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 is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch fraction rather than a millimeter value. The UTS is currently controlled by ASME/ANSI in the United States.

Origins

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

Basic profile

The basic profile of all UTS threads is the same as that of all ISO metric screw threads. Only the commonly 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. 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 threads, 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 known as 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 following equation:[1]

\begin{align} H &= \cos(30^\circ) \times P \\ &= \frac{ {\sqrt 3}}{2} \times P \\ &\approx 0.866 \times P \end{align}

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

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

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 14 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)

Unified Screw Threads — UNC, UNF and UNEF[2]:1816
Major diameter
(inch \ mm)
Thread density (d, threads per inch) and thread pitch (p) Preferred cutting tap drill size
Coarse (UNC) Fine (UNF) Extra fine (UNEF)
d (TPI) p (inch \ mm) d (TPI) p (inch \ mm) d (TPI) p (inch \ mm) Coarse Fine Extra fine
#0 0.0600 \ 1.5240 None 80 0.012500 \ 0.3175 None 364 in
#1 0.0730 \ 1.8542 64 0.015625 \ 0.3969 72 0.013888 \ 0.3528 None #53 #53
#2 0.0860 \ 2.1844 56 0.017857 \ 0.4536 64 0.015625 \ 0.3969 None #50 #50
#3 0.0990 \ 2.5146 48 0.020833 \ 0.5292 56 0.017857 \ 0.4536 None #47 #45
#4 0.1120 \ 2.8448 40 0.025000 \ 0.6350 48 0.020833 \ 0.5292 None #43 #42
#5 0.1250 \ 3.1750 40 0.025000 \ 0.6350 44 0.022727 \ 0.5773 None #38 #37
#6 0.1380 \ 3.5052 32 0.031250 \ 0.7938 40 0.025000 \ 0.6350 None #36 #33
#8 0.1640 \ 4.1656 32 0.031250 \ 0.7938 36 0.027778 \ 0.7056 None #29 #29
#10 0.1900 \ 4.8260 24 0.041667 \ 1.0583 32 0.031250 \ 0.7938 None #25 #21
#12 0.2160 \ 5.4864 24 0.041667 \ 1.0583 28 0.035714 \ 0.9071 32 0.031250 \ 0.7938 #16 #14 316 in
14 0.2500 \ 6.3500 20 0.050000 \ 1.2700 28 0.035714 \ 0.9071 32 0.031250 \ 0.7938 #7 #3 732 in
516 0.3125 \ 7.9375 18 0.055556 \ 1.4111 24 0.041667 \ 1.0583 32 0.031250 \ 0.7938 F I 932 in
38 0.3750 \ 9.5250 16 0.062500 \ 1.5875 24 0.041667 \ 1.0583 32 0.031250 \ 0.7938 516 in Q 1132 in
716 0.4375 \ 11.1125 14 0.071428 \ 1.8143 20 0.050000 \ 1.2700 28 0.035714 \ 0.9071 U 2564 in Y
12 0.5000 \ 12.7000 13 0.076923 \ 1.9538 20 0.050000 \ 1.2700 28 0.035714 \ 0.9071 2764 in 2964 in 1532 in
916 0.5625 \ 14.2875 12 0.083333 \ 2.1167 18 0.055556 \ 1.4111 24 0.041667 \ 1.0583 3164 in 12 in 3364 in
58 0.6250 \ 15.8750 11 0.090909 \ 2.3091 18 0.055556 \ 1.4111 24 0.041667 \ 1.0583 1732 in 916 in 3764 in
34 0.7500 \ 19.0500 10 0.100000 \ 2.5400 16 0.062500 \ 1.5875 20 0.050000 \ 1.2700 2132 in 1116 in 4564 in
78 0.8750 \ 22.2250 9 0.111111 \ 2.8222 14 0.071428 \ 1.8143 20 0.050000 \ 1.2700 4964 in 5164 in 5364 in
1 1.0000 \ 25.4000 8 0.125000 \ 3.1750 12 0.083333 \ 2.1167 20 0.050000 \ 1.2700 78 in 5964 in 6164 in

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

The number series of machine screws has been extended downward to include #00-90 (0.047 in = 0.060 − 0.013 in) and #000-120 (0.034 in = 0.060 − 2 × 0.013 in) screws[3] screws; however, the main standard for screws smaller than #0 is ANSI/ASME standard B1.10 Unified Miniature Screw Threads. This defines a series of metric screws named after their major diameters in mm, from 0.30 UNM to 1.40 UNM. Preferred sizes are 0.3, 0.4, 0.5, 0.6, 0.8, 1.0 and 1.2 mm, with additional defined sizes halfway between.[2]:1861 The standard thread pitch is approximately 1/4 of the major diameter. The thread form is slightly modified to increase the minor diameter, and thus the strength of screws and taps. The major diameter still extends to within 1/8 H of the theoretical sharp V, but the total depth of the thread is reduced 4% from 5/8 H = 5/8 cos(30°) P ≈ 0.541 P to 0.52 P.[2]:1858–1859 This increases the amount of the theoretical sharp V which is cut off at the minor diameter by 10% from 0.25 H to 7/8 − 0.52/cos (30°) ≈ 0.27456 H.

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 century reduced the range of sizes considerably. Now, it is less common to see machine screws larger 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 0.619 in (15.7 mm) major diameter with 20 threads per inch. UNS threads are rarely used for bolts, but rather on nuts, tapped holes, and threaded ODs. Because of this UNS taps are readily available.[4][5] Most UNS threads have more threads per inch than the correlating UNF or UNEF standard, therefore they are often the strongest thread available.[6] Because of this they are often used in applications where high stresses are encountered, such as machine tool spindles[7] or automotive spindles.[8]

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

This Standard provides essential specifications and dimensions for the gauges used on Unified inch screw threads UN [unified] and UNR [external threads only] thread form, and covers the specifications and dimensions for the thread gauges and measuring equipment listed in Tables 1 and 2. The basic purpose and use of each gauge are also described.

• ASME/ANSI B1.3-2007 Screw Thread Gauging Systems for Acceptability: Inch and Metric Screw Threads (UN, UNR, UNJ, M, and MJ)

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/no go gauge.

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