Machinery's Handbook, 31st Edition
Outside Micrometer Calipers 769 (5.9 mm) for 1.00 inch (25.0 mm) and larger micrometers. The diameter of the anvil and spindle measuring face shall be equal in diameter to within 0.0015 inch (0.038 mm). Barrel and Thimble: The diameter of the graduated surface of the barrel shall be not less than 0.406 ( 13 ⁄ 32 ) inch (10.31 mm). The angle of the bevel at the graduated end of the thimble shall not be more than 20 degrees as measured from the barrel. The beveled end of the thimble at the reading end shall be sharp but dulled to prevent injury to the operator. The major diameter opposite the reading end shall be diamond knurled to facilitate ease of operation. The radial clearance from the barrel to the reading end of the thimble shall not exceed 0.020 and shall be concentric and constant throughout the range of travel. When set at 0.000, 0.025, 0.050 inch …, etc., the reading edge shall bisect the graduation line exactly. Throughout the range of travel, there shall be no more than 2 ⁄ 3 of the line visible at any point. The barrel shall be constructed so as to prevent dust and foreign matter from coming into contact with the spindle screw. Adjusting Nut: The micrometer design shall provide an adjusting nut to compensate for wear between the spindle screw and the fixed nut. There shall as well be a means provided to compensate as well for wear of the measuring surfaces. This is generally accomplished through adjustment of the barrel or by disengaging the spindle screw from the thimble and resetting the zero indication of the micrometer. Both adjustment of the barrel and the adjusting nut shall be such that they remain secure after resetting and will not impair the original accuracy of the instrument. Disengaging the spindle screw and resetting the zero indication are more advanced adjustments and would require a recalibration of the micrometer before returning to service. Micrometers with a friction-adjusting sleeve on the barrel shall have no less than 3 1 ⁄ 2 and no more than 15 in-lb torque on the friction sleeve. Spanner wrench, keys or other specialized devices shall be provided by the manufacturer of each micrometer. Spindle Lock: The micrometer shall have a spindle lock which will arrest the spindle at the selected point and not alter the position when engaged. The spindle lock will be either a diamond knurled ring friction lock or a lever operated lock. Ratchet or Friction Stop: The ratchet or friction stop is a device that shall exert a constant and even measuring force on the spindle and effectively removes subjective “feel” of mea surements. This device is critical to obtaining an objective measurement and removes a variable in uncertainty analysis by delivering the same force between the spindle and anvil regardless of the operator. The design shall be either the ratchet or friction stop design. It shall be sufficiently knurled to enable satisfactory operation and shall exert a compressive force between the measuring faces of 1 to 1 1 ⁄ 2 lbs. up to the 6 inch size and 1 to 3 lbs. for larger sizes. Measuring Faces of Micrometer Screw Spindle: The face of the micrometer screw spin dle shall be at right angles to the axis at all positions of rotation of the screw spindle. Shall be square and flat and parallel to the anvil and shall maintain this orientation throughout the entire range of travel. The measuring face shall meet the tolerance limits as defined in Table 8a and Table 8b. Measuring Faces: All measuring faces shall be hardened, ground and lapped. The hard ness shall be no less than 62 RC and the lapped finish shall be no greater than 4 m in Ra. Utile measuring faces can be lapped to recondition them should wear from use cause the surface to exceed the tolerance limits. Carbide Measuring Faces: Some micrometer calipers are furnished with carbide mea suring faces. The carbide measuring face tip shall not be less than 0.030 inch thick and shall be an integral part of the spindle and anvil. When provided with carbide measuring faces, both spindle and anvil shall be faced with the carbide tip.
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