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How To Read a Micrometer

Micrometers are precision measuring instruments used for accurately measuring very small distances. They are used when a standard rule or a slide caliper cannot get the precise measurement needed.

Micrometers use a calibrated screw to determine these small distances. The item to be measured is placed between a stationary endpoint, the anvil, and a movable metal post, the spindle. As the spindle is screwed in and out to touch the item, the number of threads covered up or revealed is used to calculate a measurement.

Micrometer parts

The sleeve resembles a ruler with ten numbers. The gaps between the numbers are divided into quarters. The sleeve does not move. When the thimble rotates around the sleeve, it covers up, or reveals the numbers marked on the sleeve.

There are three types of micrometers commonly used:

  • Outside micrometers, used to measure outside distances
  • Inside micrometers, used to measure inside diameters
  • Depth micrometers, used to measure depths

A couple of points should be brought up before you purchase a micrometer. First, there are now digital micrometers available where the measurement can be read directly from a digital display, negating the need to learn how to read a micrometer. Secondly, micrometers are available in metric and imperial measurements. We will discuss the Imperial, or inches, version.

Quick Math Refresher
When reading a micrometer, it is necessary to know what the readings actually represent. Keep in mind our end result – a measurement in inches.

All of the following decimal numbers equal the same fraction - 1/2 of an inch:

  • .5"
  • .50"
  • .500"
  • .5000"

Decimal numbers and their meaning:

tenths 0.5
hundredths 0.05
thousandths 0.005
ten thousandths 0.0005

Each digit is a different place value.

Here is an example: 0.4567

  • Digit 1 is the tenths place - 0.4567 has four tenths
  • Digit 2 is the hundredths place - 0.4567 has five hundredths
  • Digit 3 is the thousandths place - 0.4567 has six thousandths
  • Digit 4 is the ten-thousandths place - 0.4567 has seven ten-thousandths

So, our example has four tenths, five hundredths, six thousandths, and seven ten-thousandths.

Reading An Imperial Micrometer

Micrometer Numbers:

  • Whole numbers on the sleeve = tenths of an inch
  • Quarter numbers on the sleeve = hundredths and thousandths of an inch
  • Vernier scale on the sleeve = ten-thousandths of an inch

To read a micrometer, read the micrometer scale and add the fraction of a revolution.

Micrometer lines

To read this example:

  1. Read the highest whole number visible on the sleeve: 3 = 0.300 inches
  2. Read the lines visible after the whole number and before the thimble. Multiply this number by 25: 2 lines visible x 25 = 0.050 inches.
  3. Read the line on the thimble: 17 = 0.017 inches.

Full Measurement = .300 + .050 + .017 = .367 inches

Vernier Scale
To get readings precise to ten-thousandths of an inch, some micrometers will have an extra scale of numbers on the sleeve called a vernier scale, which has ten divisions. To get this extra precision, first read the micrometer to get the thousandths reading. Then, find the line on the vernier scale which corresponds with a line on the thimble and add that amount to the thousandths reading. So, for example, if the line is “1”, add one ten-thousandth, and if the line is “2”, add two ten-thousandths.

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