Machinery's Handbook, 31st Edition
MICROMETER CALIPERS 767 Resolution: Micrometers using the inch system shall have a least significant digit of 0.001 in., 0.0001 in., or 0.00005 in. Micrometers using the SI (metric) system shall have a least significant digit of 0.01 mm, 0.002 mm, or 0.001 mm. Finish: Unless otherwise specified, the frames and graduated components of Type I Class 2 micrometers and the thimble and barrels of all inside, depth, and micrometer heads shall have either a dull (non-glare) or satin chrome finish. All other exposed surfaces, excepting Type I Class 1 frames, shall have a ground finish. The surface roughness of all ground surfaces excepting direct measuring surfaces shall not exceed 32 m in (0.8 m m) Ra, and shall be coated to prevent corrosion. The Type I Class 1 frames shall have either a smooth or ripple enamel coating. Identification: The Type I, Class 1 frame shall have a manufacturer’s pad of smooth machined finish. All instruments shall be permanently and legibly marked with the manu facturer’s name or trademark, so the source of manufacture can be easily determined, part number, and range of the instrument. Adjustments: Each type, style and class shall provide a method for adjustment due to wear of the measuring faces and/or wear between the spindle screw and the nut. Regular calibration and adjustment of the nut will maintain the accuracy of the micrometer. This is especially true in higher accuracy instruments. Micrometer Screw Spindle: The screw spindle is a critical component of the micrometer. The true accuracy of the instrument derives from the accuracy of the manufacturing of the threaded (screw) section of the spindle. The screw spindle material will be a stabilized high grade tool steel or alloy steel. The spindle screw section will be precision ground and have a hardness of no less than 62 RC on the Rockwell C scale. The exposed spindle section will have a fine ground finish with a surface finish of no greater than 32 m in (0.08 m m) Ra. The fit between the spindle and the bearing including the nut, shall be free- turning and exhibiting no side shake, end shake or backlash. Measuring Faces: The spindle and anvil of all Type I, Class 1 and Class 2, microm- eters shall be at minimum made of a stabilized high grade tool steel or alloy steel and have a hardness of no less than 62 RC. Alternatively, the measuring faces can be made of tungsten carbide with a hardness value of 92 RA on the Rockwell A scale. The tungsten carbide surfaces are preferred and provide a greatly improved life span for the measuring surfaces. Graduations: The graduations shall have a depth reduced below the surface of the barrel and thimble and shall be of contrasting color. Variations in the width of the graduated lines on the barrel, thimble or vernier shall not exceed 0.001 inch. Outside Micrometers Type I, Caliper Micrometer.— Size and Range: The size of an outside micrometer cali per is defined as the maximum cylindrical diameter measurement that can be obtained by that particular instrument. The range is defined as the span of obtainable measurements from the minimum to maximum capacity of the particular micrometer. Design: The basic features of the micrometer design shall consist of a frame, micrometer screw spindle, barrel and thimble, adjusting nut, either a lock-ring or lever type spindle lock, and a ratchet or friction stop, as shown in Fig. 1. Higher accuracy micrometers will include a vernier scale on the barrel that enables the reading of measurements to 1/10,000 of an inch (0.002 mm).
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