UNIFIED SCREW THREADS (UNC/UNF/UNEF) Thread size chart

The Unified Screw Threads have the designations of DPSC. These designations are as follows:

1. Diameter
2. Pitch
3. Series
4. Class

The designations are described in detail below:

1.Diameter

The Nomnal Diameter is measured in Inch Fraction. The diameters that are ¼” and above are measured in inch fractions. For instance, ¼”, ¾” or 1 ¼”. The diameters that are below ¼” are given with a series of numbers that range from #0 to #12. Every number is assigned to a single arbitrary diameter.

Instead of using fractions, diameters may also be indicated using the relevant decimal value. For instance, instead of ¼”, 0.250 and instead of 7/16”, 0.4375 can be used. The diameter that is described by each ‘number’ from #0 to #12 is given in the below table.

2. Pitch

The Pitch is given in TPI. For instance, a ¼-20 is in fact a thread with a pitch of 20 TPI. Instead of this, the pitch can also be denoted using distance. In this method, ¼-0.05P is a thread with a pitch of 0.05” which is equivalent to 20 TPI.

3.Series

Most of the time, the series of UST would be UNC (coarse pitch), UNF (fine pitch) or UNEF (extra fine pitch).  The ASTM B1.1 also has given definitions for several constant pitch series. These are identified by ##-UN. For instance, the 12-UN or 8-UN remains the same for the complete diameter range within this series. Although this method is in use, it is very rare.

List of symbols used in charts and formulas of Unified Inch Threads

4.Class

The class is defined using a code with two characters. The first character contains a digit between 1 and 3. The designations are 1 = Tight fit, 2 = Medium Fit and 3 = Loose Fit. The second character is a letter from the English alphabet. According to his, A = External thread and B = Internal thread.

In addition to the above designations, some additional parameters are given as well. They are:

1. Direction: the suffix -LH should be added to a left-handed thread. Always the default direction of the thread would be right-handed, which means a screw would be right-handed unless specified so.

2. The number of starts: Every thread has a single start by default; which results in an equal number of lead and pitch on the thread. The lead is indicated along with a thread that has multiple starts.