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SURFACE METROLOGY


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Surface Measurement Applications and SPC


here are cautions about using surface measure- ments in Statistical Process Control calculations, according to Tom Stewart, president of Q-DAS (Rochester Hills, MI), a Hexagon-branded company. As he describes it, calculating common SPC parameters,


such as process capability index measures Cp and Cpk, depend on data that is both normally distributed about a mean and is stable.


That assumption may not hold for surface measure- ments such as profile and roughness. “They are unilateral, because they cannot fall below zero,” he explained. This means, according to him, that they are biased towards zero and not distributed about a mean. “Our experience shows that 98% of all surface measurement data points do not fall into a normally distributed data model set; they are mixed distribution data sets,” he stated. “You can transform the data to get a capability index, but that is


counterproductive because you have changed the data to suit the analysis method.”


Recognizing this issue, Stewart makes the point that his company’s Q-DAS statistical software package finds the best fitting distribution model for any given data set and uses that in quality reporting. “We fit the analysis method to suit the data, rather than transform the data to fit a pre-determined method,” he said. “You need to use that accurate statistical data model to set proper upper and lower data limits for control of your process.” In an interview, he showed surface measurement data that had been modeled in Q-DAS using the percentile method, which is equivalent to standard deviation, but not dependent on fixed ranges. “They are not equal distance and they get smaller and smaller as you go towards the zero because the data set is tending towards zero any- ways,” he explained. “Visually it makes more sense.”


Using a percentile method for modeling surface data instead of a Gaussian method, as shown here with actual test data, is more accurate, according to Stewart from Q-DAS.


The Spread to Area Measurements While the measures for roughness, waviness, and


profile for stylus systems are predicated on a single line, the growth of optical surface measurement systems has led to development of areal surface measurements. “There are many different advantages to optical methodologies,” claimed Michael Schmidt, market development manager of Zygo (Middlefield, CT), an Ametek company. Faster scan times, a much larger data set that provides an area rather than a line, and the fact that it is noncontact means it won’t deform or mar the surface of a part. He noted that many of Zygo’s offerings are based on co- herence scanning interferometry, or CSI, that features a white


70 AdvancedManufacturing.org | June 2017


light source rather than a laser and produces 3D topography maps of a surface. He believes it has become even more powerful. “CSI in the past has been often challenged in per- forming gage-capable measurements of complex surfaces in machine-shop environments. However, recent advancements in CSI-based tools have met these challenges and exceeded the capabilities of many other optical metrology technologies. For example, robust solutions exist for measuring highly- sloped surfaces or rough surfaces,” said Schmidt. He believes noncontact scanning is becoming especially important in a number of manufacturing applications. One is in measuring surfaces created using additive manu- facturing techniques. Another is in measuring surfaces


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