The ISO metric screw thread is the most commonly used type of general-purpose
screw thread
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a ...
worldwide. They were one of the first
international standard
international standard is a technical standard developed by one or more international standards organizations. International standards are available for consideration and use worldwide. The most prominent such organization is the International Or ...
s agreed when the
International Organization for Standardization
The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Ar ...
(ISO) was set up in 1947.
The "M" designation for metric screws indicates the nominal outer diameter of the screw thread, in millimetres. This is also referred to as the "major" diameter in the information below. It indicates the diameter of smooth-walled hole that a male thread (e.g. on a bolt) will pass through easily to create a well-located connection to an internally threaded component (e.g. a nut) on the other side. That is, an M6 screw has a nominal outer diameter of 6 millimetres and will therefore be a well-located, co-axial fit in a hole drilled to 6 mm diameter.
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'' (''D''
maj in the diagram), 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 thread depth is 0.54125 × pitch. The outermost and the innermost of the height ''H'' of the V-shape are cut off from the profile.
The relationship between the height ''H'' and the pitch ''P'' is found using the following equation where ''θ'' is half the included angle of the thread, in this case 30°:
:
or
:
In an external (male) thread (e.g. on a bolt), the major diameter ''D''
maj and the minor diameter ''D''
min define ''maximum'' dimensions of the thread. This means that the external thread must end flat at ''D''
maj, but can be rounded out below the minor diameter ''D''
min. 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 ''D''
min but may be rounded out beyond ''D''
maj. In practice this means that one can measure the diameter over the threads of a bolt to find the nominal diameter ''D''
maj, and the inner diameter of a nut is ''D''
min.
The minor diameter ''D''
min and effective pitch diameter ''D''
p 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'' (the maximum thread diameter) and the pitch ''P'', both expressed in millimetres and separated by the multiplication sign, ''×'' (e.g. M8×1.25). If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it can be omitted (e.g. M8).
The length of a machine screw or bolt is indicated by a following ''×'' and the length expressed in millimetres (e.g. M8×1.25×30 or M8×30).
Tolerance classes defined in
ISO 965-1 can be appended to these designations, if required (e.g. M500– 6g in external threads). External threads are designated by lowercase letter, g or h. Internal threads are designated by upper case letters, G or H.
Preferred sizes
ISO 261 specifies a detailed list of preferred combinations of outer diameter ''D'' and pitch ''P'' for ISO metric screw threads.
[
] ISO 262 specifies a shorter list of thread dimensions – a subset of ISO 261.
The thread values are derived from rounded
Renard series
Renard series are a system of preferred numbers dividing an interval from 1 to 10 into 5, 10, 20, or 40 steps. This set of preferred numbers was proposed in 1877 by French army engineer Colonel Charles Renard. His system was adopted by the ISO in ...
. They are defined in ISO 3, with "1st choice" sizes being from the Rˈˈ10 series and "2nd choice" and "3rd choice" sizes being the remaining values from the Rˈˈ20 series.
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.
In addition to coarse and fine threads, there is another division of extra fine, or ''superfine'' threads, with a very fine pitch thread. Superfine pitch metric threads are occasionally used in automotive components, such as suspension struts, and are commonly used in the aviation manufacturing industry. This is because extra fine threads are more resistant to coming loose from vibrations.
Fine and superfine threads also have a greater
minor diameter than coarse threads, which means the bolt or stud has a greater cross-sectional area (and therefore greater load-carrying capability) for the same nominal diameter.
Spanner (wrench) sizes
Below are some common spanner (wrench) sizes for metric screw threads. Hexagonal (generally abbreviated to "hex") head widths (width across flats, spanner size) are for
DIN 934 hex nuts and hex head bolts. Other (usually smaller) sizes may occur to reduce weight or cost, including the small series flange bolts defined in ISO 4162 which typically have hexagonal head sizes corresponding to the smaller 1st choice thread size (eg. M6 small series flange bolts have 8mm hexagonal heads, as would normally be found on M5 bolts).
Standards
International
* 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
ISO 965 (ISO general purpose metric screw thread—tolerances) is an International Organization for Standardization (ISO) standard for metric screw thread tolerances. It specifies the basic profile for ISO general purpose metric screw threads (M) ...
: ISO general purpose metric screw threads —
Tolerances
Engineering tolerance is the permissible limit or limits of variation in:
# a physical dimension;
# a measured value or physical property of a material, manufacturing, manufactured object, system, or service;
# other measured values (such as t ...
[
]
** ISO 965-1: Principles and basic data
** ISO 965-2: Limits of sizes for general purpose external and internal screw threads.
** ISO 965-3: Deviations for constructional screw threads
** ISO 965-4: Limits of sizes for hot-dip galvanized external screw threads to mate with internal screw threads tapped with tolerance position H or G after galvanizing
** ISO 965-5: Limits of sizes for internal screw threads to mate with hot-dip galvanized external screw threads with maximum size of tolerance position h before galvanizing
National
* BS 3643: ISO metric screw threads
ANSI/ASME B1.13MMetric Screw Threads: M ProfileANSI/ASME B4.2-1978 (R2009) Preferred Metric Limits and Fits
DIN13, page 519
Derived standards
Japan has a
JIS metric screw thread standard that largely follows the ISO, but with some differences in pitch and head sizes.
See also
*
ASTM A325M
*
ASTM F568M ASTM F568M is an ASTM International standard for metric bolts, screws and studs that are used in general engineering applications. It is titled: ''Standard Specification for Carbon and Alloy Steel Externally Threaded Metric Fasteners''. It defines ...
*
British Association screw threads
British Association screw threads, or BA screw threads, are a 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 modelling.
They are unusual in that ...
(BA)
*
British Standard Cycle British Standard Cycle (BSC or BSCy) is a British Imperial screw thread standard. Unlike other major British imperial thread standards (British Standard Whitworth and British Standard Fine) the thread runs at a 60 degrees rather than a 55 degrees ...
(BSC)
*
British standard fine thread British Standard Fine (BSF) is a screw thread form, as a fine-pitch alternative to British Standard Whitworth (BSW) thread.
It was used for steel bolts and nuts on and in much of Britain's machinery, including cars, prior to adoption of Unified, an ...
(BSF)
*
British standard pipe thread
British Standard Pipe (BSP) is a set of technical standards for screw threads that has been adopted internationally for interconnecting and sealing pipes and fittings by mating an external (male) thread with an internal (female) thread. It has b ...
(BSP)
*
British Standard Whitworth
British Standard Whitworth (BSW) is an imperial-unit-based screw thread standard, devised and specified by Joseph Whitworth in 1841 and later adopted as a British Standard. It was the world's first national screw thread standard, and is the basis ...
(BSW) – a British thread standard with 55° profile.
*
Buttress thread
*
Engineering tolerance
Engineering tolerance is the permissible limit or limits of variation in:
# a physical dimension;
# a measured value or physical property of a material, manufactured object, system, or service;
# other measured values (such as temperature, hum ...
*
Garden hose thread
A garden hose, hosepipe, or simply hose is a flexible tube used to convey water. There are a number of common attachments available for the end of the hose, such as sprayers and sprinklers (which are used to concentrate water at one point or ...
*
List of drill and tap sizes
Below is a comprehensive drill and tap size chart for all drills and taps, imperial and metric, up to in diameter.
In manufactured parts, holes with female screw threads are often needed; they accept male screws to facilitate the building and ...
*
National pipe thread
American National Standard Pipe Thread standards, often called national pipe thread standards for short, are United States national technical standards for screw threads used on threaded pipes and pipe fittings. They include both tapered and stra ...
(NPT)
*
National thread
*
Nominal size
The distinction between real value and nominal value occurs in many fields. From a philosophical viewpoint, nominal value represents an accepted condition, which is a goal or an approximation, as opposed to the real value, which is always present. ...
* ''
Panzergewinde
The Stahlpanzerrohrgewinde (, "steel conduit thread") standard for screw threads, more often called by the shortened Panzergewinde (), was a technical standard created in Germany and subsequently used in Switzerland, Austria, and other neighbori ...
''
*
Photographic Filter thread
*
Preferred metric sizes
Preferred metric sizes are a set of international standards and de facto standards that are designed to make using the metric system easier and simpler, especially in engineering and construction practices. One of the methods used to arrive at thes ...
*
Screw thread
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a ...
*
Square thread form
The square thread form is a common screw thread profile, used in high load applications such as leadscrews and jackscrews. It gets its name from the square cross-section of the thread.Bhandari, p. 203. It is the lowest friction and most effi ...
*
Thread angle
The thread angle of a screw is the included angle between the thread flanks, measured in a plane containing the thread axis.. This is a defining factor for the shape of a screw thread
A screw thread, often shortened to thread, is a helical stru ...
*
Trapezoidal thread forms
Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews (power screws). They offer high strength and ease of manufacture. They are typically found where large loads are r ...
*
United States Standard thread
United States Standard thread (USS thread), also known as Sellers Standard thread, Franklin Institute thread and American Standard thread, is a standard for inch based threaded fasteners and washers.
The USS standard is no longer supported. It, t ...
*
Unified Thread Standard
The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a w ...
(UTS, UNC, UNF, UNEF and UNS) – a US/Canadian/British thread standard that uses the same 60° profile as metric threads, but an inch-based set of diameter/pitch combinations.
References
Bibliography
*
External links
Metric screw thread dimensions and tolerancesMetric coarse thread dimensionsMetric fine thread dimensionsDiagrams and tables of many screwthread series. In GermanDIN 931: M1,6 to M39 Hexagon head bolts (Product grades A and B)1987)
*
IS 9519: Fasteners - Hexagon products - Width across flats, Indian standard (2013)
{{ISO standards
Thread standards
Screws
ISO 68