Davidson centrifugal finishing

MECHANICAL FINISHING

Precision Finishing Processesin Centrifugal Barrel EquipmentMethods promise faster cycling in select applications.By David A. Davidson, Society of Manufacturing Engineers, 2006 Chair: DeburringTechnical Committee, Spokane, Wash.

Large numbers of parts can be uniformly edgeand surface finished in mass finishing equip-ment. Process development in recent years has

facilitated the creation of very refined surfaces byprocessing parts in a series of successively finerprocesses in high-energy centrifugal equipment.

Nearly all manufactured and fabricated compo-nents require some edge and surface finish prepa-ration prior to final assembly. Although many partstoday are still individually handled and processedwith hand tools or hand-held power tools, it hasbecome almost universally accepted that utiliza-tion of mass finishing techniques are preferred forboth their economic and technical merits, whenev-er possible. Utilizing energy developed from motionimparted to the containers or vessels in mass fin-ishing equipment, loose abrasive, or polishingmedia can produce a wide range of desirable edgeand surface effects.

Roughly machined, milled, molded, stamped, orcast parts can be transformed into attractive partsfree or burrs, sharp edges, and corners with uni-form surfaces that can range from the utilitarianto the highly aesthetic. Among the specific tasksthese processes are specified to perform are clean-ing, deburring, descaling, surface smoothing, edge-break, radius formation, removal of surface con-taminants from heat treat and other processes,preplate, prepaint or coating surface preparation,blending in surface irregularities from machiningor fabricating operations, producing reflective sur-faces with nonabrasive burnishing media, refiningand polishing surfaces, and developing superfinishor microfinish equivalent surface profiles.

CENTRIFUGAL BARREL FINISHINGAlthough many types of manufactured componentswith high-volume output are abrasively processedwith barrel and vibratory finishing equipment,many different types of parts with specialized orprecision edge and surface finish requirements areprocessed in high-energy centrifugal barrel equip-ment (see Figures 1–3).

Centrifugal barrel finishing (CBF) is a high-energy

finishing method (see Figure 4), which has come intowidespread acceptance in the last 25 to 30 years.Although not nearly as universal in application asvibratory finishing, many important CBF applicationshave been developed in the last few decades.

Similar in some respects to barrel finishing, inthat a drum-type container is partially filled withmedia and set in motion to create a sliding action ofthe contents, CBF is different from other finishingmethods in some significant ways. Among these arethe high pressures developed in terms of media con-tact with parts, the unique sliding action inducedby rotational and centrifugal forces, and accelerat-ed abrading or finishing action. As is true withother high-energy processes, because time cyclesare much abbreviated, surface finishes can bedeveloped in minutes, which might tie up conven-tional equipment for many hours.

The principle behind CBF is relatively straightfor-ward. Opposing barrels or drums are positioned cir-

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Figure 1: Centrifugal barrel finishing equipment, shown here, isused to produce very refined surface finishes.The equipment isvery versatile—it can be used for both highly abrasive and pol-ishing processes. The large, 220-liter-capacity machine to theleft can process large numbers of small parts in production, orit can process larger parts by segregated processing in individ-ual chambers or fixturing. The smaller machine has a totalcapacity of 12 liters and is useful for smaller operations such asdental laboratories, jewelry manufacturing, and precision screwmachine applications. It has a much higher degree of portabili-ty than similar equipment in that it can be rolled from locationto location, and use single-phase, 110V current. (Photo courtesyof Mass Finishing, Inc.)

cumferentially on a turret. (Most systems haveeither two or four barrels mounted on the turret;some manufacturers favor a vertical setup, and oth-ers a horizontal orientation for the turret.) As theturret rotates at high speed, the barrels are coun-terrotated, creating very high G-forces or pressures,

as well as considerable media sliding action withinthe drums.

Pressures as high as 50 Gs have been claimed forsome equipment. The more standard equipmenttypes range in size from one cubic foot (30 L) to 10cubic feet, although much larger equipment hasbeen built for some applications.

Media used in these types of processes tend to bea great deal smaller than the common sizes chosen

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Figure 2: Centrifugal barrels are being used to develop verydemanding finish specifications, such as that shown here onboth the cast acrylic denture and dental partial frame. To pro-duce precision finishes of this order, multiple process cycles inthe correct sequence are required that utilize differing mediatypes, including (1) ceramic abrasive media, (2) plastic fine fin-ishing media, and (3) dry process polishing media. Processesutilizing similar sequential steps are also specified for medical,aerospace, bearing, electronic, automotive, and jewelry applica-tions. The aesthetically desirable surface finishes developed onthe dental parts shown here also have important functionalcharacteristics, including reduced plaque adhesion and metalfatigue resistance. (Photo courtesy of Mass Finishing, Inc.)

Figure 3: Close-up photo showing acrylic dentures and ceramicmedia in processing step 1 as outlined in the caption for dentalparts shown in Figure 2. In this view, a 30-liter-capacity machineis shown with four separate removable hexagonal barrel com-partments with polyurethane linings. Typically, the abrasivecycles in this type of processing are done in conjunction with anaqueous solution in addition to the abrasive media, making fora “wet process.” The final process often makes use of soft pol-ishing media in a dry environment. (Photo courtesy of MetalFinishing, Inc.)

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for barrel and vibratory process-es. The smaller media, in such ahigh-pressure environment, arecapable of performing muchmore work than would be thecase in lower-energy equip-ment. They also enhance accessto all areas of the part and con-tribute to the ability of theequipment to develop very finefinishes. In addition to the abil-ity to produce meaningful sur-face finish effects rapidly, andto produce fine finishes, CBFhas the ability to impart com-pressive stress into criticalparts that require extendedmetal fatigue resistance.

Small and more delicate partscan also be processed with confi-dence, as the unique slidingaction of the process seems tohold parts in position relative toeach other, and there is general-ly little difficulty experiencedwith part impingement withproper media and part loading.Dry process media can be used incertain types of equipment and isuseful for light deburring, polish-ing, and producing very refinedisotropic superfinishes.

Practicality and questions ofcost effectiveness often deter-mine whether high-energymethods are selected over con-ventional barrel or vibratoryprocesses. If acceptable surface

finishes can be developed in afew hours, conventional equip-ment is usually the most eco-nomic alternative. CBF equip-ment’s strong suit is the abilityto develop surface finishes thatmay require lengthy time cyclesin conventional equipment andthe capability to develop a widerange of special surface finishesrequired for demanding andcritical applications.

REFERENCES:Davidson, D.A., “Micro-Finishing

and Surface Texture,” MetalFinishing, 100(7):10–12; 2002.

Davidson, D.A., “Mass FinishingProcesses,” 2005 Metal FinishingGuidebook and Directory,103(6A):78–89; 2005.

Hignett, J. Bernard, “CentrifugalBarrel Deburring and SurfaceConditioning–Some RecentDevelopments,” TechnicalPaper: MR79-567, Society ofManufacturing Engineers,Dearborn, Mich.; 1979.

Gillespie, Laroux, Deburring andEdge-Finishing Handbook,Society of ManufacturingEngineers, Dearborn, Mich.;1999.

For more information, contactDave Davidson at (e-mail) [email protected] or visitwww.deburringsolutions.com. mf

Figure 4: Four barrels are mounted at the periphery of alarge turret. Each barrel is loaded with media, parts,and water to approximately 50% to 90% full. Duringoperation, rotation of the large turret creates a cen-trifugal force on the media and parts inside each barrel.This force compacts the load into a tight mass, causingthe media and parts to slide against each other, remov-ing burrs and creating superior finishes.This action alsoreduces the cycle time needed to complete the finish-ing of the parts by up to a factor of 30 over conven-tional vibratory and barrel equipment. (Diagram cour-tesy of Mass Finishing, Inc.)

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