Machinery's Handbook, 31st Edition
MICROMETER DEPTH GAGES 781 with digital encoders). The graduations on the barrel shall read from the thimble end toward the base to indicate extension of the measuring rod from the base position of zero. Base: The base shall be designed with a measuring surface of no less than 3 ⁄ 8 inch, no more than 7 ⁄ 8 inch in width, and a length of either 2, 2 1 ⁄ 2 , 3, 4, 5, or 6 inches. The surface finish shall not exceed 4 m in Ra with a surface hardness of not less than 62 RC on the Rockwell C scale. The measuring surface of the base shall be flat to within 0.0001 inch per inch of length. Measuring Rods: The basic micrometer depth gage shall be equipped with three depth measuring rods to provide for the measurement of ranges of 0 to 1, 1 to 2, and 2 to 3 inches or 0 to 25, 25 to 50, and 50 to 75 mm. The depth measuring rods shall be of a design so that one rod at a time can be inserted through a hole in the micrometer screw spindle. The rod seat on the barrel end of the base shall provide a mounting surface parallel to the base mea suring surface. When inserted into position the measuring rod shall be held securely into place and pass freely and smoothly throughout the entire range of travel. The measuring rod shall have a means of compensation for wear. The measuring rods shall be straight with a runout when fully extended of no more than 0.003 inch. Additional Depth Measuring Rods: While the basic design requires that the set be pro vided with three depth measuring rods to reach the range of 0 to 3 inches, additional rods are available in expanded sets. All additional measuring rods shall be of design and performance as defined in this section. Accuracy: All micrometer depth gage micrometer heads and depth measuring rods shall meet the accuracy limits as defined in Table 7. Adjustment to out of tolerance conditions shall be performed before instrument is returned to service. Table 7. Type III, Micrometer Depth Gage Inch
Tolerance
Range of Micrometer Head (inch)
Range (inch) 0 to 3 3 to 6 6 to 9
Micrometer Head (inch)
Depth Rods (inch)
0 to 1.000 0 to 1.000 0 to 1.000
0.0001 0.00015 0.0002
0.0003 0.0004 0.00045
Metric
Tolerance
Range of Micrometer Head (mm)
Range (mm) 0 to 75
Micrometer Head (mm)
Depth Rods (mm)
0 to 25 0 to 25 0 to 25
0.002 0.003 0.004
0.006 0.008 0.009
75 to 175 175 to 275
Manufacturers’ Production Tolerance Tables The Manufacturers’ Production Tolerance Tables, Table 8a and Table 8b, represent the parameters that are utilized by the original manufacturers of the instruments. These values were defined by the Project Team B89.1.13 of the ASME Standards Committee on Dimensional Metrology. This committee is a representative group of the manufacturing industry that establishes the standards used in the production of micrometers. These stan- dards address the requirements of the American industry and the ISO efforts in the sup- port of international commerce. While these tolerance tables and values are utilized in the manufacturing of the instruments, the calibration of the individual instruments is slightly different and does not always include all the geometrical parameters that are taken into consideration during the manufacturing process.
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