- Nominal size
In manufacturing, a nominal size or trade size is a size "in name only" used for identification. The nominal size may not match any dimension of the product, but within the domain of that product the nominal size may correspond to a large number of highly standardized dimensions and tolerances.
For example, dimensional lumber sizes such as "2 by 4" refers to a board whose finished dimensions are closer to 1 1/2 inches by 3 1/2 inches. A "3 1/2 inch" floppy disk standard dimension is 90 mm, or 3.54 inches, and is advertised to hold "1.44 megabytes" although its capacity is 1,474,560 bytes. A "3/4 inch pipe" in the Nominal Pipe Size system has no dimensions that are exactly 0.75 inches. A screw thread has a number of dimensions required to assure proper function but is referred to by a nominal size and a thread design family, for example "1/4 inch, 20 threads per inch, Unified National Coarse".
A nominal size may not even carry any unit of measure, for example 120 film is a film format made by different manufacturers, interchangeable in cameras but with no particular correlation to "120" in either inch or millimetre units. American wire gauge is a series of sizes for copper wire, with an arbitrary but well-standardized relationship between the size number and the dimensions of the finished wire.
Nominal sizes may be well-standardized across an industry, or may be proprietary to one manufacturer.
Applying the nominal size across domains requires understanding of the size systems in both areas; for example, someone wishing to select a drill bit to clear a "1/4 inch screw" may consult tables to show the proper drill bit size. Someone wishing to calculate the load capacity of a steel beam would have to consult tables to translate the nominal size of the beam into usable dimensions.
- ^ R. K. Rajputpage A textbook of manufacturing technology: (manufacturing processes),Firewall Media, 2008 ISBN 8131802442 page 705
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