Tool School: More Precise Measuring

In my post about vernier calipers, I highlighted one rugged option for making highly accurate measurements. When building projects that involve things like sliding fits, interference fits, shafts and bearings, rotating parts, measuring sheet metal thickness (and the list goes on, and on), accurate and repeatable measurements in the range of 1/1000 of an inch become very important. In this Tool School, I look at another option: the micrometer. A standard micrometer is capable of the same 1/1000-inch accuracy as the vernier calipers, and micrometers that incorporate a vernier scale are capable of measurements an order of magnitude more accurate: 1/10,000 of an inch. In addition, the variety of forms micrometers take allow measurement of a far larger variety of things than would be possible with calipers. Here's how to use one.

Like the vernier calipers, micrometers use the simple fact of arithmetic that 40 * 25 = 1000 to great advantage. Lets look at how.

The "40": The essential element of a micrometer is a threaded shaft with a thread pitch of exactly 40 threads per inch. Thread pitch refers to the measurement of the number of threads in one inch on something that is threaded, like a bolt, or the shaft in a micrometer. In the case of the micrometer, this 40 tpi thread pitch means that one full revolution of the shaft will move it in or out exactly 1/40th of an inch.

The "25": Full revolutions are further divided equally into 25 divisions. Each full revolution of the shaft of the micrometer entails 24 marks indicating 25 divisions passing the fixed line (lets call it the "zero" point) on the body of the micrometer.

So, if each revolution of the handle is 1/40th of an inch and each mark passing the "zero point" on the body of the tool represents 1/25th of each of those revolutions, then it follows that each mark passing the zero point represents 1/25 * 1/40 = 1/1000 of an inch. Hence the 1/1000" accuracy of a micrometer.

There are versions of the micrometer that include a 10-division vernier scale on the sleeve of the tool. Using this vernier scale, the micrometer can provide accuracy to another order of magnitude, 1/10th (because it is a 10-division scale) of 1/1000th or 1/10,000th of an inch.

As with so many things, using the micrometer is far simpler than the math behind of its operation might suggest. Simply turn the handle, or thimble, of the micrometer until it is positioned snugly against the item to be measured. Micrometers often include some form of torque clutch on the handle which will begin to freewheel after the thimble has been tightened sufficiently.

With the thimble in position, read the measurement. First, read the number of full divisions showing on the sleeve. Each division corresponds to one full revolution: 1/40th of an inch or 0.025-inch. To aid in reading the tool, every 0.1-inch is typically also marked. In the micrometer pictured here, the reading in this step is 0.125-inch. Next, read the scale marked on the thimble. The numbers on this thimble correspond to 1/1000th of an inch, or 0.001-inch. In our example, this is 14 or 0.014-inch. To obtain the full measurement, simply add these two readings; 0.139-inch in this example.

For a micrometer with a vernier scale, as in the micrometer pictured below, the next significant digit (0.0001) is read from the first line on the thimble to directly align with a mark on the vernier scale.

It should be noted you need to periodically calibrate micrometers to ensure that the measurement they provide actually corresponds to the standard version of that measurement in the real world. And it's worth noting that these tools are more delicate than vernier calipers.

To learn more, a great reference is "The Starrett Book For Student Machinists."