Reading the Vernier Scale on a Micrometer

The enlarged scale in figure 2-38 can help you understand how to take a complete micrometer read- ing to the nearest thousandth of an inch. As you can see, the thimble has been turned through 12 spaces of its scale, and the 12th graduation is lined up with the reference line on the sleeve. When the value on the sleeve scale is added to the value on the thimble scale that is lined up with the reference line on the sleeve scale, the space between the anvil and spindle must be 0.712 inch (seven hundred and twelve-thousandths of an inch). Occasionally you attain a reading in which the horizontal reference line of the sleeve scale falls be- tween two graduations on the thimble scale, as shown in figure 2-39. Note the horizontal reference line is closer to the 15 mark than the 14 mark. To read this measurement to THREE decimal places, simply round off to the 15 mark as shown in example A of figure 2-39. To read this measurement to FOUR deci- mal places, estimate the number of tenths of the dis- tance between thimble scale graduations the horizontal reference line has fallen. Each tenth of this distance equals one ten-thousandth (0.0001) of an inch. Add the ten-thousandths to the reading as shown in example B of figure 2-39. Reading the Vernier Scale on a Micrometer Many times you are required to work to excep- tionally precise dimensions. Under these conditions it is better to use a micrometer that is accurate to ten- thousandths of an inch. This degree of accuracy is obtained by the addition of a vernier scale. The vernier scale of a micrometer (fig. 2-40) fur- nishes the fine readings between the lines on the Figure 2-38.—Enlarged micrometer scale. Figure 2-39.—Estimating a micrometer reading. thimble rather than requiring you to interpolate (or estimate) the reading. The 10 spaces on the vernier are equivalent to 9 spaces on the thimble. Therefore, each unit on the vernier scale is equal to 0.0009 inch and the difference between the sizes of the units on each scale is 0.0001 inch. When a line on the thimble scale does not coin- cide with the horizontal reference line on the sleeve, you can determine the additional spaces beyond the readable thimble mark by finding which vernier mark matches up with a line on the thimble scale. Add this Figure 2-40.—Vernier scale of a micrometer. 2-26