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)
| Major diameter |
Threads per inch (Pitch) | Tap drill size - Preferred sizes | ||||
|---|---|---|---|---|---|---|
| Coarse (UNC) | Fine (UNF) | Extra fine (UNEF) | Coarse | Fine | Extra fine | |
| #0 = 0.0600 (1.5240) | – | 80 | 3/64 in | |||
| #1 = 0.0730 (1.8542) | 64 | 72 | #53 | #53 | ||
| #2 = 0.0860 (2.1844) | 56 | 64 | #50 | #50 | ||
| #3 = 0.0990 (2.5146) | 48 | 56 | #47 | #45 | ||
| #4 = 0.1120 (2.8448) | 40 | 48 | #43 | #42 | ||
| #5 = 0.1250 (3.1750) | 40 | 44 | #38 | #37 | ||
| #6 = 0.1380 (3.5052) | 32 | 40 | #36 | #33 | ||
| #8 = 0.1640 (4.1656) | 32 | 36 | #29 | #29 | ||
| #10 = 0.1900 (4.8260) | 24 | 32 | #25 | #21 | ||
| #12 = 0.2160 (5.4864) | 24 | 28 | 32 | #16 | #14 | |
| 1/4 = 0.2500 (6.3500) | 20 | 28 | 32 | #7 | #3 | |
| 5/16 = 0.3125 (7.9375) | 18 | 24 | 32 | F | I | |
| 3/8 = 0.3750 (9.5250) | 16 | 24 | 32 | 5/16 in | Q | |
| 7/16 = 0.4375 (11.1125) | 14 | 20 | 28 | U | 25/64 in | |
| 1/2 = 0.5000 (12.7000) | 13 | 20 | 28 | 27/64 in | 29/64 in | |
| 9/16 = 0.5625 (14.2875) | 12 | 18 | 24 | 31/64 in | 33/64 in | |
| 5/8 = 0.6250 (15.8750) | 11 | 18 | 24 | 17/32 in | 37/64 in | |
| 3/4 = 0.7500 (19.0500) | 10 | 16 | 20 | 21/32 in | 11/16 in | |
| 7/8 = 0.8750 (22.2250) | 9 | 14 | 20 | 49/64 in | 13/16 in | |
| 1 = 1.0000 (25.4000) | 8 | 12 | 20 | 7/8 in | 59/64 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" + 0.060". For example, a number 10 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 fasteners of 0.3 to 1.4 millimetres (0.0118 to 0.0551 inch) diameter. These sizes are intended for watches, instruments, and miniature mechanisms and are interchangeable with threads made to ISO Standard 68. 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.
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.
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 OD's. Because of this UNS taps 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.
Read more about this topic: Unified Thread Standard
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