A micrometer (often called a mike) is a precision measuring instrument or tool. Outside, inside, and depth micrometers are made to divide an inch into 1,000 equal parts. The vernier micrometer is made to divide an inch into 10,000 parts.

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0 - 1 Outside Micrometer

Outside micrometer are used to measure the thickness of materials and parts and the diameter of shafts.

A standard outside micrometer has a U-shaped frame, an anvil, a spindle, a sleeve, a barrel or hub, and a thimble.

Outside micrometers are made in sizes up to 24 inches. Regardless of the size of the micrometer, the length of the spindle is always 1 inch. The frames are made larger. Micrometer sizes are classified as 0 to 1 inch, 1 to 2 inches, 2 to 3 inches, 9 to 10 inches, etc., so each size of micrometer measures just 1 inch.

The basic element of the outside micrometer is an accurately ground spindle cut to 40 threads per inch. One revolution of the spindle produces a movement of one-fortieth of an inch, or 0.025 inch. There is a mark on the micrometer sleeve for every 0.025-inch increment. The thimble over the sleeve is divided into 25 parts, each representing 0.001 inch. When the zero graduation of the thimble is aligned with an engraved line along the sleeve, the reading is a multiple of 0.025 inch (0.025, 0.050, 0.075). If some mark other than zero on the thimble is opposite the line on the sleeve, the number of thousandths on the thimble is added to the reading on the sleeve. Thus, if the sleeve reads 0.450 (4 plus two small divisions) and if the 17 mark on the thimble aligns with the line on the sleeve, the measurement is 0.450 + 0.017, or 0.467 inch.

How to read an outside micrometer

Each of the 25 marks on the thimble represents 0.001 inch. Each complete turn on the thimble equals 0.025 inch. Each division on the sleeve is equal to 0.025 inch. Four divisions are equal to 0.100 inch (4 x 0.025 = 0.100). Therefore, four complete rotations of the thimble will equal 0.100 inch.

Note the number of lines on the sleeve that are exposed, starting with zero. Count the number of 0.100 lines that show. Add to this the value of the 0.025 lines such as 0.025, 0.050, or 0.075. To this reading, add the number of divisions on the thimble from zero to the line that coincides with the index line on the sleeve. The index line is the long line that runs lengthwise to the sleeve. All thimble readings are taken from the line on the thimble that lines up with or coincides with this index line.

Outside Micrometer

When the end of the thimble is somewhat beyond or between the sleeve graduations, you must add the thimble reading from zero to the sleeve reading to get the total reading. Three divisions past the zero line on the thimble represent 0.003 inch.

Add the reading on the thimble to the reading on the sleeve, or 0.175 + 0.003. This equals 0.178, the total reading.

Now look at the exercise to see if you can determine the readings.

Micrometer Reading Example