Reading Micrometers - 2: The Barrel Reading WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor. The first step in learning how to read a micrometer is to understand how it works. The Micrometer spindle threads into the body of the micrometer like a screw unto a nut. And just like a screw and a nut rotating about each other it causes one or the other of them to move an exact distance in relationship to each other. To be exact the movement is one pitch of the thread for each revolution. For a standard micrometer the number of threads over one inch is exactly 40. A little division then shows that the pitch is .025 inches and therefor the exact movement of the spindle inside of the micrometer body is .025 inch. Exactly! When the anvil and spindle are closed tight the thimble would cover all of the numbers and marks shown and the 0 (zero) on the thimble would be right on the reading line of the barrel. Each revolution of the thimble backwards would then expose exactly one of the .025 marks. Reading a micrometer entails adding up the .025 marks that become exposed. To help keep track of the number exposed there will be numbers 1 through 9 that also become exposed when the thimble is moved back far enough. These numbers represent tenths of an inch such as .100, .200, .300, and so on . On the next page you will see how to read measurements that lie between the marks. A typical alignment exposes most of the mark (left) when the thimble zero aligns with the reading line on the barrel. For some micrometers the alignment fully exposes the mark (left) when the thimble zero aligns with the reading line on the barrel. For some micrometers the alignment almost completely covers the mark (left) when the thimble zero aligns with the reading line on the barrel. As shown not all mics align the same so the best thing to do is get familiar with your pair by closing them and seeing how things line up and what marks are exposed (or not). In that way you can figure out how to read the mic in other positions. R.S. label: micrometer, history, anvil, thimble, spindle, frame, barrel, ratchet, adjustment, holding, handling, reading, vernier , metric, electronic