ISO metric screw thread
Encyclopedia
The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread
. They were one of the first international standard
s agreed when the International Organization for Standardization
was set up in 1947.
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 relationship between the height H and the pitch P is found using the following equation:
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
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, H or G.
ISO 262 specifies a shorter list of thread dimensions – a subset of ISO 261.
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.
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 straight thread and the latter called a tapered thread...
. They were one of the first international standard
International standard
International standards are standards developed by international standards organizations. 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. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
was set up in 1947.
Basic profile
The relationship between the height H and the pitch P is found using the following equation:
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, × (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). Tolerance classes defined in ISO 965-1ISO 965
ISO 965 is a International Organization for Standardization standard for metric screw thread tolerances. It specifies the basic profile for ISO general purpose metric screw threads conforming to ISO 261.The tolerance system refers to the basic profile in accordance with ISO 68-1...
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, H or G.
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.
| Nominal diameter D (mm) ISO 261 |
Pitch P (mm) |
||
|---|---|---|---|
| 1st choice ISO 262 |
2nd choice |
coarse | fine |
| 1 | 0.25 | ||
| 1.2 | 0.25 | ||
| 1.4 | 0.3 | ||
| 1.6 | 0.35 | ||
| 1.8 | 0.35 | ||
| 2 | 0.4 | ||
| 2.5 | 0.45 | ||
| 3 | 0.5 | ||
| 3.5 | 0.6 | ||
| 4 | 0.7 | ||
| 5 | 0.8 | ||
| 6 | 1 | ||
| 7 | 1 | ||
| 8 | 1.25 | 1 | |
| 10 | 1.5 | 1.25 or 1 | |
| 12 | 1.75 | 1.5 or 1.25 | |
| 14 | 2 | 1.5 |
| Nominal diameter D (mm) |
Pitch P (mm) |
||
|---|---|---|---|
| 1st choice |
2nd choice |
coarse | fine |
| 16 | 2 | 1.5 | |
| 18 | 2.5 | 2 or 1.5 | |
| 20 | 2.5 | 2 or 1.5 | |
| 22 | 2.5 | 2 or 1.5 | |
| 24 | 3 | 2 | |
| 27 | 3 | 2 | |
| 30 | 3.5 | 2 | |
| 33 | 3.5 | 2 | |
| 36 | 4 | 3 | |
| 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.
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.
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 965ISO 965ISO 965 is a International Organization for Standardization standard for metric screw thread tolerances. It specifies the basic profile for ISO general purpose metric screw threads conforming to ISO 261.The tolerance system refers to the basic profile in accordance with ISO 68-1...
: ISO general purpose metric screw threads — Tolerances- 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 B1.13M: Metric Screw Threads: M Profile
See also
- Acme thread formAcme thread formTrapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews . They offer high strength and ease of manufacture. They are typically found where large loads or high accuracy are required, as in a vise or the leadscrew of a lathe...
- British Association screw threadsBritish Association screw threadsBritish 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 modelling....
(BA) - British Standard CycleBritish Standard CycleBritish Standard Cycle or is a British Imperial screw thread standard. Unlike other major British imperial thread standards the thread runs at a 60 degrees rather than a 55 degrees angle...
(BSC) - British standard fine threadBritish standard fine threadBritish standard fine thread is a fine screw thread alternative to British Standard Whitworth thread.It is used for steel bolts and nuts on much British machinery, including automobiles, prior to adoption of the metric standards...
(BSF) - British standard pipe threadBritish standard pipe threadThe 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.-Types:...
(BSP) - British Standard WhitworthBritish Standard WhitworthBritish Standard Whitworth is one of a number of imperial unit based screw thread standards which use the same bolt heads and nut hexagonal sizes, the others being British Standard Fine thread and British Standard Cycle...
(BSW) – a British thread standard with 55° profile. - Buttress threadButtress threadThe buttress thread form, also known as the breech-lock thread form, refers to two different thread profiles. One is a type of leadscrew and the other is a type of hydraulic sealing thread form...
- Garden hose thread
- National pipe threadNational pipe threadNational Pipe Thread Tapered Thread is a U.S. standard for tapered threads used on threaded pipes and fittings. The taper rate for all NPT threads is measured by the change of diameter over distance. The angle between the taper and the center axis of the pipe is 1° 47′ 24″...
(NPT)
- National thread
- PanzergewindePanzergewindeThe Stahlpanzerrohrgewinde standard for screw threads, more often called by the shorter name Panzergewinde , is a technical standard created in Germany and subsequently used in various countries, such as the German-speaking zone and neighboring European countries...
- Screw threadScrew threadA 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 straight thread and the latter called a tapered thread...
- Square thread formSquare thread formThe square thread form is a common thread form for leadscrews. It gets its name from the square cross-section of the thread.-Advantages & disadvantages:...
- Thread angleThread angleThe thread angle of a screw is the angle between the threads. This is a defining factor for the shape of a screw thread. Standard values include:...
- United States Standard threadUnited States Standard threadUnited States Standard thread , also known as Sellers Standard thread, Franklin Institute thread and American Standard thread, is a standard for inch based threaded fasteners and washers....
- Unified Thread StandardUnified Thread StandardThe Unified Thread Standard defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada...
(UTS, UNC, UNF, UNEF and UNS) – a US/Canadian/British thread standard that uses the same 60° profile, but an inch-based set of diameter/pitch combinations.