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The Unified Thread Standard (UTS) defines a standard
Standardization

Standardization is the process of developing and agreeing upon Standard . A standard is a document that establishes uniform engineering or technical specifications, criteria, methods, processes, or practices....
 thread form and series—along with allowances, tolerances, and designations—for 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 ....
s commonly used in the United States
United States

The United States of America is a Federal government constitutional republic comprising U.S. state and a federal district. The country is situated mostly in central North America, where its Contiguous United States and Washington, D.C., the Capital districts and territories, lie between the Pacific Ocean and Atlantic Oceans, Borders of the U...
 and Canada
Canada

Canada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean....
. It has the same 60° profile as the ISO metric screw thread
ISO metric screw thread

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 standards agreed when the International Organization for Standardization was set up in 1947....
 used in the rest of the world, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch
Inch

An inch is the name of a Units of measurement of length in a number of different systems, including Imperial units, and United States customary units....
 fraction rather than a round millimeter value.






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The Unified Thread Standard (UTS) defines a standard
Standardization

Standardization is the process of developing and agreeing upon Standard . A standard is a document that establishes uniform engineering or technical specifications, criteria, methods, processes, or practices....
 thread form and series—along with allowances, tolerances, and designations—for 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 ....
s commonly used in the United States
United States

The United States of America is a Federal government constitutional republic comprising U.S. state and a federal district. The country is situated mostly in central North America, where its Contiguous United States and Washington, D.C., the Capital districts and territories, lie between the Pacific Ocean and Atlantic Oceans, Borders of the U...
 and Canada
Canada

Canada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean....
. It has the same 60° profile as the ISO metric screw thread
ISO metric screw thread

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 standards agreed when the International Organization for Standardization was set up in 1947....
 used in the rest of the world, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch
Inch

An inch is the name of a Units of measurement of length in a number of different systems, including Imperial units, and United States customary units....
 fraction rather than a round millimeter value. The UTS is currently controlled by ASME
American Society of Mechanical Engineers

The American Society of Mechanical Engineers is a professional body, specifically an engineering society, focused on mechanical engineering.The ASME was founded in 1880 by Alexander Lyman Holley, Henry Rossiter Worthington, John Edison Sweet and Matthias N....
/ANSI
American National Standards Institute

The American National Standards Institute or ANSI is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States....
 in the United States.

Origins

See the "History of standardization" section of the screw thread article
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 ....
.


Basic profile


Each thread in the series is characterized by its major diameter Dmaj and its pitch P. UTS 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 pitch P is the distance a screw advances during a 360° rotation. It is related to a TPI (teeth per inch or threads per inch) figure by:

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


The standard designation for a UTS thread is a number indicating the nominal (major) diameter of the thread, followed by the pitch measured in threads per inch. For diameters smaller than ¼ inch, the diameter is indicated by an integer number defined in the standard, for all other diameters, the inch figure is given.

This number pair is optionally followed by the letters UNC, UNF or UNEF if the diameter-pitch combination is from the “coarse”, “fine” or “extra fine” series, and may also be followed by a tolerance class.

Example:
  • 6-32 UNC 2B (major diameter: 0.1380 inch, pitch: 32 tpi)


Preferred sizes


major
diameter
[in] 		threads per inch
major
diameter
[mm] 		tap drill size
coarse
UNC		 fine
UNF 		extra fine
UNEF		 coarse
UNC		 fine
UNF 		extra fine
UNEF
#0 = 0.0600 – 80 1.5240 3/64 in  
#1 = 0.0730 64 72 1.8542 #53 #53  
#2 = 0.0860 56 64 2.1844 #50 #50  
#3 = 0.0990 48 56 2.5146 #47 #45  
#4 = 0.1120 40 48 2.8448 #43 #42  
#5 = 0.1250 40 44 3.1750 #38 #37  
#6 = 0.1380 32 40 3.5052 #36 #33  
#8 = 0.1640 32 36 4.1656 #29 #29  
#10 = 0.1900 24 32 4.8260 #25 #21  
#12 = 0.2160 24 28 32 5.4864 #16 #14  
1/4 20 28 32 6.3500 #7 #3  
5/16 18 24 32 7.9375 F I  
3/8 16 24 32 9.5250 5/16 in Q  
7/16 14 20 28 11.1125 U 25/64 in  
1/2 13 20 28 12.7000 27/64 in 29/64 in  
9/16 12 18 24 14.2875 31/64 in 33/64 in  
5/8 11 18 24 15.8750 17/32 in 37/64 in  
3/4 10 16 20 19.0500 21/32 in 11/16 in  
7/8 9 14 20 22.2250 49/64 in 13/16 in  
1 8 12 20 25.4000 7/8 in 59/64 in  


Formula for number screw sizes: Diameter = Screw Size # × 0.013" + 0.060"
Example: #10 × 0.013" + 0.060" = 0.190" diameter

Sometimes "special" thread profiles (UNS) are used, for example a 0.619 inch (15.723 mm) major diameter with 20 teeth per inch. This gives a stronger assembly fit of external and internal threads. These may be design considerations in aerospace, defense and automobile applications.

Gauging


A screw thread gauging system comprises a list of screw thread characteristics that must be inspected to establish the dimensional acceptability of the screw threads on a threaded product and the gauge(s) which shall be used when inspecting those characteristics.

Currently this gauging UTS is controlled by:

  • ASME/ANSI B1.2-1983 Gauges And Gauging For Unified Inch Screw Threads
  • ASME/ANSI B1.3M-1992 Screw Thread Gauging Systems Dimensional Acceptability Inch Metric Screw Threads


These Standards provide essential specifications and dimensions for the gauges used on Unified inch screw threads (UN, UNR, UNJ thread form)on externally and internally threaded products. It also covers the specifications and dimensions for the thread gauges and measuring equipment. The basic purpose and use of each gauge are also described. It also establishes the criteria for screw thread acceptance when a gauging system is used.

Tolerance classes

A classification system exists for ease of manufacture and interchangeability of fabricated threaded items. Most (but certainly not all) threaded items are made to a classification standard called the Unified Screw Thread Standard Series. This system is analogous to the fits
Engineering fit

Fit refers to the mating of two mechanical components. Manufactured parts are very frequently required to mate with one another. They may be designed to slide freely against one another or they may be designed to bind together to form a single unit....
 used with assembled parts.

Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads.

Class 1 threads are loosely fitting threads intended for ease of assembly or use in a dirty environment.

Class 2 threads are the most common. They are designed to maximize strength considering typical machine shop capability and machine practice.

Class 3 threads are used for closer tolerances.

Thread class refers to the acceptable range of pitch diameter for any given thread. The pitch diameter is indicated as Dp in the figure shown above. There are several methods that are used to measure the pitch diameter. The most common method used in production is by way of a Go-NoGo gauge
Go-NoGo gauge

A Go-NoGo gauge refers to an inspection tool used to check a workpiece against its allowed Tolerance s. Its name derives from its use: the gauge itself has two tests; the check involves the workpiece's having to pass one test and 'fail' the other ....
.

See also

  • 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....
  • William Sellers
    William Sellers

    William Sellers was a mechanical engineer, manufacturer, and inventor who filed more than 90 patents, most notably the design for the United_States_Standard_thread....
  • British Standard Whitworth
    British Standard Whitworth

    British Standard Whitworth is one of a number of imperial unit based screw thread standards which use the same bolt heads and nut hexagonal sizes, some others being British Standard Fine thread and British Standard Cycle....
  • ISO metric screw thread
    ISO metric screw thread

    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 standards agreed when the International Organization for Standardization was set up in 1947....
  • 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....


External links


  • standard and drill sizes
  • standard and drill sizes
  • (US units)
  • (US units)
  • Additional information and spanner jaw size table.