix. A screw thread is in fact a helix.

Screw Threads Helix

Screw Threads The screw thread gives us a very quick method of assembling parts. It can also be used as a power source as in a car jack or a vice.

Screw Threads Power Source

Screw Threads Screw Thread Terms There are several terms associated with screw threads the main ones are as follows:

Screw Threads Crest Root Flank Pitch Lead Major Diameter Minor Diameter

Screw Threads Screw Thread Terms

Screw Threads Screw Thread Terms Crest This is the most prominent part of the thread where its flanks meet. Root This is the bottom of the thread groove where the flanks meet.

Screw Threads Screw Thread Terms Flank This is the straight part of the thread between the root and the crest. Pitch This is the distance between corresponding points on adjacent threads, measured parallel to the axis.

Screw Threads Screw Thread Terms Lead This is the distance the screw moves in one revolution along its axis. On a single start thread the lead is equal to its pitch.

Screw Threads Screw Thread Terms

Screw Threads Screw Thread Terms Major Diameter This is the largest diameter of the screw thread when measured over the crest of the thread.

Screw Threads Screw Thread Terms Minor Diameter This is the smallest diameter at the root of the screw thread. It is equal to the major diameter minus twice the thread depth.

Screw Threads Screw Thread Terms

Screw Threads Types of Screw Threads The main types of screw threads used are: ISO Metric Threads Buttress Threads Square Threads ACME Threads

Screw Threads ISO Metric Threads This is an international screw thread system which replaced the British Standards (BSW, BSS and BA threads).

Screw Threads ISO Metric Threads ISO metric threads are designated by the letter M, followed by the nominal size and pitch. Both of these sizes are given in millimetres. E.G.: M10 x 1.25: Nominal size 10mm and pitch of 1.25mm.

Screw Threads ISO Metric Threads

It is used in the screws of quick release vices. Screw Threads Buttress Thread This thread is used to apply thrust in one direction only and is often used as a power source. It is used in the screws of quick release vices.

Screw Threads Buttress Thread

Screw Threads Buttress Thread

Screw Threads Square Thread This thread is suitable for applying thrust in both directions. It is used in screw jacks, bench vices and valve spindles.

Screw Threads Square Thread

Screw Threads Square Thread

Screw Threads ACME Thread The sloped flanks of this thread allow easy engagement of half nuts. These threads are often used in the leadscrews of lathes.

Screw Threads ACME Thread

Screw Threads ACME Thread

Questions What is the correct name for a screw thread? Make a neat sketch of a screw thread and label the following, Crest, Root, Pitch, Flank and Thread angle. Make a neat sketch of an ACME thread. What is the main advantage of a square thread.