Thousandth of an inch

Thousandth of an inch
ID-1cards likepayment cardshave a thickness of 0.03 inches (30 thou).
General information
Unit system	Imperial/USunits
Unit of	Length
Symbol	thou, mil
Conversions
1 thouin...	... is equal to...
imperial/USunits	0.001in
SI units	25.4μm

Athousandth of an inchis a derivedunitoflengthina system of units using inches.Equal to1⁄1000of aninch,a thousandth is commonly called athou/ˈθaʊ/(used for both singular and plural) or, particularly in North America, amil(pluralmils).

The words areshortened formsof the English and Latin words for "thousand" (millein Latin). In international engineering contexts, confusion can arise becausemilis a formal unit name in North America butmilormillis also a common colloquial clipped form ofmillimetre.The units are considerably different: amillimetreis approximately 39 mils.

Contexts of use

The thou, or mil, is most commonly used inengineeringandmanufacturingin 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
  • For example, most plasticID cardsare about 30 thou (0.76 mm) in thickness.
  • Card stock thickness in the United States, where mils are also called points.
  • Gauge(diameter) of strings in stringed instruments
• Manufacturing dimensions andtolerances,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 ofprinted circuit boards(PCBs).However, the component dimensions are now typically provided inmillimetres,because they are sold worldwide.
  • Tolerance specifications onhydraulic cylinders

There are also compound units such as "mils per year" used to express corrosion rates.

Mil-measurements

The word "mil" appears in the following different units:

• linear mil= 0.0254 mm (0.001 in)
• square mil= 0.00064516 mm²(1.0×10⁻⁶sq in)
• circular mil= 0.0005067 mm²(7.854×10⁻⁷sq in) = π/4 square mil
• angular mil≈ 1milliradian≈ 0.057296 degrees (also known asNATO mil)

Tenths

In areas ofmachiningwhere the thou is used, 0.0001 inches (2.54micrometres) 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".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

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: inplug gaugeandgauge blockmanufacturing or calibration, they are typically expressed in millionths of an inch or, alternatively, in micrometres; innanotechnology,nanometresorpicometresare used.

Usage notes onmilversusthou

In the United States,milwas once the more common term,but as use of the metric system has become more common,thouhas replacedmilamong most technical users toavoid confusion with millimetres.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 internationalinches(1 international inch is equal to 1,000 thou)
• 0.0254mm,or 25.4μm(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 toJoseph Whitworthwho 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...

Whitworth's main point was to advocatedecimalizationin place of fractions based on successive halving; but in mentioning thousandths, he was also broaching the idea of a finer division than had been used previously. Until then, workers such asmillwrights,boilermakers,andmachinistsin theAnglospheremeasured only in traditional fractions of an inch, divided via successive halving, usually only as far as 64ths (1,1⁄2,1⁄4,1⁄8,1⁄16,1⁄32,1⁄64). Each 64th is about 16 thou. Communication about sizes smaller than a 64th of an inch was subjective and hampered by a degree ofineffability—while phrases such as "scant 64th" or "heavy 64th" were used, they were imprecise. Dimensions and geometrycouldbe controlled to high accuracy, but this was done by comparative methods: comparison against templates or other gauges, feeling the degree of drag of calipers, or simply repeatably cutting, relying on the positioning consistency of jigs, fixtures, and machine slides. Such work could only be done in craft fashion: on-site, by feel, rather than at a distance working from drawings and written notes. Although measurement was certainly a part of the daily routine, the highest-precision aspects of the work were achieved by feel or by gauge, not by measuring (as in determining counts of units). This in turn limited the kinds ofprocess designsthat could work, because they limited the degree ofseparation of concernsthat could occur.

The introduction of the thou as a base unit for machining work required the dissemination ofvernier calipersand screwmicrometersthroughout the trade, as the unit is too small to be measured with practical repeatability usingrulesalone. (Most rule markings were far too wide to mark a single mil, and even if suchdividingis accomplished, it is unclear to the naked eye, being discernible but not useful for measuring.) During the following half century, such measuring instruments, previously expensive rarities, became widespread, everyday tools among machinists. Bringing moremetrologyinto machining made possible, for example, designing an assembly in the form of anengineering drawing,then having themating partsmade at different firms who did not have any contact with (or even awareness of) each other—yet still being sure that their products would have the desiredfit.