Machinery's Handbook, 31st Edition
770 Outside Micrometer Calipers Graduations: Reference , Graduation and Vernier lines on the barrel and thimble shall be clearly cut to a width of not more than 0.010 inch and all vernier and graduation lines shall be within 0.001 inch in uniform size. The lines and numbers shall have distinct and clean edges and shall be clearly legible under ordinary service conditions. The barrel and thimble shall be graduated as follows: Inch Reading Micrometer: The graduations on the thimble of the English (inch) reading micrometer shall be divided into 25 divisions each representing 0.001 inch. They shall be numbered as 0, 1, 2, 3, …, 23, and 24. Zero, then, represents 25 graduations or one full rev olution. The barrel is graduated with a longitudinal reference line that runs parallel to the axis of travel of the screw spindle. This reference line is graduated into 40 divisions each representing 0.025 or one full revolution of the thimble. The first graduation shall be num bered 0 (zero) and shall be the zeroing point for all consecutive measurements. After that every fourth division shall be numbered 1, 2, 3, and so on to 9, and 0. These divisions rep resent 0.100, 0.200, 0.300 and so on throughout the 1.000 inch length of travel. The 0.100 in. lines shall meet the reference line and extend above the line in a manner to designate them as the major 0.100 divisions. The 0.050 in. division lines shall meet the reference line and extend above the line but shall be half the length of the major 0.100 in. lines. The 0.025 in. graduation lines shall also meet the reference line and extend above the line to half the length of the 0.050 division lines. This method establishes a readily visible refer- ence to the exact cumulative value of the reading. The vernier lines for the measurement of 1/ 10,000 th inch (0.0001) shall be engraved on the upper part of the barrel parallel to the axis of travel of the spindle and extend a sufficient length along the barrel as to provide a reading at any point throughout the entire length of travel. The 10 vernier spaces are equal to nine divisions on the thimble and are numbered 0, 1, 2, 3, …, 9, and 0. Metric Reading Micrometer: The graduations of metric (SI) reading micrometers are configured in the same manner as the English reading micrometers. However, it must be stressed that manufacturers have developed variations of the basic principal that are equally accurate, and in some cases may be considered a genuine improvement. Slanted minor graduation lines, and split major minor graduations, have both been employed with great success. However, the principle for division of the scale is the same, and provides the amplification of the spindle screw movement necessary to obtain precision and repeat - able measurements with the micrometer. The metric micrometer thimble is graduated into 50 divisions, each representing 0.01 mm. These are numbered as 0, 5, 10, 15, …, 45. The barrel is graduated along the reference line at 0.5 mm intervals. The reference line runs parallel to the axis of travel of the spindle screw. The first of the graduations lines is designated as 0 (zero) and every 10th graduation thereafter is numbered as 5, 10, 15, 20, and 25. The graduation lines representing 1.0 mm shall be meet the reference line and be distinguished by either length or position. The graduation lines representing 0.05 mm shall be distinguished from all other lines again by either length or position, as in the En- glish scale system. The vernier scale shall be as described previously in the section Inch Reading Micrometer . Accuracy: All micrometers shall be calibrated at a controlled temperature in a manner and method developed to assure performance parameters are acceptable and within the limits of tolerances as defined in Table 21a and Table 21b. Proper maintenance and periodic calibration are mandatory to maintaining a high confidence level in all precision measuring instruments. Reference Setting Standards: The Type I, Class 1, Styles A and B, and Type I, Class 2, Style A micrometers are furnished with a reference standard for checking and setting the zero reading between the regularly scheduled calibration. These reference standards do not constitute a complete and thorough calibration but may be used to verify the zero setting of the instrument. These setting standards are themselves calibrated to assure adherence to tolerance, and act as a transfer standard during the calibration interval. The
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