Threads and screws

Threads A thread is a helical structure used to convert between rotary and linear movement or force. Screws are the most common of all machine elements. A screwed connection is easy to assemble, and can be dis- and reassembled hundreds of times.

Manufacturing threads Threads are generally made by removing material from a cylinder, or less commonly, adding material onto a cylinder. Threads can be made in many ways, including cutting, casting, forming, grinding, lapping and rolling. acturing)

Shape The cross-sectional shape of a thread is often called its form or threadform. It may be square, triangular, trapezoidal, round or other shapes.

Shape The triangular threads, V- threads are used for making screws. The most common shape of threads. Square, round or trapezoid threads are used for transferring force and movement in machines.

Handedness The helix of a thread can twist in two possible directions. Most threads are right-handed. The screw is tightened by turning it clockwise into the hole or nut. For special applications, you need left-handed threads. For example when the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to fretting induced precession.

Angle The angle characteristic of the cross-sectional shape is often called the thread angle. For most V-threads, this is standardized as 60 degrees, but any angle can be used. The thread angle for metric and unified threads is 60°.

ISO metric screw threads Metric threads are standardised by ISO. Metric threads are classified by outer diameter and pitch. The coarse metric thread (pitch is a standard “coarse” value) is designated by the letter M followed by the value of the nominal (outer) diameter: M8 w_thread w_thread

Fine metric threads The fine metric thread is designated by the letter M followed by the value of the nominal diameter and the pitch: M8X1,25

ISO metric screw thread

Unified thread standard The most common screw thread dimensioned in inches, used in USA and Canada. Classified by outer diameter and pitch. Pitch is measured in threads per inch. Coarse threads, example: 9/16-12 UNC Fine threads, example: 9/16-18 UNF Also National Pipe Thread (NPT) ard

Other standards British standard Whitworth British standard pipe thread British Association screw threads Power screw threads Many others

Measuring threads If you do not know what size threads are, you will have to measure them. You need to measure the major or minor diameter. Use caliper. You also need to measure the pitch. Use pitch gauge.

Pitch gauge