Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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these system adjustments are helpful, using a "factor" to change the original calibration causes a loss of NIST traceabil- ity. Thus, to obtain maximum accuracy, thickness standards should be made from the same plating bath and substrate materials as the parts that will be measured. Otherwise, to con- firm the accuracy a different measurement method should be used for comparison pur- poses. Most eddy current instruments also include the magnetic induction capability for this purpose. EDDY CURRENT ADVANTAGES Eddy current instruments are rugged, with the electronics vir- tually sealed, and can be used directly at the end of a plating line or other hostile work envi- ronments. Another benefit: eddy current is much less affected by subtle changes of the Eddy current instruments are rugged enough to use at the end of a plating line. Photo courtesy of Micro Metal Finishing. magnetic properties of steel or iron substrates. Most low-carbon steel substrates can be measured using a single calibration and only one set of standards would be needed for measuring thousands of different parts such as screws, washers, bolts or brackets. Curved and irregularly shaped parts can be measured with little additional error and typically without the necessity of parts fixturing. This is due to the small mea- surement areas (field effect) of microprobes. Plating thickness can also be mea- sured on rough, uneven surfaces without affecting he overall accuracy. Thickness distribution can also be scanned by sliding the probe along critical surfaces of the parts. With PSEC, there is a high degree of measurement precision (repeatability) even on thin metallic coatings. Since most plating specifications are typically less than 0.5 mils, this is an important feature. For example, vacuum deposits for EMI/RFI applications, such as aluminum or copper on plastic substrates are commonly below 0.1 mils, but can be accurately measured even below 1 micron by eddy cur- rent using the appropriate probe and thickness standards. Platings such as zinc can be measured even under a coat of paint with no loss of accuracy. The "lift off" of the probe from the surface will not affect accuracy of the zinc measurement. EDDY CURRENT PROBES A wide variety of specially designed probes are available from the manufacturers for use in measuring virtually any application or part. Probes with different tip 439

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