Figure 2-22.—Nomenclature of an outside micrometercaliper.Figure 2-23.—Sleeve and thimble scales of a micrometer.numbered 1, 2, 3, 4, and so on, representing 0.100inch,0.200 inch, and so on. When you turn the thimbleso its edge is over the first sleeve line past the 0 on thethimble scale, the spindle has opened 0.025 inch. If youturn the spindle to the second mark, it has moved 0.025inch plus 0.025 inch, or 0.050 inch.When the beveled edge of the thimble stopsbetween graduated lines on the sleeve scale, you mustuse the thimble scale to complete your reading. Thethimble scale is divided into 25 equal parts; each partor mark represents 1/25th of a turn. And, 1/25th of0.025 inch equals 0.001 inch. Note that in figure 2-23every fifth line on the thimble scale is marked 5, 10,15, and so on. The thimble scale permits you to takevery accurate readings to the thousandths of an inch.Figure 2-24.—Enlarged micrometer scale.The enlarged scale in figure 2-24 can help youunderstand how to take a complete micrometer readingto the nearest thousandth of an inch.The thimble is turned far enough to expose the 7 onthe sleeve scale but not far enough to expose the firstmark after the 7. Therefore, the measurement must bebetween 0.700 inch and 0.725 inch, Exactly how farbetween 0.700 inch and 0.725 inch must be read on thethimble scale.As you can see, the thimble has been turnedthrough 12 spaces of its scale, and the 12th graduationis lined up with the reference line on the sleeve. Whenthe value on the sleeve scale is added to the value onthe thimble scale that is lined up with the reference lineon the sleeve scale, the space between the anvil andspindle must be 0.712 inch (seven hundred and twelvethousandths of an inch).MICROMETER-READING EXERCISE.—Occasionally you attain a reading in which thehorizontal reference line of the sleeve scale fallsbetween two graduations on the thimble scale, as shownin figure 2-25. Note the horizontal reference line iscloser to the 15 mark than to the 14 mark. To read thismeasurement to THREE decimal places, simply roundoff to the 15 mark, as shown in example A of figure2-25. To read this measurement to FOUR decimalplaces, estimate the number of tenths of the distancebetween thimble scale graduations the horizontalreference line has fallen. Each tenth of this distanceequals one ten-thousandth (0.0001) of an inch. Add theten-thousandths to the reading as shown in example Bof figure 2-25.2-9

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