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178
Reading Rulers
UNIT
22
Objectives
Information in this unit will enable you to:
▪ Discuss how the US Customary System works.
▪
Describe how the metric system works.
▪ Explain how to read US Customary, decimal, and metric rulers.
The US Customary System
Most of the measurements machinists encounter are linear measuremnts. That means
they are measurements of distance.
The US Customary System of measurents is the most widely used in the
United States, although use of the metric system is increasing. It was adapted from
the English system after the Revolutionary War. The US Customary System is based
on yards, feet, and inches for linear measure.
US Customary Units of Linear Measure
1 foot 12 inches
1 yard 3 feet
1 mile 5,280 feet
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The inch is the unit that is usually used in the machine shop. An inch can be
divided into fractional parts of an inch for smaller distances. The divisions (called
graduations) can be seen on a common ruler.
Reading a US Customary Ruler
The most common measuring instrument is the ruler, sometimes called a rule. Steel
rules are used in the machine shop. Rulers can have three kinds of graduations. The
graduations on a US Customary ruler divide inches into halves, quarters, eighths,
sixteenths, thirty-seconds, and sometimes sixty-fourths. Each graduation level divides
the next larger graduation in half. For example, 1/16 is half of 1/8, and 1/32 is
half of 1/16. The whole-inch graduations are the longest on the ruler. The 1/2-inch