Machinery's Handbook, 31st Edition
790
CALIBRATION OF OUTSIDE MICROMETERS Note : The Minimum Use Specifications column of Table 10 represents the principal parameters required for performance of the calibration. References to specific equipment are recommendations only, and in no way constitute an endorsement other than as examples of the appropriate accuracy required to maintain the correct Test Accuracy Ratio ( TAR ) and confidence level in the calibration procedure. It is understood that equivalent equipment produced by other manufacturers is capable of equally satisfactory performance of this procedure. Note : If the UUT uses the metric scale, use test points equivalent to those defined for the English scale so as to test the micrometer at five places in the circumference of the drum and five equally spaced intervals throughout the range of travel of the micrometer spindle screw. Table 12. Types of Monochromatic Light Source
Wave Length
Length/Fringe
Light Source
µ in
nm
µ in
nm
Helium-Neon Laser (bright red) Sodium Vapor (yellow) Mercury Vapor (green) Helium Discharge (yellow-orange)
24.9 632.8 12.5 316.4 23.2 589.3 11.6 294.7 21.5 546.1 10.8 273.1
23.1 587.6 11.6 293.8 Preliminary Operations.— Ensure a clean and well illuminated work area, free from drafts, excessive humidity with a stable temperature that does not fluctuate more than ±2° per hour. It is recommended that a stable temperature of 68° F is maintained in the calibration work area. Note: The calibration of OD Micrometers and similar precision measurement devices is accomplished through the use of gage blocks as the length standard. Due to the thermal reactivity of gage blocks, environmental stability is a major consideration in maintaining the confidence in the accuracy of the measurements taken during the calibration. Calibration of UUT with an accuracy of ±0.001 inch or greater can be accurately cali brated in an environment with a temperature fluctuation of ±4°F. Calibration of UUT with a higher accuracy, such as ±0.0001 inch, must be accomplished in a stable environment of no greater than 68°F with a temperature fluctuation of no more than ± 2°F per hour. Ensure that the UUT and all associated standards have been allowed to stabilize in the controlled environment for a minimum of 8 hours prior beginning the calibration proce dure. This is especially critical for UUT in excess of 10 inches. Closely examine the UUT for any burrs or nicks on the measuring surfaces that could in any way damage the highly polished precision surface finishes of the gage blocks and optical flats. Damage to these surfaces could degrade or potentially ruin the standard. Thoroughly clean the UUT and rotate the micrometer through the full range of travel, ensuring smooth movement with no binding or indication of drag or misalignment of the spindle through the support bearing. Note: It is recommended that well-fitting cotton or nitrile gloves be used to reduce ther mal transfer and protect from corrosive elements of direct contact. Calibration Procedure, Flatness Test (Outside Micrometers).— UUT used in preci sion work or any re-lapped anvil or spindle require a flatness test. If the instrument is known to be for general purpose; i.e. tool box issue, diameter measurement where flatness is not a factor, measurement is a non-quantified value, etc., a flatness test would not be required. Calibration records should reflect “flatness not calibrated”. Ball anvil and tubing micrometers require a flatness test on the spindle only. • Connect the Monochromatic Light to a power source and turn on. Allow a few moments to warm up. • Position the UUT in the monochromatic light field and place the optical flat on the anvil. Apply slight pressure and gently seat the optical flat until 3 to 5 bands are displayed on the surface. It is important to not apply greater than necessary force or to cause damage to the
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