Measurement Fundamentals - 6: The Machinist's Inch
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.

Long ago engineers and machinist realized the awkwardness and limitations of using fractions. Especially very small fractions with large denominators. So, by convention, the machine trades borrowed a little idea from the Metric System: they decided to break the inch into10ths, 100ths and 1000ths and then to represent these inch divisions as decimals. This now makes the inch as easy to work with as a millimeter. For example 1/10 inch is automatically converted to decimals as 0.100 and 3/16 inch is automatically converted to 0.1875. However one thing that has evolved from this has made the machinist's inch even more unique. The shop trades have dropped the 10th and 100th divisions of an inch and refer to everything in thousandths of an inch. For example .12 is automatically converted to .120 and 2.1 is converted to 2.100. Decimals of the inch are always expressed as three place decimals (thousandths) regardless of the blueprint dimension. The animation in this paragraph has an audio track. Click on it and listen carefully.

The thousandths method works fine until measurements even smaller are required. For example if a measurement reads 0.0625 there would be no way to express the last digit using our shop lingo since the last digit is in the ten-thousandths range. The machinist handles this by adding a tag of tenths to his measurement. The above decimal would be pronounced, "sixty two and five tenths" or alternately "sixty two thousandths and five tenths" and .1021 would be pronounced, "one hundred two and one tenth" or alternately, "one hundred two thousandths and one tenth"

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label: linear, angular, measurement, systems, inch, metric, convert, fractions, machinist inch