Machinery's Handbook, 31st Edition
690
Measuring Instruments MEASURING, INSTRUMENTS, AND INSPECTION METHODS Reading Verniers and Micrometers
Reading a Vernier.— A general rule for taking readings with a vernier scale is as follows: Note the number of inches and subdivisions of an inch that the zero mark of the vernier scale has moved along the true scale, and then add to this reading as many thousandths, hundredths, or whatever fractional part of an inch the vernier reads to, as there are spaces between the vernier zero and that line on the vernier coinciding with one on the true scale. For example, if the zero line of a vernier that reads to thousandths is slightly beyond the 0.5 inch division on the main or true scale, as shown in Fig. 1, and graduation line 10 on the vernier exactly coincides with one on the true scale, the reading is 0.5 + 0.010 or 0.510 inch. In order to determine the reading or fractional part of an inch that can be ob- tained by a vernier, multiply the denominator of the finest subdivision given on the true scale by the total number of divisions on the vernier. For example, if one inch on the true scale is divided into 40 parts or fortieths (as in Fig. 1 ), and the vernier into twenty-five parts, the vernier will read to thousandths of an inch, as 25 3 40 = 1000. Similarly, if there are sixteen divisions to the inch on the true scale and a total of eight on the vernier, the latter will enable readings to be taken within 1 ⁄ 128 of an inch, as 8 3 16 = 128.
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Fig. 1. Inch Vernier
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Fig. 2. Protractor with Vernier Scale If the vernier is on a protractor, note the whole number of degrees passed by the vernier zero mark and then count the spaces between the vernier zero and the line coinciding with a graduation on the protractor scale. If the vernier indicates angles within five minutes or one-twelfth degree (as in Fig. 2), the number of spaces multiplied by 5 will, of course, give the number of minutes to be added to the whole number of degrees. The reading of the protractor set as illustrated would be 14 whole degrees (the number passed by the zero mark on the vernier) plus 30 minutes, as the graduation 30 on the vernier is the only one to the right of the vernier zero that exactly coincides with a line on the protractor scale. It will be
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