ISO metric screw thread#ISO 261

Image:ISO and UTS Thread Dimensions.svg

The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1.{{cite book | title=ISO 68-1:1998 ISO general purpose screw threads – Basic profile – Part 1: Metric screw threads | publisher=International Organization for Standardization | url=http://www.iso.org/iso/catalogue_detail.htm?csnumber=3707}} 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 a cross-section along the thread axis, the sides of the V have an angle of 60° to each other. The thread depth is 0.54125 × pitch. The outermost {{frac|1|8}} and the innermost {{frac|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 where θ is half the included angle of the thread, in this case 30°:{{harvnb|Oberg|Jones|Horton|Ryffel|2000|p=1706}}

:$H\; =\; \backslash frac\; \{1\}\{\backslash tan\backslash theta\}\; \backslash cdot\; \backslash frac\; \{P\}\{2\}\; =\; \backslash frac\{\backslash sqrt\; 3\}\{2\}\backslash cdot$

P \approx 0.8660 \cdot P

or

:$P\; =\; 2\backslash tan\backslash theta\backslash cdot\; H\; =\; \backslash frac\; \{2\}\{\backslash sqrt\; 3\}\; \backslash cdot\; H\; \backslash approx\; 1.1547\; \backslash cdot\; H$

Because only {{frac|5|8}} of this height is cut, the difference between major and minor diameters is {{frac|5|4}} × 0.8660 × P = 1.0825 × P, so the tap drill size can be approximated by subtracting the thread pitch from the major diameter.

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

:$\backslash begin\{align\}$

D_\text{min} &= D_\text{maj} - 2\cdot\frac58\cdot H = D_\text{maj} - \frac{ 5 {\sqrt 3}}{8}\cdot P \approx D_\text{maj} - 1.082532 \cdot P \\[3pt]

D_\text{p} &= D_\text{maj} - 2\cdot\frac38\cdot H = D_\text{maj} - \frac{ 3 {\sqrt 3}}{8}\cdot P \approx D_\text{maj} - 0.649519 \cdot P

\end{align}

Tables of the derived dimensions for screw diameters and pitches defined in ISO 261 are given in ISO 724.

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 a dash or sometimes the multiplication sign, × (e.g. M8-1.25 or 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).{{r|iso965-1|p=17}}

The length of a machine screw or bolt is indicated by an × and the length expressed in millimetres (e.g. M8-1.25×30 or M8×30).{{cn|date=May 2021}}

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.{{r|iso965-1|p=17}}

{{See also|Preferred metric sizes#ISO_261 and 262 fastener diameters}}

ISO 261 specifies a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads.{{cite book | title=ISO 261:1998 ISO general purpose metric screw threads – General plan | publisher=International Organization for Standardization | date=17 Dec 1998 | url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=4165}}[http://www.emuge-franken-bg.com/attachments/article/97/15%20Gewindetabellen.pdf ISO & DIN13] ISO 262 specifies a shorter list of thread dimensions – a subset of ISO 261.{{cite book | title=ISO 262:1998 ISO general purpose metric screw threads – Selected sizes for screws, bolts and nuts | publisher=International Organization for Standardization | date=17 Dec 1998 | url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=4167}}

{{noteslist}}

The thread values are derived from rounded Renard series. 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.{{cite web |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950018571.pdf |title=Final report |website=ntrs.nasa.gov |access-date=2017-07-07 |archive-date=2017-03-14 |archive-url=https://web.archive.org/web/20170314024016/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950018571.pdf |url-status=dead }} 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.

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).{{cite web |url=https://www.iso.org/standard/56451.html |title=ISO 4162:2012 |publisher=International Organization for Standardization |access-date=2022-12-23 }}

• 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 general purpose metric screw threads — Tolerances

{{cite book

| title=ISO 965-1:2013 ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data

| publisher=International Organization for Standardization

| date=15 Sep 2013

| url=https://www.iso.org/standard/57778.html

}}

• 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

• BS 3643: ISO metric screw threads
• [https://webstore.ansi.org/RecordDetail.aspx?sku=ASME+B4.2-1978+(R2009) ANSI/ASME B1.13M]: [https://web.archive.org/web/20071231051947/http://catalog.asme.org/Codes/PrintBook/B113M_2005_Metric_Screw.cfm Metric Screw Threads: M Profile]
• [https://webstore.ansi.org/RecordDetail.aspx?sku=ASME+B4.2-1978+(R2009) ANSI/ASME B4.2-1978 (R2009)]: Preferred Metric Limits and Fits
• [http://www.emuge-franken-bg.com/attachments/article/97/15%20Gewindetabellen.pdf DIN13, page 519]

Japan has a JIS metric screw thread standard that largely follows the ISO, but with some differences in pitch and head sizes.

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• ASTM A325M
• ASTM F568M
• British Association screw threads (BA)
• British Standard Cycle (BSC)
• British standard fine thread (BSF)
• British standard pipe thread (BSP)
• British Standard Whitworth (BSW) – a British thread standard with 55° profile.
• Buttress thread
• Engineering tolerance
• Garden hose thread
• List of drill and tap sizes
• National pipe thread (NPT)
• National thread
• Nominal size
• Panzergewinde
• Photographic Filter thread
• Preferred metric sizes
• Screw thread
• Square thread form
• Thread angle
• Trapezoidal thread forms
• United States Standard thread
• Unified Thread Standard (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.

{{div col end}}

{{reflist}}

• {{Citation | last1 = Oberg | first1 = Erik | last2 = Jones | first2 = Franklin D. | last3 = Horton | first3 = Holbrook L. | last4 = Ryffel | first4 = Henry H. | title = Machinery's Handbook | place = New York | publisher = Industrial Press Inc. | year = 2000 | edition = 26th | isbn = 0-8311-2635-3 | postscript =.}}

• [http://www.emuge-franken-bg.com/attachments/article/97/15%20Gewindetabellen.pdf Diagrams and tables of many screwthread series. In German]
• [http://www.sinofastener.com/uppics/201010161010087778.pdf DIN 931: M1,6 to M39 Hexagon head bolts (Product grades A and B)] {{Webarchive|url=https://web.archive.org/web/20221210050204/http://www.sinofastener.com/uppics/201010161010087778.pdf |date=10 December 2022 }}(1987)
• IS 9519: Fasteners - Hexagon products - Width across flats, Indian standard (2013)
• [https://www.threadspecification.com/m/ Metric screw thread profile, dimensions and tolerances]
• [https://www.metricbolt.com/metric-bolt-sizes/ Metric bolt sizes, threads and materials]

{{ISO standards}}

Category:Thread standards

Category:Screws

Category:ISO 68