Inch-based and Metric-based Micrometers

Inch-based and metric-based outside micrometers are precision instruments used to measure the thickness of parts from small washers to large brake rotors. They work the same and are similar in design. Place a part between the spindle and the anvil and turn the spindle until a slight drag is felt. Overtightening the spindle can affect the reading.

Inch-based Micrometers

They are different. The inch-based micrometer above is graduated in thousandths of an inch. Each numbered division equals one-tenth of an inch (0.10"). The divisions on the bottom of the spindle are each 25 thousandths of an inch (0.025"). The divisions on the thimble are each one one-thousandths of an inch (0.001").

Reading an Inch-based Micrometer

An accurate reading is taken by adding these three numbers. The micrometer above reads .228 inches. The number 2 on the top of the spindle indicates two hundredths (0.20") of an inch. The division on the bottom indicates twenty-five thousandths (0.025") of an inch. The three divisions on the thimble read three one-thousandths (0.003") of an inch. The three add up to two hundred twenty-eight thousandths of an inch (0.228").

Reading a Metric-based Micrometer

Metric micrometers read differently. The sleeve of a metric micrometer has markings on the top and the bottom, and the thimble has increments of .01 millimeters. It has whole millimeters on the top and half-millimeters on the bottom. There are 50 divisions of .01mm on the thimble of a metric micrometer.

The above micrometer has six whole millimeters on the top and a half-millimeter on the bottom. The thimble indicates 0. The three together equal 6.50mm.

Some metric micrometers have whole digits on the bottom and half millimeters on the top. The micrometer on the bottom reads 6.55mm. Start with the six whole millimeters on the bottom, which equals 6.0 millimeters. Then add in the one .50 millimeters on the top of the sleeve; this equals 6.50 millimeters. Finally, add the five .01 millimeters on the thimble for a total of 6.55 millimeters.