A screw thread is a ridge of uniform section in the form of a helix on the external or internal surface of a cylinder. Threads on bolts, screws and studs are examples of "External threads", whilst those in nuts or tapped holes are "Internal threads".
There are so many types of thread sizes, pitches and tolerance classes in both metric and inch configurations that they require some definition and terminology for the specifier. The designation for standard metric coarse threads such as M8 or M12, refers to the nominal diameter of the fastener and the hole required through which it will pass.
The thread form or cross-section is made up of three elements: root, crest, and flank. The root is at the bottom of the thread and the crest at the top of the thread, with the flank joining them.
It cannot be assumed that all 60° thread angled fasteners can be interchanged with other thread type systems with a similar thread angle.. Pitch and tolerances are very important and are specified for best practice engineering.
The following table listing can be cross-referenced to the appropriate international standards for thread forms.
International Organisation for Standardisation (ISO) - ISO Metric Threads
Deutches Institut fur Mormung (DIN) - ISO Metric Threads
American National Standards Institute (ANSI) - Unified and Metric Threads
British Standards Institution (BSI) - ISO Metric, Unified National & British Threads
Thread Form System Thread Angle
ISO Metric Coarse
ISO Metric Fine 60°
Unified National Coarse
Unified National Fine
National Pipe Taper Fine 60°
British Standard Whitworth
British Standard Fine
British Standard Pipe - Taper Thread 55°
British association 47.5°
Reference to the appropriate international standards will provide the specifications for thread tolerances and other information on the geometry of fastener threads.
Threads are either ROLLED or CUT. Rolled threads are formed by passing the material between two thread rolling dies and applying a squeezing pressure to swage the required thread form without actually removing any material. This produces continuous grain flow in the material.
Cut threads are produced by removing metal from the body of the fastener with a cutting tool. This process is generally used for lower volume production runs or material that is too hard to thread roll.
NUTS usually have cut threads, which are produced with a thread tap after the hole has been punched or drilled out.
On an EXTERNAL THREAD, as in a bolt, the diameter at the thread crest is the major diameter and the diameter at the root is the minor diameter.
Both unified and metric threads are available with coarse or fine pitch threads and both have a 60° thread angle. Technical research has determined that this 60° thread angle provides the best mechanical performance for a threaded fastener.
THREAD PITCH (P) is the distance between the threads. It is measured parallel to the thread axis, between corresponding points on adjacent threads.
With metric threads the pitch is expressed in millimetres whilst in inch sizes the pitch is expressed in threads per inch (TPI).
PITCH DIAMETER is the diameter of a theoretical cylinder that passes between the top and bottom of the threads. It is the measuring element of the thread that controls and references the dimensions of the thread profile and defines thread size.
The normal choice of thread should always be coarse and is assumed if no other reference id given. Major considerations for selection of fine pitch will hinge upon:
Materials being clamped
Loading and stress on the joint
Fatigue life considerations
Assembly productivity and efficiencies
Degree of vibration experienced by the assembly in the working environment.