Thread Gages

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 has the same 60° profile as the ISO metric screw thread used in the rest of the world, but the characteristic dimensions of each UTS thread (outer diameter 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.

Origins

Basic profile

Each thread in the series is characterized by its major diameter D _maj 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 0.125 and the innermost 0.25 of 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-start, it is equal to the lead, which is the axial distance that the screw advances during a 360° rotation (or spatial period). Pitch is related to a spatial frequency (threads per inch, or TPI) 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 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 .

The minor diameter D _min and effective pitch diameter D _p 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

Formula for number screw sizes: Diameter = Screw size # × 0.013" + 0.060"

Sometimes "special" thread profiles (UNS) are used, for example a 0.619 in (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 and threads are readily available. Most UNS threads have more threads per inch than the correlating UNF or UNEF standard, therefore they are often the strongest thread available. Because of this they are often used in applications where high stresses are encountered, such as machine tool spindles or automotive spindles.

Gaging

A screw thread gaging 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 gage(s) which shall be used when inspecting those characteristics.

Currently this gaging UTS is controlled by:

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

These Standards provide essential specifications and dimensions for the gages 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 gages and measuring equipment. The basic purpose and use of each gage are also described. It also establishes the criteria for screw thread acceptance when a gaging 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 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 gage.

Related standards

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

See also

• British Association screw threads
• William Sellers
• British Standard Whitworth
• ISO metric screw thread
• National pipe thread
• British standard pipe thread
• United States Standard thread

References

1. ^ McMaster-Carr p. 2520 , http://www.mcmaster.com/#catalog/115/2520/=3dxzrr , retrieved 2009-08-28   .
2. ^ McMaster-Carr p. 2501 , http://www.mcmaster.com/#catalog/115/2501/=3dy2xw , retrieved 2009-08-28   .
3. ^ Schwaller 2004, p. 55.
4. ^ Special connections , http://www.nuraseal.com/Filton/Special_Connections/Special_Connections.htm , retrieved 2009-08-28   .
5. ^ Hub components , http://i.b5z.net/i/u/1080235/f/parts_kit_cat_8-08/Hub_Components.pdf , retrieved 2009-08-29   .

Bibliography

• Schwaller, Anthony E. (2004), Total Automotive Technology (4th ed.), Cengage Learning, ISBN 9781401824761 , http://books.google.com/books?id=tpzOoC_G4HAC   .

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