History of Surface Finish - BC MacDonald & Co.

The digital revolution allowed us to create instruments that digitizethe analog signal from the stylus and generate the typicalsurface profile trace we are familiar with today. With the profilein hand it became possible to apply mathematical tools toanalyze a wide variety of profile characteristics. Today there areliterally hundreds of different ways to analyze surface characteristics.Source: MahrWith today’s digital surface instruments, analysis of 2-D digitalprofiles has gone way beyond simple mathematical averages.It is now possible to compute sophisticated functional characteristics,such as the ability of surfaces to bear loads, retainlubrication, seal against leaks, and even support the growth andattachment of human bone. Source: MahrThere followed a concerted effort tobetter understand the role of surfacetexture and to quantify it in ways thatwere meaningful to manufacturing.Initially, optical microscopes wereemployed to provide a magnifiedview of surface characteristics. Again,measurement was comparative andsomewhat subjective, but the variousdegrees of magnification and the differingfields of view afforded led tothe concept of sampling lengths andfrequency that are central to surfaceanalysis today. Other early efforts tocapture information about a surfaceutilized a trace stylus and a mechanical“amplifier” which used linkageto replicate the trace onto a smokedglass surface.In 1933, E. J. Abbot developed whatis widely believed to have been thefirst analog surface instrument. Thisused a stylus to contact the part andprovided an actual number to quantifytexture. Abbot also co-developed theAbbot-Firestone curve. This used asimple curve to represent the surfaceand made contact area the basis of thecurve, allowing the calculation of amaterial-to-air ratio as a function ofdepth. This was the first instrument tolink form with function in a numericalmanner simple enough to quantify asurface and help control manufacturingprocesses.Modern InstrumentsWith the ability to trace surface texturecame the realization that, withinthe limitations of a probe or stylustip and the device used, any parttrace would include an amalgam ofa nearly infinite number of differentfrequencies present in the path beingtraced. These, in turn, reflected characteristicsimposed by the manufacturingprocess. Just how to segmentand analyze this data occasionedmuch theoretical work during thenext decades. This led in the early1960s to the development of 2RCfilters, a growing number of commercialinstruments, and the popularityof basic roughness parameters suchas Ra, Rq and Rz.In 1933, E. J. Abbot developed what is widely believed to have been the first analog surfaceinstrument which used a stylus to contact the part and provided an actual numberto quantify texture. He also co-developed the Abbot-Firestone curve. This uses a simplecurve to represent the surface and make the contact area the basis of the curve, allowingthe calculation of a material-to-air ratio as a function of depth. This was the firstinstrument to link form with function in a numerical manner simple enough to quantifya surface and help control manufacturing processes. Source: Mahr

| 50 Years of Quality |For your Surface Finish needscontact B.C. MacDonald & Co.800-875-4243The single largest event to changehow surfaces are measured was yet tocome however. The digital revolutionallowed us to create instruments thatdigitize the analog signal from thestylus, and generate the typical surfaceprofile trace we are familiar with today.With the profile in hand it becamepossible to apply mathematical tools toanalyze a variety of profile characteristics.Today there are literally hundredsof different ways to analyze surfacecharacteristics, some codified intonational and international standards,others specific to various industries oreven individual companies. Moreover,analysis of two-dimensional (2-D)digital profiles has gone way beyondsimple mathematical averages, and it isnow possible to compute sophisticatedfunctional characteristics, such as theability of surfaces to bear loads, retainlubrication, seal against leaks, andeven support the growth and attachmentof human bone.Digitization also is blurring the linesbetween what was once considered surfacemeasurement and part geometry.Metrologists today often categorizethe data from a measured surface intothree categories—roughness, wavinessand form.Shorter wavelength data tends toreflect surface roughness characteristicsimposed by machining operations,such as turning, grinding or polishing.Waviness involves longer wavelengthdata and might reflect instabilities inthe machining process, such as imbalancein a grinding wheel or wornspindle bearings. Long wavelengthdata tends to reflect errors, such aslack of straightness in the guidewaysof a machine or misalignment ofmachine axes.These long wavelength errors are usuallythought of as form characteristics,such as roundness, straightness or flatness.Perfect straightness, for example,could be described as a line, or wave,with an amplitude of zero. Furthermore,what is perceived as a short wavelengthand what is a long wavelength are nowgenerally understood to vary dependingon the size of the part and the intendedfunction of the surface.In addition, while old analog instrumentsgathered only the data neededfor a specific measurement, moderndigital instruments gather a wholespectrum of data from each trace,making subsequent analysis of otherwavelengths possible without the needto remeasure the part.Future DimensionsAs dimensional tolerances have grownsteadily tighter over the years, and theneed for documentation and traceabilitybecome ever greater, the roleof surface finish measurement in themanufacturing process has grown dramatically.It has been estimated that inthe 1940s, the proportion of the toleranceband taken up by surface irregularitieswas roughly 15%. Today thatproportion is sometimes 50% or morefor precision components.Future developments in surfacemetrology will no doubt make theprocess even more important. Asmicroprocessors gain power and theability to process enormous amountsof data becomes economically feasible,optical systems are coming back intovogue and opening up new avenues ofexploration. With optical systems thereis the opportunity to measure a surfacein three-dimensional (3-D) rather thanthe typical 2-D measurements madewith stylus tracing instruments.It is generally believed that since surfacesexist in 3-D and function in 3-D,measurement of these surfaces using3-D instrumentation should allowbetter linkage between the measurementsand the suitability of a surface toperform a desired function. But thereis a great deal of work still to be donebefore this becomes part of the typicalsurface measurement specification.Currently there is a lot of researchbeing done with various optical techniques,and it is clear that differenttechnologies measuring the same surfacesometimes “see” different thingsin the structure of the surface.Work to create standardized measurementand analysis methods forthese optically measured surfaces iscritical to lay the groundwork for theuse of 3-D parameters. So there is moreto come on how to properly read a surface,but we are standing on the thresholdof the next revolution.Patrick Nugent is vice president of metrology systemsat Mahr Federal Inc. (Providence, RI). Formore information, e-mail pat.nugent@mahr.comor visit www.mahr.com.