Machinery's Handbook, 31st Edition
CALIBRATION OF OUTSIDE MICROMETERS 791 optical flat by pressing down if a burr is detected. Scratching the surface of the optical flat can ruin the flat. Caution is required. • Using the optical flat and monochromatic light, observe interference bands on anvil face. • Count the number of bands between the lowest ends and the crest of 1 band. Multiply this number by the frequency of the monochromatic light being used. Confirm that the calculated deviation is within the tolerance limits defined in Table 13. 1 band = 11.6 millionths ( µ in) using helium discharge light source (Table 12). Record the band count, frequency and result. Interpreting Optical Flat Fringe Patterns Fig. 23a. Fig. 23d. Example, Optical Flat Fringe Patterns: Fig. 23a through Fig. 23d illustrate fringe patterns produced by an optical flat in contact with several test surfaces. The following illustrates how to interpret the fringe patterns observed to determine deviations in flatness of the unit under test (UUT). Fig. 23b. Fig. 23c. a) Convex surface (Fig. 23a ): side edges low 11.6 µ in b) Convex surface (Fig. 23b ): side edges low 5.7 µ in. c) Nearly flat ( Fig. 23c ): side edges low 5.7 µ in. d) Complex surface (Fig. 23d ) both convex and concave: hollow in center, higher each side of center and lower at side edges: error 3 µ in. • Turn over the UUT and repeat preceding steps for the spindle. If this is inconvenient due to size of UUT, remove the spindle to take the reading. Calculate in the same manner as above. Record the reading. • If UUT is an interchangeable anvil micrometer, repeat measurement for each anvil. • Note: If anvil or spindle do not create an image, or are too worn to produce an image, they will need to be sent out to the appropriate vendor for lapping. Table 13. Anvil/Spindle Flatness Tolerances per USAF Range Tolerance Inches Millimeters English UUT (inch) Metric UUT (millimeters)
0 to 3 4 to 9
0 to 75 100 to 225
0.00005 0.00008 0.0001 0.000025
0.001 0.002 0.0025
10 and above
250 and above
All with a resolution of 50 in. or 0.001 mm 0.0006 Parallelism Test (Outside Micrometers).— The anvil and spindle parallelism test is performed on 0–1 inch / 0–25mm micrometers where a high degree of accuracy is re- quired to be maintained. Prior to the parallelism test, the flatness test must be performed to the satisfaction of the tolerance of the UUT. • It is critical to the accurate measurement that no dirt or dust particles are present. It is highly recommended that several measurements be taken to verify the results of the measurement. Cleaning the anvil and spindle and optical parallel with lint free lens tissue will aid in assuring the accuracy of the measurement.
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