Measurement Fundamentals - 5: Using Fractions of an 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.
Most trades use fractions of an inch for measurement. A carpenter can cut a 2x4 to a length of 36-1/4 inches and a welder can mark a length of pipe as 40-1/2 inches. Fractions play a big role in measurement accuracy. The tradesmen can use larger fractional denominators to make more precise measurements. For example, by using a larger denominator the carpenter can measure more precisely and cut the 2x4 to 36-3/16, an increase in accuracy of 1/16th inch. The larger the denominator used the more precise the measurement can be but the more difficult it would be to fabricate accurately.

The machinist also works in fractions of an inch. But, since a machined part is nearly always more precise in design than a piece of wood or a length of welded pipe, the machinist will use very large denominators in order to achieve fine measurements.

The machinist uses fractions of an inch as small as 64ths. A 64th can be visually discerned using a scale. However, anything smaller than a 64th becomes very hard to read . The photo below is somewhat enlarged but with clear vision and a good quality scale you can determine that the scale is reading 1-17/64. If the blueprint called out a dimension of 1-35/128 it would be very difficult to detect. So other instruments must be used to accurately measure these very fine dimensions.

There is also another limitation to using fractions with denominators based on these divisions; they are difficult to work with mathematically. Adding, subtracting, multiplying, and dividing fractions with so many different denominators is far more time consuming than doing the same math operations using decimals - as does the Metric System.

Note: a fraction is nothing more than a form of division and can be converted to decimals at will. For example 3/4 is 3 divided by 4 which is .750 decimal.

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