How to Read a Micrometer Screw

Micrometers are precision measuring instruments capable of recording measurements more than 10 times thinner than a human hair. A typical micrometer resembles a C-clamp, with the top of the "C," called the anvil, being a fixed point and the bottom point, called the spindle, moving up and down via a screw mechanism. To measure with a micrometer, you twist the handle, known as the thimble, until the spindle and anvil contact whatever it is you're measuring. Where the top of the thimble stops on its ruler-like shaft, known as the sleeve, indicates the measurement taken.

• 1

  Spin the thimble counterclockwise until it moves all the way down to the bottom of the sleeve and the "0" on the thimble lines up with the ruler line on the sleeve. Note the ruler-like markings on the sleeve. You'll see ten large lines, usually noted with a number one through ten, with three smaller lines between each. The net effect is that you have ten sections all divided into four smaller sections. Each of the large numbers represents 0.100 inch; each of the small hash marks represents 0.025 inch.

• 2

  Note the numbered hash marks on the thimble, which start at zero and go to 25. Each of the hash marks on the thimble represents 0.001 inch. As such, a full rotation of the thimble equals 0.025 inch and four full rotations equal 0.100 inch, or one number up on the sleeve.

• 3

  Measure your object by placing one side of it against the anvil and slowly spinning the thimble until the spindle contacts the other side. Look at where the thimble stopped on the sleeve. Decode the readings, and add the values together.

  Example: The thimble stops at the third hash mark past the #2 mark, with the thimble at the 8 position. Add together 0.200--the #2 hash mark--plus 0.075--indicated by the third mark position, since each small mark equals 0.025--to wind up with a measurement of 0.275. Add in the 0.008 indicated by the thimble position and you end up with a final measurement of 0.283 inches.