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Green Mass Finishing with DryAbrasive and Polishing Media

Novel finishing process allows refined surface edge effectswhile avoiding conventional wet-waste discharge.

All mass finishing processes utilizea loose or free abrasive material

referred to as media within a contain-er or chamber of some sort. Energy isimparted to the abrasive media massby a variety of means to impartmotion to it and to cause it to rubagainst or wear away at part surfaces.

Although by definition, the termmass finishing is used generally todescribe processes in which partsmove in a random manner through-out the abrasive media mass, equip-ment and processes that utilize looseabrasive media to process parts thatare fixtured all fall under this heading.

Most current mass finishing appli-cations—such as barrel, vibratory,centrifugal and spindle-finishprocesses—produce edge and surfaceconditioning on parts by processingthem in machinery work-chamberswith abrasive media and treatedwater. The water, treated with spe-cial chemical compounds, helps pro-mote smooth roll and flow of mediainside the work-chamber; rinse andclean the parts; and prevent sludgebuild-up and redeposition of abra-sive or metallic particulate on partsurfaces during the process.

The type of compound used totreat the water can have a materialeffect on the resultant surface fin-ish. Some compounds are formulat-ed to improve the abrasive cuttingaction of media. Other compoundshave been formulated to comple-ment the action of burnishingmedia, such as metallic media,porcelain or vitreous aluminum-oxide nuggets in developing reflec-

tive surfaces by reducing surfaceroughness with the compressiverolling action of the media.

One drawback this type ofmechanical finishing entails is thecreation of a wet waste effluent ordischarge that must be treated priorto disposal. Depending on the meth-ods used and the materials being fin-

ished, treatment can be complex,problematical and expensive.

In a very well-rounded discussionof the potential problems that maybe involved, LaRoux Gillespie, in hisMass Finishing Handbook1, noted thatthe treatment requirements can beformidable. Waste from commonmass finishing operations that usewater and compounds can include:

1) Fine, abrasive particles in asludge 2) Metal fines or dissolved metalfines 3) Fine, abrasive particles sus-pended in water 4) Oils—dissolved, dispersed, etc. 5) Plastic resins 6) Vitrified clay.

The amount of effluent created bybatch-oriented mass finishing sys-tems, such as barrel and centrifugalbarrel equipment, can be modestwhen compared to industrial-sizedvibratory systems. These kinds of sys-tems are draining effluent andadding make-up water and com-pound on a continuous basis. A 30-cubic-ft. capacity vibratory finishingmachine can develop a 30-gallon-per-hour waste stream that may includetwo to six pounds of dissolved or sus-pended solids per every hour of oper-ation. The treatment of the waste-stream is often made more compli-cated by the use of chelating com-pounds in the compound solution,which makes separation of dissolvedor suspended metals difficult.

In recent years, one alternative hasbeen developed is combining drymedia with high-energy methods toavoid the need for utilizing water-based processes. Although manymass finishing applications mightnot lend themselves to an all “dry”approach, there are some that do.Plus, if a dry approach can be uti-lized, waste disposal considerationscan be greatly simplified by this“green” approach, thereby obviatingthe need for effluent treatment andsecondary part drying operations.

One characteristic that has placeddry media at a disadvantage relativeto conventional wet process media isits relatively light weight or bulkdensity. Plastic or ceramic media canweigh two to three times more per agiven unit of volume. Also, the abra-sive is an integral part of the mediamass, so fresh abrasive particles arecontinuously being exposed as themedia surface is being worn away. Anumber of approaches are beingused to overcome this handicap:

1) Newer dry media formulationsutilizing nylon impregnated withabrasive.

TECHNICALLYspeaking

BY DAVID A. DAVIDSON, SOCIETY OF MANUFACTURING ENGINEERS

Figure 1: Some dry media is made up of natural materi-als, such as the wooden shapes (top row), and the agri-cultural by-products (bottom row). These kinds ofmaterials are treated with abrasives, such as pumice, orpolishing materials, such as jeweler's rouge or otherabrasive materials similar to those found in buffingcompound formulations. Photo by Tyha Davidson.

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ing. A closely related set of processeswould be fixture-centric processing,such as the spin-, drag-, spindle-, andturbo-finish methods (see Figure 2).The fixture methods produce resultsby imparting motion to parts directlythat are fixtured on spindle fixtures (byeither dragging, rotating or developinga planetary motion) and are immersedin loose abrasive or polishing media.The force with which part edges andsurfaces are interacted with loosemedia can be considerably higher thanthat developed by mass-media process-es, whereby parts are placed randomly

2) The use of high energy cen-trifugal equipment.3) Fixturing parts inside finishingmachine work chambers to increasethe force and flow of media contactagainst part surfaces.

ACCELERATING CYCLE TIMESWITH FIXTURING.Included in these mass-finishingmethods are traditional barrel tum-bling, vibratory and centrifugal finish-

within the media mass, and aredependent on the loose media motionto achieve the surfacing results.

Fixturing parts in more convention-al barrel or vibratory methods is alsocommon. This is done for a variety ofreasons, including the need to preventany part-on-part contact but also toincrease the amount of force flow ofmedia against part surfaces; to accel-erate cycle times; and produce morepronounced surface finish effects.

Part applications for fixture finish-ing in conventional equipment varywidely. To cite a few examples: Somemanufacturers of brass musical instru-ments (trumpets, french horns, trom-bones) fixture brass instrument assem-blies in barrel or vibratory chambersand flow soft polishing granulatemedia through the assemblies toreplace multiple buffing operations.

Similarly, some manufacturers ofmedical and surgical implant devicesfixture the devices in high-energy cen-trifugal barrels, and produce veryrefined surfaces on cobalt chrome andtitanium substrates by processing thedevices through a sequence of succes-sively finer loose abrasive operations.

SEQUENTIAL PROCESSINGOne trait that many of today’s moresophisticated mass finishing opera-tions share is a reliance on multiple-step sequential processing. In thistype of processing, very rough sur-faces can be brought to a highly pol-ished or micro-finished state. This isdone by initially processing the partswith coarse abrasive material, and then

TECHNICALLYspeaking

Figure 2: The Turbo-Finish dry process (spindle-finishmethod) is a unique surface- and edge-preparationmethod designed to produce vibratory or barrel-finish-style isotropic surface finishes on parts with media sim-ilar to that used in dry blasting operations. The highspindle speeds used in the process can develop edgecontour and refined surface conditions very rapidly,resulting in smooth surface conditions. Photo courtesyof Turbo-Finish Corporation.

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methods were developed concurrent-ly in Europe.2

The method was developed prima-rily to mitigate the high labor costsassociated with hand-buffing largenumbers of consumer-oriented arti-cles, such as eyewear and jewelry.This technique was widely acceptedas a standard method for producingvery refined consumer acceptableproduct finishes that had previously

following up with a sequence of finerabrasives (see Figure 1). Each of the sub-sequent steps uses an abrasive materialthat has been calculated to clear andblend-in the abrasive pattern left in thesurface by the preceding step.

To borrow an analogy from thewood finishing industry, almosteveryone understands that to pro-duce fine finishes in woodworkingapplications that it is necessary to usesanding operations with successivelyfiner abrasive grits to produce cabinetor furniture quality surfaces. Thesame principle holds true in massfinishing (or even hand-finishing)metal parts, when very smooth orpolished surfaces are required.

One time-honored method for pro-ducing very refined surfaces is drybarrel processing. This technologywas originally developed and heavilyutilized in the northeastern UnitedStates as early as the 1920s; similar

been the sole province of those buff-ing methods, and it is still utilizedfor these types of applications.

This sequential principle has beenadapted for use in other types ofequipment for other part finishingapplications. Where reflective sur-faces are desired on parts being fin-ished in vibratory equipment, it isnot unusual now to see secondaryvibratory processes with burnishingmedia or dry process polishingmedia develop those surfaces (seeFigure 3).

Many processes have been devel-oped for centrifugal disk and centrifu-gal barrels whereby three or moresteps are utilized in order to bring partsurfaces to very low micro-inch sur-face profiles, or to develop very reflec-tive surfaces for cosmetic reasons.

One recent development in usingthese techniques reflects an under-standing this equipment is capable ofprocessing much larger work-piecesthan had previously been thought.Larger, single-piece parts are being

TECHNICALLYspeaking

Figure 3: Turbo-Finish decorative and functional surfacefinish effects. The parts in this group have been fixtureprocessed via dry spindle finishing to prepare surfacesfor further decorative or functional effects. Photo cour-tesy of Turbo-Finish Corporation.

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nylon with various abrasives impreg-nated within the media shape.3 Thesemedia types are generally used inhigh energy finishing equipment,which has been modified from stan-dard designs to accommodate andevacuate the dust developed as a by-product of the process. Some studiesmade at the university level in Japan,where the media was developed, indi-cate that this kind of process can alsobe used to develop beneficial com-pressive stress and promote surfaceintegrity on critical parts.

German manufacturers of “drag-finish” equipment are utilizing dryfinishing and polishing materials todevelop edges and surfaces on cut-ting tools that both improve the sur-face finish the tools produce as wellas substantially increase the serviceor operational life of the tools them-selves. The driving force behind thedevelopment of these materials hasbeen specifically the need to developwaterless technologies that do nothave associated effluent stream treat-ment costs.

DRY PROCESSING OF MINIATURE PARTS

One area where dry processes havehad particular success is in the high-energy processing of a variety ofsmall, even miniature machined parts(such as Swiss turning or screwmachine applications). In oneextreme example, one large manufac-turer had been machining large vol-umes of precision speedometer shaftsusing several dozen tumbling barrelsto surface finish the shafts in a 48-hour cycle to a 10Ra finish. This wetprocess was creating an effluentwaste stream in excess of 10,000 gal-lons per day, which was highly objec-tionable to the municipal authorities.

This entire operation wasreplaced by two 120 liter capacitycentrifugal barrel finishingmachines utilizing a walnut shell-abrasive mixture. The end result:cycle times were reduced from 48hours to two 2-hour cycles; effluentdischarge was reduced from 10,000gallons per day to zero; and endingsurface roughness on the shafts wasreduced from 10 Ra to 4-5 Ra.

run in barrel compartments by beingfixtured within the barrel compart-ment, or in some cases are being runin barrel compartments that havebeen over-filled with media deliber-ately to cushion the part during therotational movement.

NEWER DRY MEDIA FORMULATIONSIn recent years, waterless processeshave been developed by using specialdry media made primarily from

SUMMARY Dry media mass finishing processescan be used to produce refined edgeand surface effects. These processesare not as well known as conventionalprocesses that make use of aqueoussolutions in concert with abrasivemedia. Although not universal inpotential applications, these materialsand methods should be investigatedas a potential method for green fin-ishing by companies seeking to avoidrather than treat wet waste dischargefrom mass finishing operations.

REFERENCES1. Gillespie, LaRoux, Mass Finishing

Handbook, (New York, NY:Industrial Press) 2007, pp. 200-203.

2. Koichi Kitajima, AtsushiYamamoto, M. Sasai, FinishingCharacteristics and ResidualStress on Workpiece Surface inDry Barrel Finishing,Department of MechanicalEngineering, University ofKansai, 3-3-35 Yamate-Cho,Suita-Shi, Osaka Prefecture,Japan, 2007, www.scientific.net

3. Massarsky, M. L., Davidson, D.A., “Turbo-Abrasive Machiningand Turbo-Polishing in theContinuous Flow ManufacturingEnvironment”, SME TechnicalPaper MR99-264, CONFER-ENCE PROCEEDINGS: 3rdInternational Machining andGrinding Conference, Cincinnati,OH, Oct 4-7, 1999, Dearborn,MI: Society of ManufacturingEngineers, 1999

4. Davidson, D. A., “Micro-Finishing and Surface Textures”,Metal Finishing”, (White Plains,NY: Elsevier Science) July, 2002

BIODavid A. Davidson is a member of theSociety of Manufacturing Engineers, Deburring, Edge-Finish and SurfaceConditioning Technical Group. He can bereached via e-mail: ddavidson@debur-ringsolutions.com

TECHNICALLYspeaking

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