Thousandth of an inch
Thousandth of an inch | |
---|---|
Symbol | thou, mil |
Conversions | |
1 thou in ... | ... is equal to ... |
imperial and US customary systems | 0.001 in |
SI units | 25.4 μm |
A thousandth of an inch is a derived
The unit of measurement mil appears as:
- linear mil = 0.0254 mm (0.00100 in)
- square mil = 0.00064516 mm2 (1.0×10−6 sq in)
- circular mil = 0.0005067 mm2 (7.854×10−7 sq in)
- angular mil ≈ 1 milliradian≈ 0.057296 degrees (also known as NATO mil)
The words are
Contexts of use
The thou, or mil, is most commonly used in engineering and manufacturing in non-metric countries. For example, in specifying:
- The thickness of items such as paper, film, foil, wires, paint coatings, latex gloves, plastic sheeting, and fibers
- Manufacturing dimensions and tolerances, such as:
- In the manufacture of older automobile engines. A typical example is the thickness of the head gasket, or the amount of material to be removed from the head to adjust the compression ratio of the cylinders.
- In the servicing of older automobile engines. Typical examples include a spark-plug gap or ignition points gap.
- The manufacture of millimetres, because they are sold worldwide.
- Tolerance specifications on hydraulic cylinders
There are also compound units such as "mils per year" used to express corrosion rates.[7]
A related measurement for area known as the circular mil, is based on a circle having a diameter of one mil.
Tenths
In areas of machining where the thou is used, 0.0001 inches (2.54 micrometres) is often treated as a basic unit and can be referred to as "one tenth", meaning "one tenth of a thou" or "one ten thousandth".[8] Other common terms used in machining with Imperial units involve adding tenths together to achieve a specific tolerance or measurement. For example, "five tenths," is typically a measurement or tolerance of five ten-thousandths of an inch, and written as 0.0005 inches. "Three tenths," as another example, is written as 0.0003 inches [9]
Machining "to within a few tenths" is often considered very accurate, and at or near the extreme limit of tolerance capability in most contexts. Greater accuracy (tolerance ranges inside one tenth) apply in only a few contexts: in
Usage notes on mil versus thou
In the United States, mil was once the more common term,[10][11] but as use of the metric system has become more common, thou has replaced mil among most technical users to avoid confusion with millimetres.[10][11][12] Today both terms are used, but in specific contexts one is traditionally preferred over the other.
Equivalents in other units of length
1 thou is equal to:
- 0.001 international inches (1 international inch is equal to 1,000 thou)
- 0.0254 mm, or 25.4 μm (1 millimetre is about 39.37 thou)
History of usage
The introduction of the thousandth of an inch as a base unit in engineering and machining is generally attributed to Joseph Whitworth[13] who wrote in 1857:
... instead of our engineers and machinists thinking in eighths, sixteenths and thirty-seconds of an inch, it is desirable that they should think and speak in tenths, hundredths, and thousandths ...[14]
Whitworth's main point was to advocate
The introduction of the thou as a base unit for machining work required the dissemination of
See also
- Angular mil
- Circular mil
References
- ^ "mil". Lexico UK English Dictionary. Oxford University Press. Archived from the original on July 13, 2021.
- ^ "Paper weight comparison and more". paper-paper.com. Micro Format. Retrieved 2016-01-18.
- ^ Carruthers, John; Hurwitz, Tobia (2014). Guitar Care and Gear Essentials. Canada: Alfred Music Publishing. p. 146.
- )
- ^ "The Gerber Format Specification" (PDF). Ucamco. July 2014. p. 30. Retrieved 2014-08-27.
- ISBN 1-877750-24-7. Retrieved 2021-07-10 – via Smithsonian Chips.
- ^ "Corrosion Rate Conversion", Corrosionist.com
- ^ "...the smallest move of one-tenth (not 0.1 but 0.0001 of an inch)...", Dan Nelson, The CNC Toolbox, p89
- ^ "Beginner's Guide To Reading Machine Shop Numbers & Values". machinistguides.com. Amazon Services LLC Associates Program. Retrieved 2023-04-28.
- ^ a b Mil at How Many? A Dictionary of Units of Measurement by Russ Rowlett
- ^ a b University of Queensland: PCB design FAQ
- ^ Thou at How Many? A Dictionary of Units of Measurement by Russ Rowlett
- ^ Edkins, Jo. "Small units". Imperial Measures of Length. Jo Edkins. Retrieved 2009-09-23.
- ^ "A Paper on Standard Decimal Measures of Length", Manchester, 1857