MECHANICAL FINISHING Developments in Mass Finishing Technology by David A. Davidson Deburring/Surface Finishing Specialist; E-mail; [email protected]MASS FINISHING FUNDAMENTALS Mass finishing describes a group of industrial processes by which large lots of manufactured parts can be processed in bulk economically to achieve a variety of surface effects. These eco- nomical processes, in contrast with hand debur- ring methods, develop these effects with a high degree of part-to-part and lot-to-lot uniformity and consistency. These effects might include edge break, edge contour, surface smoothing and improvement, tool mark blending, burnishing, polishing, superfinishing, and microfinishing. In these types of processes, energy is imparted to an abrasive-embedded or abrasive-coated loose material known as media that is contained with- in the work chamber of a finishing machine. Energy is then transferred from work chamber motion to the media and to the work-pieces placed in the media by way of a random rubbing or scrubbing action. This achieves some sort of edge or surface improvement and refinement. The surface and edge effects produced are typi- cally nonselective in nature, unless a part has been partially masked or fixtured. While edge geometries can be modified (contoured) to some extent, it would be a mistake to consider these processes for substantial material removal oper- ations that are best left to traditional grinding and machining methods. Typical dry media used for dry finish and polish applications in barrel, vibratory, and centrifugal high speed equipment is shown in Figure 1. The top row shows media shapes manufactured from hardwood, the bottom row various granular materials from agricultural sources. These media are made effective by treating them with very fine abrasive powders that can, on properly pre- pared surfaces, produce very refined and highly reflective surfaces on many metal and plastic substrates. Because of their relatively light- weight bulk density, these materials are not typ- ically specified for use in every-day general deburring application, where ceramic and plastic media shapes are more commonly used. However, in some circumstances, special blends of these materials have been paired for use with high- energy mass finishing equipment because of the environmental advantages of processing parts in a waterless system. Additionally, some manufac- turers have developed specialized abrasive and plasic resin shapes to be able to perform some abrasive operations in a high energy dry environ- ment. Finishing plastic components to achieve high-gloss surface finishes can be accomplished in barrel sys- tems with dry media. Multistage processing with a sequence of steps, each utilizing successively finer abrasive materials, is crucial to developing low micro- inch R a finishes such as those shown in Figure 2. This type of sequential step finishing has been adopted and utilized with other mass finishing methods, to produce similar surface effects, on metal parts as well. PART FIXTURING AND SURFACE FINISHING Included in these mass finishing methods are tradi- tional barrel tumbling, vibratory, and centrifugal July/August 2003 49 Figure 1. Photo courtesy of Tyha S. Davidson. Figure 2. Photo courtesy of PEGCO Process Laboratories. Davidson.qxd 7/16/2003 1:54 PM Page 1
Transcript
MECHANICAL FINISHING
Developments in Mass Finishing Technologyby David A. Davidson
MASS FINISHING FUNDAMENTALSMass finishing describes a group of industrialprocesses by which large lots of manufacturedparts can be processed in bulk economically toachieve a variety of surface effects. These eco-nomical processes, in contrast with hand debur-ring methods, develop these effects with a highdegree of part-to-part and lot-to-lot uniformityand consistency. These effects might include edgebreak, edge contour, surface smoothing andimprovement, tool mark blending, burnishing,polishing, superfinishing, and microfinishing. Inthese types of processes, energy is imparted to anabrasive-embedded or abrasive-coated loosematerial known as media that is contained with-in the work chamber of a finishing machine.Energy is then transferred from work chambermotion to the media and to the work-piecesplaced in the media by way of a random rubbingor scrubbing action. This achieves some sort ofedge or surface improvement and refinement.The surface and edge effects produced are typi-cally nonselective in nature, unless a part hasbeen partially masked or fixtured. While edgegeometries can be modified (contoured) to someextent, it would be a mistake to consider theseprocesses for substantial material removal oper-ations that are best left to traditional grindingand machining methods.
Typical dry media used for dry finish and polishapplications in barrel, vibratory, and centrifugalhigh speed equipment is shown in Figure 1. Thetop row shows media shapes manufactured from
hardwood, the bottom row various granularmaterials from agricultural sources. These mediaare made effective by treating them with veryfine abrasive powders that can, on properly pre-pared surfaces, produce very refined and highlyreflective surfaces on many metal and plasticsubstrates. Because of their relatively light-weight bulk density, these materials are not typ-ically specified for use in every-day generaldeburring application, where ceramic and plasticmedia shapes are more commonly used. However,in some circumstances, special blends of thesematerials have been paired for use with high-energy mass finishing equipment because of theenvironmental advantages of processing parts ina waterless system. Additionally, some manufac-turers have developed specialized abrasive andplasic resin shapes to be able to perform someabrasive operations in a high energy dry environ-ment.
Finishing plastic components to achieve high-glosssurface finishes can be accomplished in barrel sys-tems with dry media. Multistage processing with asequence of steps, each utilizing successively finerabrasive materials, is crucial to developing low micro-inch Ra finishes such as those shown in Figure 2. Thistype of sequential step finishing has been adopted andutilized with other mass finishing methods, to producesimilar surface effects, on metal parts as well.
PART FIXTURING AND SURFACE FINISHINGIncluded in these mass finishing methods are tradi-tional barrel tumbling, vibratory, and centrifugal
July/August 2003 49
Figure 1. Photo courtesy of Tyha S. Davidson. Figure 2. Photo courtesy of PEGCO Process Laboratories.
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finishing. A closely related set of processes would befixture-centric processing such as the spin-finish,drag-finish, spindle-finish, and the turbo-finishmethods. The fixture methods produce results byimparting motion to parts that are fixtured (byeither dragging, rotating, or developing a planetarymotion) and are immersed in loose abrasive or pol-ishing media. The force with which part edges andsurfaces interact with loose media can be consider-ably higher than that developed by mass mediaprocesses, where parts are placed randomly withinthe media mass, and are dependent on the loosemedia motion to achieve the surfacing results.Fixturing parts in more conventional barrel orvibratory methods is also not uncommon. This isdone for a variety of reasons, including the need toprevent any part-on-part contact but also toincrease the amount of force flow of media againstpart surfaces, to accelerate cycle times, and producemore pronounced surface finish effects.
Part applications for fixture finishing in conven-tional equipment vary widely. For example, somemanufacturers of brass musical instruments (trum-pets, French horns, trombones) fixture brass instru-
ment assemblies in barrel or vibratory chambersand flow soft polishing granulate media through theassemblies to replace multiple buffing operations.Similarly, some manufacturers of medical and sur-gical implant devices fixture the devices in high-energy centrifugal barrels and produce very refinedsurfaces on cobalt chrome and titanium substratesby processing the devices through a sequence of suc-cessively finer loose abrasive operations. Fixturedprocessing in vibratory equipment can accommo-date even very large structural parts. This is animportant application for large structural aerospaceparts. The method can be used to reduce the need forcostly manual deburring and finishing methods onairframe components. More importantly, with theproper sequence of abrasive and nonabrasive opera-tions, it can be used to develop very significant com-pressive stress and work hardening characteristicsto the parts, enhancing their wear and fatigue fail-ure resistance dramatically.
OLD DOG — NEW TRICKS, SEQUENTIAL PROCESSINGOne trait that many of today’s more sophisticatedmass finishing operations share is a reliance on
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multiple-step sequential processing. In this type ofprocessing, very rough surfaces can be brought to ahighly polished or microfinished state. This is doneby initially processing the parts with coarse abra-sive material, and then following up with a sequenceof finer abrasives. Each of the subsequent steps usesan abrasive material that has been calculated toclear and blend in the abrasive pattern left in thesurface by the preceding step. To use a commoneveryday analogy, almost everyone understandsthat to produce fine finishes in woodworking appli-cations, it is necessary to use sanding operationswith successively finer abrasive grits to producecabinet or furniture quality surfaces. The same prin-ciple holds true in mass finishing (or even hand-fin-ishing) metal parts when very smooth or polishedsurfaces are required.
One time-honored method for producing veryrefined surfaces is dry barrel processing. This tech-nology was originally developed and heavily utilizedin the northeastern U.S. as early as the 1930s; simi-lar methods were developed concurrently in Europe.
The method was developed primarily to miti-gate the high labor costs associated with hand
buffing large numbers of consumer-oriented arti-cles such as eyewear and jewelry. This techniquewas widely accepted as a standard method forproducing very refined consumer acceptableproduct finishes that had previously been thesole province of those buffing methods. It is stillutilized for these types of applications. Thissequential principle has been adapted for use inother types of equipment for other part finishingapplications. Where reflective surfaces aredesired on parts being finished in vibratoryequipment, it is not unusual now to see second-ary vibratory processes with burnishing media ordry process polishing media develop those sur-faces. Many processes have been developed forcentrifugal disk and centrifugal barrels wherethree or more steps are utilized in order to bringpart surfaces to very low micro-inch surface pro-files or to develop very reflective surfaces for cos-metic reasons.
MASS FINISHING PROCESSES AND COMPRESSIVESTRESS EFFECTSEven simple tumbling can develop residual stresses
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that can provide some functional improvements toservice life in certain components. High-energymass finishing methods can magnify this effectmany times. In the early 1990s some researcherspioneered the use of scanning electron microscope(SEM) analysis to evaluate surface finishes fordurability and functionality.
This early work showed that it was possible toimprove functionality and service life of many dif-fering types of components by a two-fold improve-ment in metal surface profile and integrity.
Processes, such as peening, are commonly usedfor metal surface integrity improvement to miti-gate crack propagation points and improve serv-ice life by improving wear and metal fatigueresistance.
It was found that high-energy loose media sequen-tial finishing could develop not only compressivestresses but very level or negatively skewedplateaued surfaces. This provided a great deal morebearing load surface to parts that interacted withother part surfaces.
In one application, stamping dies used for formingaluminum can tops were given a useful life of
approximately ten times the value of parts that hadnot been surface finished with this method. Anotherapplication cited by Richard Gilliam in a technicalpaper describing centrifugal barrel processing notedextensive cycling tests conducted by a spring manu-facturer. “This ability to improve resistance tofatigue failure is graphically demonstrated by theresults of some tests made by a manufacturer ofstainless steel coil springs. A group of springs wastaken from a standard production run. Half of thesample was finished in the manufacturer’s usualmanner of barreling followed by shot peening, whilethe other half was CBF-treated for 20 minutes. Thesprings were then tested to failure by compressingthem to a stress change from 0 to approximately50,000 psi. The results showed that all the springsfinished by the conventional method failed between160,000 and 360,000 cycles. The springs that hadbeen processed by CBF failed at between 360,000and 520,000 cycles, an average improvement of60%.”
Substantial compressive stress effects can also begenerated in lower-energy types of equipment withvery dense metal media. In commenting on this,
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John Rogers, Process Laboratory Manager for theAbbott Ball Co., Inc., noted some points made in acompany publication: “Steel media is smooth andheavy. It is not abrasive in action. Rather, themedia’s weight and strength increases the smooth-ness and pressure of its finishing action. Workpieceskeep their tolerances intact, gain compressivestress, and achieve the ultraclean, microscopicallysmooth surface.
The wide selection of media shapes and sizesallow full control over the type and amount of con-tact obtained between the media and the part.Steel media is heavy, weighing approximately 300lb/ft3. The media mass forms a dense cushion thatproduces rapid finishes yet does not harm fragileparts.
As steel media impinges on a part, its surface iswork-hardened. The working action imparts com-pressive stress as a beneficial byproduct of the fin-ishing process. In many instances, the process canreplace steel shot peening as a work-hardening step.Parts processed with steel media have longer cyclelives and greater resistance to wear as a result ofthis compressive stress action.
ELECTROCOAGULATION: AN EMERGING WASTEWATERTREATMENT TECHNOLOGYOne of the most pressing problems faced by thoseinvolved in surface finishing is compliance with anincreasingly stringent maze of regulations designedto protect underground resources from industrialcontaminants. Penalties for failure to adhere towaste effluent treatment and disposal can be dra-conian. This can be a particularly vexing problem forthose involved in both the plating and mass finish-ing industries because of the difficulty and complex-ity involved in removing the heavy metals in sus-pension or solution in their wastewater effluentstream before discharging the water back to themunicipality. A variety of strategies involving chem-ical treatment and the use of flocculent along with afinal dewatering process, such as filtration or evap-oration, are commonly used. All of the strategiesemploy a method for separating water from dis-solved or suspended solids by coagulation, agglom-erating solid particles together to precipitate themor make them much more filterable. One emergingtechnology for these kinds of applications is knownas “Electrocoagulation” — using electric current
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instead of chemical flocculent consumables to pro-duce this effect.
A leading proponent of the technology, Scott W.Powell of Powell Water Systems, contrasted the dif-ference between chemical and electrical coagulationmethods in a recent technical paper: “Coagulationcaused by altering the charge on metal ions, organ-ics, and colloidal particles creates a large particlethat can be settled or filtered out. Chemical coagu-lation typically uses a dissolved salt. Part of the saltwill attach to the material in the water to be coagu-lated. The other part of the ion typically remains inthe solution. Chemical coagulation creates a hydrox-ide sludge that attracts water. The hydrophilicsludge holds water, which increases the volume ofsludge generated and increases the dewateringtime.
“Electrocoagulation adds electrons to the solutionby passing alternating current or direct currentthrough the solution from the power grid. The elec-trons destabilize the material in the water creatingoxide sludge when sufficient activation energy ispresent. … Heavy metal ions converted to metaloxides will pass the leach tests making them non-
hazardous. Metal oxides can be smelted to recoverthe metals in a usable form.”
The bottom-line for finishers and platers is thepossibility of deploying a much less complex andless costly alternative to other wastewater treat-ment methods.
TURBO-FINISH MACHINES AND TURBO-ABRASIVEMACHININGDr. Michael Massarsky, the inventor of the Turbo-Finish process, initially developed the process forimproving edge and surface finish methods forrotating parts in the aircraft engine industry. Theprocess replaces much of the manual deburring for-merly required on these types of parts. TAMmachines could be likened to free abrasive turningcenters. They utilize fluidized bed technology to sus-pend abrasive materials in a specially designedchamber. Parts interface with the abrasive materialon a continuous basis by having part surfacesexposed and interacted with the abrasive bed byhigh-speed rotational or oscillational movement.This combination of abrasive envelopment and high-speed rotational contact can produce important
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functional surface conditioning effects. Deburringand radius formation happen quickly. Unlike buff,brush, belt, and polish methods, or even roboticdeburring, abrasive operations on rotating compo-nents are performed on all features of the partsimultaneously. This produces a feature-to-featureand part-to-part uniformity that is extraordinary.
TAM processes share characteristics common toboth machining and mechanical finishing processes.A much higher degree of control is possible than isthe case with conventional finishing processes. TAMprocesses can utilize very sophisticated computercontrol technologies to create processes which arecustom tailored to the needs of specific parts. Likemachining processes, the energy to produce the cut-ting or abrasive action that develops the desiredsurface effect arise primarily from the rotationalenergy of the part itself. Unlike both machiningprocesses and manual deburring processes withtheir single point of contact, TAM processes performabrasive machining or grinding on all features of thepart by abrasive media envelopment.
TAM processes were developed originally toaddress deburring and surface conditioning prob-lems on complex rotating components within theaerospace industry. Aerospace parts, such as turbineand compressor disks, fan disks, and impellers, pose
serious edge finishing problems. Manual methodsused in edge finishing for these parts were costlyand time-consuming. Even more importantly,human intervention, no matter how skillful at thisfinal stage of manufacturing, is bound to introducesome measure of nonuniformity in both effects andstresses in critical areas on the part. TAM providesa method whereby final deburring, radius forma-tion, and blending in of machining irregularitiescould be performed in a single machining operation.This machining operation can accomplish in a fewminutes what in many cases took hours to performmanually. It soon became obvious that the technolo-gy could address edge-finishing needs of other typesof rotationally oriented components such as gears,turbo-charger rotors, bearing cages, pump impellers,propellers, and many other rotational parts.Nonrotational parts can also be processed by fixtur-ing them to the periphery of disk-like fixtures.
Another important feature of the process is its useof high-intensity small abrasive particle contact toproduce surface effects. This results in the ability toprocess intricate or complex part shapes easily.Although the abrasive material used for processingis similar in some respects to grinding and blastingmaterials, TAM produces an entirely different andunique surface condition. One of the reasons for this
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is the muitidirectional and rolling nature of abra-sive particle contact with part surfaces. Unlike sur-face effects created with pressure or impact meth-ods, such as air or wheel blasting, TAM surfaces arecharacterized by a homogeneous, finely blended,abrasive pattern developed by the nonperpendicularnature of abrasive attack. Unlike wheel or beltgrinding, surface finishes are generated without anyperceptible temperature shift at the area of contactand the microtextured random abrasive pattern is amuch more attractive substrate for subsequent coat-ing operations than linear wheel or belt grindingpatterns.
TAM processing can be especially useful when partsize, shape, or complexity preclude the use of othermechanical finishing processes. TAM deburrs anddevelops edge and surface finishing effects very rapid-ly and has unique metal improvement and compres-sive stress generation capabilities. Aqueous wastetreatment and disposal costs are avoided by a com-pletely dry abrasive operation. The process is primari-ly intended for external surface and edge preparation,although some simpler interior areas and channels canbe processed as well. Complex geometric forms can beeasily accessed. Repeatability and uniformity can beeven further enhanced with PLC or computer-con-trolled processing, and with all features of the partreceiving identical and simultaneous abrasive treat-ment, feature-to-feature, part-to-part, and lot-to-lotuniformity on parts can be extraordinary.
BIBLIOGRAPHYDavidson, D.A., “Mass Finishing Processes,” 2002 Metal
Finishing Guidebook and Directory, New York,Elsevier Science; 2002
Boitsov, V.B. et al., “Calculation of Residual StressFormation at Vibro-Strengthening,” Dynamics,Strength & Wear Resistance of Machines, (Electronicedition), Cheliabinsk State University Press, Russia,(abstract-English, Full text Russian, Vol. 5, pp. 69-72,December 1998)
Massarsky, M.L. and D.A. Davidson, “Turbo-AbrasiveMachining-A New Technology for Metal and Non-Metal Part Finishing,” The Finishing Line, Dearborn,MI: Society of Manufacturing Engineers, Associationfor Finishing Processes, October 30, 2002, pp. 1–18(Electronic Edition, pdf file)
Massarsky, M.L. D.A. Davidson “Turbo-AbrasiveMachining and Finishing,” Metal Finishing,95(7):29–31; 1997
Gilliam, R,. “The Future of Mass Finishing – AComparison Between Finishing Methods,” ChironInternational Corp., Huntington Beach, CA; 2000
Powell, S.W., “Water Reuse Eliminates GovernmentRequired Treatments for Wastewater Discharges,”Clean Tech 2003 – 10th Annual InternationalCleaning Technology Exposition, Conference:McCormick Place, Chicago, IL., Mar. 3-5, 2003,Conference Proceedings, pp. 270-272
Rogers, J., “Steel Media and Its Finishing Applications”,Abbott Ball Co., Hartford, CT: , pp. 1–5; 2000 MF
ABOUT THE AUTHORMr. Davidson is a deburring and surface finishing specialist and consultant. He has contributed technicalarticles to Metal Finishing and other technical and trade publications and is the author of the MassFinishing section in the current Metal Finishing Guidebook and Directory. He has also written and lecturedextensively for the Society of Manufacturing Engineers, Society of Plastics Engineers, AmericanElectroplaters and Surface Finishers Association, and the Mass Finishing Job Shops Association. Mr.Davidson’s specialty is finishing process and finishing product development.
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