Machinery's Handbook, 31st Edition
792 CALIBRATION OF OUTSIDE MICROMETERS • Position the thinner optical parallel between the anvil and spindle of the UUT and adjust the UUT until both surfaces are in contact with the optical parallel. Do not exert greater than necessary force in attempting to minimize interference band count. Adjust the optical flat on the anvil end of the UUT to obtain the minimum band image. The optimum number of bands anticipated here is four. This provides an easily read image to calculate the parallelism of the anvil and spindle. Note the number of bands on the anvil face as count A . • Without changing the position of the UUT, carefully turn and observe the number of bands on the spindle face. Note this as count B . • Compute the parallelism of the anvil and spindle over the smaller optical flat as follows. Use the band width of the monochromatic light that is being used; 11.6 × ( A + B ) = Parallelism (microinches) where A = count A ; B = count B ; and 11.6 is the fringe/length of the light source in microinches (Table 12). • Remove the thinner optical flat and replace with the 0.0125 inch thicker one and repeat the measurement process. To calculate the anvil to spindle parallelism inclusive of the rotation of the spindle, calculate the difference between the two optical flat measurements. • Verify that the difference between the two sets of measurements is within ± 50 μ inch or ± 12 μ m. • If only one optical flat is available the same measurement can be achieved by using the measurement technique described above but calculated as 11.6 × (A − B) = Parallelism (microinches). The exception being that this method will not take into consideration the rotation of the spindle. Linearity Test (Outside Micrometers).— 1) Set the UUT to indicate zero reading using the ratchet or friction thimble, and assure that the reading meets the applicable tolerance limits. If an Out of Tolerance (OOT) reading is identified at this time, assure there is no foreign particle or burr on the anvil or spindle. An accurate zero indication must be estab lished at this time as all other measurements will be affected by any error at zero. Note: The micrometer anvil and spindle can be closed over a sheet of clean white paper damp with isopropyl alcohol, which can then be drawn out slowly to remove any obstruct ing matter. Repeat with a dry paper and observe any smear or smudge that would indicate dirt or foreign matter. Clean until paper comes through with no residue. 2) For Interchangeable Anvil Micrometers the appropriate length gage block will be used to establish the zero indication. 3) The recommended size gage blocks used to test the micrometer spindle thread are very important. The recommended test points achieve the testing of the accuracy of the micrometer throughout the entire length and at points evenly distributed around the diam eter of the spindle thread. This verifies the accuracy of the lead error of the thread as well as identifying any sections that may have experienced excessive wear or damage. The recommended sizes are as follows: a) X .210, X .420, X .605, X .815 and 1( X ).000 (inch) and 5.10, 10.20, 15.30, 20.40, and 25.00 (mm). The micrometer should be tested at zero and full scale. X represents the applicable length for the UUT range. b) When wringing gage blocks together to achieve the desired size, the stack must soak for 1 hour when calibrating UUT with 0.0001 inch (0.002 mm) resolution in a range over 8.000 inches (200 mm). The soak time does not apply to sizes over 8.000 in (200 mm) that do not require wringing. 4) Select the first gage block stack and measure it using the ratchet or friction thimble. Do not rely on feel as the correct and controlled loading of measuring force by the ratchet or friction thimble will give the most accurate and repeatable reading. All other methods are subjective and open to operator influence. Several clicks of the ratchet or applying force until the friction thimble slips and assuring the micrometer is gently seated on the
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