Unified Thread Standard

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

The standard was originally adopted by the Screw Thread Standardization Committees of Canada, the United Kingdom, and the United States on Nov 18, 1949 in Washington, D.C., and applied to screw threads used in the above countries with the hope they would be adopted universally. The standard was not widely taken up in the UK, who continued to use their own BA (British Association) standard and then migrated to ISO metric screw threads. The original UTS standard may be found in ASA (now ANSI) publication, Vol. 1, 1949.

UTS consists of Unified Coarse (UNC), Unified Fine (UNF), Unified Extra Fine (UNEF) and Unified Special (UNS).

The International Organization for Standardization's ISO metric screw thread preferred series, based on round millimeter dimensions, is the standard that has been adopted world-wide and has displaced all former standards, including UTS. In the USA, where UTS is still prevalent, over 40% of products contain ISO metric screw threads. Of the above mentioned countries, the UK has completely abandoned its commitment to UTS in favour of the ISO metric threads, and Canada is in between.

Technical information

Form and pitch

UTS thread form and pitch technical specifications are currently controlled by ASME/ANSI industry standards in the United States:

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

$P=1/TeethPerInch$
$H=0.866025*P$
$H1=0.541266*P$
$d1=d+1.082532*P$
$d2=d+0.433013*P$
$D=d$
$D1=d1$
$D2=d2$

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.

Class of thread

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 shown as d2 in figure 1 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.

A table of pitch diameters and thread classes can be found

here.

External links

Unified Coarse standard and drill sizes
Unified Fine standard and drill sizes
International Thread Standards
Spanner Jaw Sizes Additional information and spanner jaw size table.
[1] Pitch, Major, Minor Dia. Table for UN,UNC,UNF

http://www.efunda.com/designstandards/screws/screwun.cfm