Machinery's Handbook, 31st Edition
692 Measuring Instruments The smallest inch graduation on the bar is 0.025 inch, and the 25 vernier divisions occupy the same length as 49 bar divisions, which is 1.225 inches. Therefore, one vernier division equals one twenty-fifth of 1.225 inches = 0.04 × 1.225 = 0.049 inch. Thus, the difference between the length of two bar divisions and a vernier division is 0.050 − 0.049 = 0.001 inch. The vernier scale is graduated for direct reading to 0.001 inch. In the example, the vernier zero is past the 1.075 graduation on the bar, and the 0.004 graduation on the vernier coin- cides with a line on the bar. Thus, the total reading is 1.079 inches.
Fig. 2. Dual Metric-Inch Vernier Reading a Micrometer.— The spindle of an inch-system micrometer has 40 threads per inch, so that one turn moves the spindle axially 0.025 inch (1 4 40 = 0.025), equal to the distance between two graduations on the frame. The 25 graduations on the thimble allow the 0.025 inch to be further divided, so that turning the thimble through one division moves the spindle axially 0.001 inch (0.025 4 25 = 0.001). To read a microm- eter, count the number of whole divisions visible on the scale of the frame, multiply this number by 25 (the number of thousandths of an inch that each division represents) and add to the product the number of that division on the thimble coinciding with the axial zero line on the frame. The result will be the diameter expressed in thousandths of an inch. As the numbers 1, 2, 3, etc., opposite every fourth subdivision on the frame indicate hundreds of thousandths, the reading can easily be taken mentally. Suppose the thimble were screwed out so that graduation 2 and three additional subdivisions were visible (as shown in Fig. 3), and that graduation 10 on the thimble coincided with the axial line on the frame. The reading then would be 0.200 + 0.075 + 0.010, or 0.285 inch.
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Fig. 3. Inch Micrometer Fig. 4. Inch Micrometer with Vernier Some micrometers have a vernier scale on the frame in addition to the regular gradua tions so that measurements within 0.0001 part of an inch can be taken. Micrometers of this type are read as follows: First determine the number of thousandths, as with an ordinary micrometer, and then find a line on the vernier scale that exactly coincides with one on the thimble; the number of this line represents the number of ten-thousandths to be added to the number of thousandths obtained by the regular graduations. The reading shown in the illustration, Fig. 4, is 0.275 + 0.020 + 0.0003 = 0.2953 inch.
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