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Better Finishing Through Chemistry Microsurfacing Is an Alternative Finishing Process with Benefits That Extend Beyond Parts with Supersmooth Surfaces

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Better Finishing Through Chemistry Microsurfacing Is an Alternative Finishing Process with Benefits That Extend Beyond Parts with Supersmooth Surfaces JUNE 2007 / VOLUME 59 / NUMBER 6 ➤ BY DR. QI WANG AN D D O N S C H U S T E R , H O U G H T O N I NTERNATIONAL I NC. Better Finishing Through Chemistry Microsurfacing is an alternative finishing process with benefits that extend beyond parts with supersmooth surfaces. etter part performance However, microsurfacing can take Cheesman introduced conversion and greater wear resis- place inside a common vibratory bowl. agents into chemically assisted me- tance are two of the ben- The technique combines various acids chanical finishing. He stated that “with Befits that manufacturers with mechanical forces so surface use of surface conversion agents, which can achieve by applying microsurfac- imperfections are removed layer by form a friable layer on a metal surface, ing to impart supersmooth finishes. layer—each layer is of atomic thick- the metal removal on the parts surface Used more and more to finish parts, ness—to impart a supersmooth finish. is not only accelerated but is selective, microsurfacing—with its vibratory The modern concept of microsur- avoiding the phenomenon of pits and equipment, media and chemical formu- facing was invented and patented by scratches deepening.” lation—is also an improved process, William D. Cheesman in 1963. Prior The surface conversion agents in- offering reduced cycle times, chemical to this invention, chemically assisted cluded various phosphoric acids usage and manufacturing costs. surface finishing typically utilized and their salts, nitro compounds and As examples, microsurfacing in- strong acids, which etched deeper into organic acids, such as citric and ox- creased the life of one company’s bear- the existing pits and scratches on alic acids. The inventor practiced the ings by 500 percent and saved a major the processed metal parts. To hand-tool manufacturer more than $10 overcome this problem, million in 1 year after it switched from grinding to this finishing technique. To perform microsurfacing well, though, manufacturers need to All images: Houghton consider the media, chemicals, workpiece alloy and amount of material to be removed in order to achieve the best surface finish for wear per- formance and load-bearing properties. Microsurfacing—also called superfinishing, engineered surfacing and isotropic finishing—is a chemi- cally assisted finishing method that can eliminate directional lines on metal surfaces caused by abrasive grit in the grinding wheel or lapping compound. The technique complements or re- places conventional grinding, honing Before and after photos and lapping processes. of cementite exhaust system components for Earlier superfinishing processes re- racecars, which are polished quired costly equipment, like special- inside and out with the ized grinding and sanding machines. microsurfacing process. technique in a so-called barrel finisher using abrasive media. To help understand the process fur- ther, imagine a vibrating bowl filled with metal parts and selected media where a chemical solution is continu- ously fed. The feed rate varies with bowl size and workload needs. The chemicals in the solution react with the metal surface to form a soft film capable of preventing the layer of metal underneath from further react- ing with the solution. The media in the vibratory bowl then wipes off the film through churning and tumbling mo- tions, exposing a fresh metal surface, which in turn reacts with the chemical solution to reform the soft film. This process eventually renders parts with supersmooth surfaces. The amount of material removed depends on part tol- erances and on the depth of the direc- tional lines if grinding or lapping was performed prior to superfinishing. This series of mild steel plungers (used in the bottling industry) shows how An expansion beyond vibratory microsurfacing progressively removes carbon deposits, enabling the parts to processes’ use of conversion agents function significantly longer before requiring cleaning and maintenance. was carried out by Whirlpool Corp. in 1980. The patented process virtually Why Microsurfacing? Traditional grinding uses abrasives covers all surface finishing processes Microsurfacing can dramatically that leave lines in the surface of the utilizing conversion coatings. It does improve the performance of processed metal or, worse, possibly embed grind- so by applying a compound solution parts. The technique has been increas- ing media in the workpiece’s surface. capable of forming a friable, relatively ingly adopted as a final finishing pro- Abrasives cut away burrs, remove impervious conversion coating on the cess over the last 2 decades, so more scale and oxides and remove surface surface and rubbing the conversion and more manufacturers are finding material for dimensional sizing, form- coating with a conforming surface that the process not only improves the ing the shape and imparting the desired while retaining the solution between appearance of a part, but also exponen- surface roughness. Abrasives, however, the surfaces to remove the conversion tially increases its strength and dura- can become embedded in a compo- coating by abrasion. bility—or “wearability.” This increase nent’s surface and decrease the part’s Although not new, microsurfacing is mainly attributed to three factors. wear performance and fatigue life. and its related chemistry have ad- Foremost, microsurfacing makes In critical wear surface applications, vanced in recent years. For example, a surfaces smoother, and generally the contaminants must be removed by criterion for measuring progress in the speaking, the smoother the finish, the chemical or electrochemical means. In field is the average surface roughness better the part’s performance. This was noncritical wear surface applications, (Ra) of processed parts. A few years the case for the previously mentioned the contaminants usually don’t need to ago, a typical vibratory microsurfac- bearing application. The other two be removed, but the parts do not per- ing process produced surface finishes factors are, however, less obvious, one form optimally or last as long. from 10 to 15 microns Ra. Since then, being the ambient temperature associ- In contrast, microsurfacing reduces the technique has benefited from the ated with the process and the other the need to use abrasives to remove development of heavier media and being the isotropic surface finishing of burrs, scale and oxides and refine wear variable- and higher-speed vibratory the processed parts. surfaces. This reduction gives an ad- bowls with programmable controls In contrast to other finishing pro- vantage to manufacturers for produc- for better process control and more cesses, such as grinding, microsurfac- ing wear surfaces that are metallurgi- consistent results. Also, the chemicals ing does not produce sufficient heat to cally clean of embedded abrasives. have been adjusted to take advantage cause the surface layers to plasticize. Some grinding with abrasives may be of those changes. Now, the process This precludes surface contamination needed, though, before microsurfac- imparts finishes from 2 to 3 microns by abrasives and preserves the struc- ing when the surfaces are not equal to Ra—practically as smooth as glass. tural integrity of the metal’s surface. one another with respect to their final dimensions. quire longer-lasting parts. Before and after shots of Also, the motion pattern of the Microsurfacing will a ratchet wrench that was media during microsurfacing renders soon be a main way for microsurfaced. processed parts with isotropic surface Tier 1 and 2 automotive finishes, where the finish is the same suppliers to secure large Microsurfacing, on the when measured along axes in all direc- contracts with automotive other hand, does not need tions. The combination of these three manufacturers. The finish- to be designated to one par- factors—smoothness, processing at ing process is already being ticular part or family of parts, ambient temperature and isotropic fin- applied in the U.S. to gears thus increasing equipment ish—is the key to the enhanced perfor- and bearings and in Europe utilization and improving mance of microsurfaced metal parts. for other moving parts. Even ROI. Microsurfacing, which was origi- though existing superfinish- With today’s improve- nally developed for mass-finishing ing equipment will need to ments in vibratory equip- environments, has proven to be a cost- be upgraded to meet increas- ment, media and chemical effective alternative to mechanical ingly stringent finish stan- formulation, cycle times of manufacturing operations like surface dards, microsurfacing still microsurfacing have been grinding, lapping and shot blasting. provides a cost-effective al- reduced 50 percent com- In addition to superfinishing, the ternative to investing in spe- pared to 5 years ago. For microsurfacing technology can be ex- cialized grinding, lapping or example, microsurfacing of tended to regular finishing operations, other finishing equipment. hand tools used to take 7 such as deburring and general clean- The savings could be up to $1 hours. The cycle time is 3 to ing. million in equipment alone, 4 hours today. The microsurfacing technique has depending on the company’s finishing The reduced cycle times have led to been used by the U.S. military for de- requirements. Vibratory bowls typi- proportionate reductions in chemical cades and was first implemented com- cally cost from $1,000 to $500,000, use. In addition, new techniques and mercially as a final finishing technique depending on part size and annual
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