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The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread
Screw thread

A screw thread is a helix structure used to convert between rotational and linear movement or force.A screw thread may be thought of as an inclined plane wrapped around a cylinder or cone ....
. They were one of the first international standard
International standard

International standards are standards developed by international standards organisations. International standards are available for consideration and use, worldwide....
s agreed when the International Organization for Standardization
International Organization for Standardization

The International Organization for Standardization , widely known as ISO , is an international standard-setting body composed of representatives from various national standards organizations....
 was set up in 1947.

Basic profile
The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. Each thread is characterized by its major diameter D and its pitch P.






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The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread
Screw thread

A screw thread is a helix structure used to convert between rotational and linear movement or force.A screw thread may be thought of as an inclined plane wrapped around a cylinder or cone ....
. They were one of the first international standard
International standard

International standards are standards developed by international standards organisations. International standards are available for consideration and use, worldwide....
s agreed when the International Organization for Standardization
International Organization for Standardization

The International Organization for Standardization , widely known as ISO , is an international standard-setting body composed of representatives from various national standards organizations....
 was set up in 1947.

Basic profile


The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. Each thread is characterized by its major diameter D and its pitch P. ISO metric threads consist of a symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other. The outermost 1/8 and the innermost 1/4 of the height H of the V-shape are cut off from the profile.

The height H and the pitch P are related through the 60° angle via

In an external (male) thread (e.g., on a bolt), the major diameter Dmaj and the minor diameter Dmin define maximum dimensions of the thread. This means that the external thread must end flat at Dmaj, but can be rounded out below the minor diameter Dmin. Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions, therefore the thread profile must end flat at Dmin but may be rounded out beyond Dmaj.

The minor diameter Dmin and effective pitch diameter Dp are derived from the major diameter and pitch as

Designation

A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (Dmaj in the diagram above) and the pitch P, both expressed in millimetres and separated by the multiplication sign “×”.

Example: M8×1.25

If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it is omitted.

Example: M8

Tolerance classes defined in ISO 965-1 can be appended to these designations, if required.

Example: M10 – 6g in external threads.

Preferred sizes

ISO 261, titled ISO general purpose metric screw threads -- General plan, is an international standard that defines a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads.

ISO 262, titled ISO general purpose metric screw threads -- Selected sizes for screw
Screw

A screw is a shaft with a helix groove or screw thread formed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force....
s, bolt
Bolted joint

| | |}Bolted joints are one of the most common elements in construction and machine design. They consist of cap Screw#Bolt or studs that capture and join other parts, and are secured with the mating of screw threads....
s and nut
Nut (hardware)

A nut is a type of hardware fastener with a screw thread hole. Nuts are almost always used opposite a mating screw#Bolt to fasten a stack of parts together....
s, defines a shorter list of thread dimensions – a subset of the combinations listed in ISO 261 – which is listed below:
Nominal diameter
D (mm)		Pitch
P (mm)		Bolt Head
1st
choice		2nd
choice		coarsefineHeight
H (mm)		Flat size
A/F (mm)		External diameter
A/C (mm)
1 0.25
1.2 0.25
1.40.3
1.6 0.35
1.80.35
2 0.4
2.5 0.45
3 0.5 2.125 5.5 6.4
3.50.6
4 0.7 2.925 7 8.1
5 0.8 3.65 8 9.2
6 1 0.75 4.15 10 11.5
7 1
8 1.251 5.65 13 15
10 1.5 1.25 or 1 7.18 17 19.6
12 1.751, 1.5 or 1.258.18 19 22.1
14 2 1.5
16 2 1.510.18 24 27.7
18 2.5 2 or 1.5
20 2.5 2 or 1.5 13.21530 34.6
22 2.5 2 or 1.5
24 3 2 15.21536 41.6
27 3 2
30 3.5 2 19.26 46 53.1
33 3.5 2
36 4 3 23.26 55 63.5
39 4 3
42 4.5 3
45 4.5 3
48 5 3
52 5 4
56 5.5 4
60 5.5 4
64 6 4


The "coarse" pitch is the commonly-used default pitch for a given diameter. In addition, one or two smaller "fine" pitches are defined, for use in applications where the height of the normal "coarse" pitch would be unsuitable (e.g., threads in thin-walled pipes). The terms "coarse" and "fine" have in this context no relation to the manufacturing quality of the thread.

See also

  • Unified Thread Standard
    Unified Thread Standard

    The Unified Thread Standard defines a standardization thread form and series?along with allowances, tolerances, and designations?for screw threads commonly used in the United States and Canada....
     – a US/Canadian thread standard that uses the same 60° profile, but an inch-based set of diameter/pitch combinations.
  • BSW - a (now obsolete) British thread standard (Whitworth) with 55° profile.
  • British Association screw threads
    British Association screw threads

    British Association or BA screw threads are a largely obsolete set of small screw threads, the largest being 0BA at 6 mm diameter. They were, and to some extent still are, used for miniature instruments and modeling....
  • National pipe thread
    National pipe thread

    National Pipe Thread Tapered is a United States standard for tapered threads used to join Pipe and fittings. ANSI/American Society of Mechanical Engineers standard B1.20.1 covers threads of 60-degree form with flat crests and roots in sizes from 1/16 inch to 24 inch Nominal Pipe Size [NPS] ....
  • British standard pipe thread
    British standard pipe thread

    The British Standard Pipe thread is a family of standard screw thread types that has been adopted internationally for interconnecting and sealing pipe ends by mating an external with an internal thread....


International standards

  • ISO 68-1: ISO general purpose screw threads — Basic profile — Metric screw threads.
  • ISO 261: ISO general purpose metric screw threads — General plan.
  • ISO 262: ISO general purpose metric screw threads — Selected sizes for screws, bolts and nuts.
  • ISO 965-1: ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data.


Equivalent national standards

  • BS 3643: ISO metric screw threads
  • ANSI/ASME B1.13M:


External links

  • Additional background information and spanner
    Wrench

    A wrench or spanner is a tool used to provide a mechanical advantage in applying torque to turn screw, Nut or other items designed to interface with a wrench....
     jaw size table.