The Inch Based Micrometer The illustration above shows a basic micrometer as used on outside, inside and depth micrometers (along with many others). The thimble rotates relative to the barrel on a threaded shaft that has 40 threads per inch. Therefore one complete revolution of the thimble moves it axially 1/40th, or .025 inch. The barrel scale is divided into increments of .025 inch, with each .100 inch numbered for convenience. The thimble scale is divided into increments of .001 inch, with each .005 inch numbered around the thimble as 0, 5, 10, 15, and 20.

The following example is presented to illustrate the four step addition method. To read the indicated value, proceed as follows: Step #1. Record the whole number that represents the minimum size the micrometer could read. Assume the illustration above is on a 3 to 4 inch outside or inside micrometer. Write it down as shown here:

3.000

Step #2: Using the end of the thimble as an index line, record the whole number of .100 inch increments to the left of the index line. The illustration above shows that there are seven .100 inch increments to the left of the index line, which equals .700 inch. Record this value as shown:

3.000
0.700

Step #3: Still using the end of the thimble as an index line, record the whole number of .025 inch increments to the left of the index line. The illustration above shows that there is one .025 inch increments to the left of the index line, which equals .025 inch. Record this value as shown:

3.000
0.700
0.025 Step #4. Using the axial line on the barrel as the index line, read the number of the .001 increment from the thimble that is closest to the index line. The illustration above shows that there the mark representing sixteen lines up with the index line, which equals .016 inch. Write it down as shown here:

3.000
0.700
0.025
0.016