Quantifying lubricant thickness quickly and accurately can determine or eliminate whether lubricant issues are the cause of forming issues. But, measuring lubricant thickness has not been easy and has been the cause of much discussion and dissension.
Stampers, processors and mills are routinely tasked with accurately measuring oil-films ranging between 7-120 Milligrams per square foot (mg/ft2) in order to meet specification for making good parts. As a thickness instead of a weight, this equates to only 2 - 40 millionths of an inch! Making things even more challenging is that measurements have to be made instantly, portably, on a number of different substrates, and the inspection equipment must be easy to operate with virtually no training. In reality, this is but one of the many difficult challenges facing the stamping industry, metals mills and coil processors.
Lubricant thickness has been determined for many years from weighing a sample, cleaning off the lubricant and then re-weighing (weigh/strip /weigh). The average thickness of lubricant can then be calculated from those numbers. Mills and automotive plants have used this method because of the availability of extremely accurate scales/balances, but the weigh/strip/weigh method is not without issues. The weight difference between coated and uncoated samples can be less than .008 grams. Adding all of the uncertainties resulting from imperfect sample size, scale error, uneven lube distribution and improper handling normally reduces the accuracy to below acceptable limits. Plus, the weigh/strip/weigh method is destructive, slow and untraceable to any national standard.
Fortunately, advancements in electronics technology are occurring rapidly and many promising new technologies have been introduced. This is good news and just in time for measuring at the tightened lubricant-thickness specification requirements that have increasingly become difficult to confirm and control.
New measurement devices are primarily using infra-red, beta backscatter or ultra-violet principles for oil-film measurement. Each of these new technologies have their strengths and weaknesses, so choosing the best one will depend on any number of factors, depending on the specific conditions. Choosing the wrong method can be an expensive mistake, so doing some homework beforehand is worth the effort. Gone are the days when companies can say “I will use this method—because my customer uses it.” Having both companies using an incorrect method
doesn’t help anyone, so the best choice of instrumentation should be based on facts and careful evaluation.
Hear more from Mike at PMA's Metal Stamping Technology and Tool & Die Conference, December 6-7, 2016 in Chicago, IL.