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"Threads per inch" redirects here. For the textile term, see Glossary of textilemanufacturing#T.

The Unified Thread Standard (UTS) defines a standard thread form and series—alongwith allowances, tolerances, and designations—for screw threads commonly used in theUnited States and Canada. It has the same 60° profile as the ISO metric screw thread usedin the rest of the world, but the characteristic dimensions of each UTS thread (outerdiameter and pitch) were chosen as an inch fraction rather than a round millimeter value.The UTS is currently controlled by ASME/ANSI in the United States.

Contents:

1. Origins2. Basic profile3. Designation4. Gaging5. Tolerance classes6. Related standards7. See also8. References9. External links

1. Origins

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

2. Basic profile

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The basic profile of all UTS threads is the same as that of all ISO metric screw threads. Only thecommonly used values for Dmaj and P differ between the two standards.

Each thread in the series is characterized by its major diameter Dmaj and its pitch, P. UTSthreads consist of a symmetric V-shaped thread. In the plane of the thread axis, the flanksof the V have an angle of 60° to each other. The outermost 0.125 and the innermost 0.25of the height H of the V-shape are cut off from the profile.

The pitch P is the distance between thread peaks. For UTS threads, which are single-startthreads, it is equal to the lead, the axial distance that the screw advances during a 360°rotation. UTS threads do not usually use the pitch parameter; instead a parameter knownas threads per inch (TPI) is used, which is the reciprocal of the pitch.

The relationship between the height H and the pitch P is found using the followingequation: [1]

In an external (male) thread (e.g., on a bolt), the major diameter Dmaj and the minor


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diameter Dmin define maximum dimensions of the thread. This means that the externalthread 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 diametersare minimum dimensions, therefore the thread profile must end flat at Dmin but may berounded out beyond Dmaj.

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

3. 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 diameterssmaller than ¼ inch, the diameter is indicated by an integer number defined in thestandard, for all other diameters, the inch figure is given.

This number pair is optionally followed by the letters UNC, UNF or UNEF if thediameter-pitch combination is from the “coarse”, “fine” or “extra fine” series, and mayalso 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 sizeCoarse (UNC) Fine (UNF) Extra fine (UNEF) Coarse Fine Extra fine #0 = 0.0600 - 801.5240 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 40 443.1750 #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 28 326.3500 #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


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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 31/64in 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 in7/8 9 14 20 22.2250 49/64 in 13/16 in 1 8 14 20 25.4000 7/8 in 59/64 in

The following formula is used to calculate the major diameter of a numbered screw greaterthan or equal to 0: Major diameter = Screw # × 0.013" + 0.060". For example, a number10 calculates as: #10 × 0.013" + 0.060" = 0.190" major diameter.

A Unified Miniature screw thread series is defined in ANSI standard B1.10, for fastenersof 0.3 to 1.4 millimetres (0.0118 to 0.0551 inch) diameter. These sizes are intended forwatches, instruments, and miniature mechanisms and are interchangeable with threadsmade to ISO Standard 68. [2] These screw sizes are denoted by multiple zeroes, i.e. #000.The formula for number sizes smaller than size #0 is given by Major diameter = 0.060" -Zero size × 0.013", with the zero size being the number of zeroes after the first. So a #00

screw is .047" dia, #000 is .034" dia, etc.[citation needed]

The number series of machine screws once included more odd numbers and went up to#16 or more. Standardization efforts in the late 19th and the early part of the 20th centuryreduced the range of sizes considerably. Now, it is less common to see machine screwslarger than #14, or odd number sizes other than #1, #3 and #5. Even though #14 and #16

screws are still available, they are not as common as sizes #0 through #12.[citation needed]

Sometimes "special" diameter and pitch combinations (UNS) are used, for example a 0.619in (15.7 mm) major diameter with 20 teeth per inch. UNS threads are rarely used for bolts,but rather on nuts, tapped holes, and threaded OD's. Because of this UNS taps are readilyavailable. [3] [4] Most UNS threads have more threads per inch than the correlating UNF orUNEF standard, therefore they are often the strongest thread available. [5] Because of thisthey are often used in applications where high stresses are encountered, such as machinetool spindles [6] or automotive spindles. [7]

4. Gaging

A screw thread gaging system comprises a list of screw thread characteristics that must beinspected to establish the dimensional acceptability of the screw threads on a threadedproduct and the gage(s) which shall be used when inspecting those characteristics.

Currently this gaging UTS is controlled by:


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ASME/ANSI B1.2-1983 Gages And Gaging For Unified Inch Screw ThreadsASME/ANSI B1.3M-1992 Screw Thread Gaging Systems Dimensional AcceptabilityInch Metric Screw Threads

These Standards provide essential specifications and dimensions for the gages used onUnified inch screw threads (UN, UNR, UNJ thread form) on externally and internallythreaded products. It also covers the specifications and dimensions for the thread gagesand measuring equipment. The basic purpose and use of each gage are also described. Italso establishes the criteria for screw thread acceptance when a gaging system is used.

5. Tolerance classes

A classification system exists for ease of manufacture and interchangeability of fabricatedthreaded items. Most (but certainly not all) threaded items are made to a classificationstandard called the Unified Screw Thread Standard Series. This system is analogous to thefits used with assembled parts.

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

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

Class 2 threads are the most common. They are designed to maximize strength consideringtypical 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. Thepitch diameter is indicated as Dp in the figure shown above. There are several methodsthat are used to measure the pitch diameter. The most common method used in productionis by way of a Go-NoGo gage.

6. Related standards

ASME/ANSI B1.1 - 2003 Unified Inch Screw Threads, UN & UNR Thread FormASME/ANSI B1.10M - 2004 Unified Miniature Screw ThreadsASME/ANSI B1.15 - 1995 Unified Inch Screw Threads, UNJ Thread Form


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7. See also

British Association screw threadsWilliam SellersBritish Standard WhitworthISO metric screw threadNational pipe threadBritish standard pipe threadUnited States Standard thread

8. References

Oberg et al. 2000, p. 1706.1.Ryffel 1988, p. 1547.2.McMaster-Carr p. 2520, http://www.mcmaster.com/#catalog/115/2520/=3dxzrr,retrieved 2009-08-28.

3.

McMaster-Carr p. 2501, http://www.mcmaster.com/#catalog/115/2501/=3dy2xw,retrieved 2009-08-28.

4.

Schwaller 2004, p. 55.5.Special connections, http://www.nuraseal.com/Filton/Special_Connections/Special_Connections.htm, retrieved 2009-08-28.

6.

Hub components, http://i.b5z.net/i/u/1080235/f/parts_kit_cat_8-08/Hub_Components.pdf, retrieved 2009-08-29.

7.

8. 1. Bibliography

Ryffel, Henry H., et al. (eds) (1988), Machinery's Handbook (23rd ed.), New York:Industrial Press, ISBN 978-0-8311-1200-4.Schwaller, Anthony E. (2004), Total Automotive Technology (4th ed.), CengageLearning, ISBN 9781401824761, http://books.google.com/books?id=tpzOoC_G4HAC.

9. External links

Unified Coarse standard and drill sizesUnified Fine standard and drill sizesUnified Coarse/Fine diameters and tap drill sizes (US units)


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Unified Coarse/Fine tap drill sizes (US units)Imperial Metric fastening size conversion chartsInternational Thread StandardsConversion chart Whitworth/BSF/AF and metricSpanner Jaw Sizes Additional information and spanner jaw size table.Unified Screw Threads Series

Categories: All articles with unsourced statements, Articles with unsourced statementsfrom December 2009, Mechanical standards, Screws, Threading (manufacturing)

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