MICROMETER-READING EXERCISE

When the beveled edge of the thimble stops between graduated lines on the sleeve scale, you must use the thimble scale to complete your reading. The thimble scale is divided into 25 equal parts; each part or mark represents 1/25th of a turn. And, 1/25th of 0.025 inch equals 0.001 inch. Note that in figure 2-23 every fifth line on the thimble scale is marked 5, 10, 15, and so on. The thimble scale permits you to take very accurate readings to the thousandths of an inch. The enlarged scale in figure 2-24 can help you understand how to take a complete micrometer reading to the nearest thousandth of an inch. The thimble is turned far enough to expose the 7 on the sleeve scale but not far enough to expose the first mark after the 7. Therefore, the measurement must be between 0.700 inch and 0.725 inch. Exactly how far between 0.700 inch and 0.725 inch must be read on the thimble scale. 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). MICROMETER-READING EXERCISE.— Occasionally you attain a reading in which the horizontal reference line of the sleeve scale falls between two graduations on the thimble scale, as shown in figure 2-25. Note the horizontal reference line is closer to the 15 mark than to 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 2-25. To read this measurement to FOUR decimal places, estimate the number of tenths of the distance 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 2-25. 2-9 Figure 2-23.—Sleeve and thimble scales of a micrometer. Figure 2-24.—Enlarged micrometer scale. Figure 2-22.—Nomenclature of an outside micrometer caliper. Figure 2-25.—Reading sleeve and thimble scales of a micrometer.