AIRLINE/AIRCRAFT PARTS FINISHING

Surface Condition Impacts Part PerformanceBurrs, edges can negatively influence function of components.By David A. Davidson, Society of Manufacturing Engineers, Chair: Deburring, Edge-Finishand Surface Conditioning Technology Committee

The role of mass finishing processes—such asbarrel tumbling, vibratory, centrifugal, andspindle finishing—as a method for removal of

burrs, developing edge contour, and smoothing andpolishing parts has been well established and docu-mented for many years. These processes have beenused in a wide variety of part applications to promotesafer part handling (by attenuation of sharp partedges); improve the fit and function of parts whenassembled; and produce smooth, even micro-finishedsurfaces to meet either functional or aesthetic crite-ria or specifications. Processes for developing specif-ic edge and/or surface profile conditions on parts inbulk are used in industries as diverse as the jewelry,dental, and medical implant sectors on up throughthe automotive and aerospace fields.

Less well known and less clearly understood is therole specialized variants of these types of processescan play in extending the service life and perform-ance of components in demanding manufacturing oroperational applications.

Industry has always been looking to improve sur-

face condition to enhance part performance, and thistechnology has become much better understood inrecent years. Processes are routinely utilized tospecifically improve life of parts and tools subject tofailure from fatigue and to improve their perform-ance. These improvements are mainly achieved byenhancing part surface texture in a number of dif-ferent, and sometimes complementary, ways.

In his recently published “Mass FinishingHandbook” author LaRoux Gillespie a chapter titled“Process Side Effects” notes some of these potentialimprovements and comments on negatives that canbe caused by incorrect process selection:

“In addition to removing burrs and improving sur-face finishes, mass finishing can at the same time

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Figure 1: Increasingly sophisticated methods for measuring sur-face condition have been developed in recent years to assistengineers in analyzing and understanding surface conditionsand textures and their relationship to part performance. Often,these methods are used to understand how surface finish tex-tures meet operational requirements after parts have beenmachined and finished. As in the case shown above, they arealso used in forensic applications to measure current surfacecondition in terms of determining potential remaining opera-tional service life. The diagram shown here depicts a 1.5 mm x7.5 mm of a gear tooth wear area. A computer-enhanced 3-Dcharacterization is shown on the diagram to the left; the twodiagrams to the right show a 2-D surface profile trace.The partin question is part of a gear-box built by Hamilton Sundstrandfor the space shuttle. (Photo courtesy of Jack Clark; Zygo Corp.,Middlefield, Conn.

Figure 2A and 2B: In before-and-after comparisons, burrremoval, hole-edge radius, and interior surface finish developedby Abrasive Flow Machining method (AFM). In many applica-tions, developing edge and interior hole surface quality are CTQ(critical to quality) and overall performance of the part, espe-cially if non-turbulent air flow and air-flow efficiency are impor-tant part attributes. Easily discerned in the comparison of thetwo close-up photographs is the isotropic surface finish charac-teristic of the finished part. (Photo courtesy of Extrude-HoneCorp., Irwin, Pa.)

Figure 3 — Surface finish values of the small holes seen along theedge of the foil area of the blade here are critical to cooling of theblade. Improved and less turbulent flow due to high quality ofinterior hole surfaces can be critical to function and performanceof the part. (Photo courtesy of Extrude-Hone, Irwin, Pa.)

change other key attributes of parts, some for theworse and others for the better. In addition toremoving burrs, mass finishing can:• Radius or blunt part edges;• change part dimensions (0.000050 in.–0.003 in.);• change a part’s surface finish;• compact a part’s surface pores;• clean a part’s oily and dirty surfaces;• remove oxides and heavy scale from parts;• change a part’s flatness;• prevent soldering (if wrong abrasives are used);• create large compressive stresses in part;• improve or worsen corrosion rates;• change part luster;• change part color;• change friction;• and decontaminate radioactive surfaces.

AEROSPACE EDGE/SURFACE QUALITYCONCERNSSometimes, to fully understand the significance ofedge and surface quality issues, it is important tounderstand the magnitude of the consequenceswhen edge and surface condition receive insufficientattention. Gillespie, when summarizing some pointsmade in an aerospace forum regarding edge andsurface quality issues, noted that important serviceand operational considerations can be heavilyimpacted by edge and surface condition quality:

Fatigue life, stresses, and strain: Fatigue lifeincreases with decreasing surface roughness, andsmoother surfaces have less preload loss when theyare part of a mechanically fastened joint. Burrsincrease stress concentration at hole edges, whichalready have three times the net section stress at theedge. Therefore, removing burrs decreases stress con-centration, which increases fracture resistance andfatigue life. Lastly, burrs can interfere with properseating of mechanical fasteners, so removing them

reduces damage to fasteners and clamped componentsduring assembly.

Sharp corners increase stress concentration, soincreasing radii decreases stress concentration,which increases fracture resistance and fatigue life.If water creeps under interfaces via higher surfaceroughness and fills up a cavity or interface, thenfreezes, it could create high stresses and/or acceler-ate material fracture, not to mention stress corro-sion cracking at scores from the hidden, trappedwater/chemicals.

One author notes, “Sharp corners, burr holes, etc.increase not only the stress but the strain as well.Looking at the strain we can have three differentsituations:1. The strain can be inside the linear behavior.

(Under the yield limit).2. The strain can be between the ultimate and the

yield limit.3. The strain can reach the ultimate limit.If the third situation is going to occur, the cracks

can develop because of material failure. In this case,the crack can also reach the material’s “criticalvalue.” For this reason, round the corners, deburringthe holes, and finishing the surfaces will help topass from the third to the first situation.”

Almost without exception fatigue cracks start atthe surface of a part rather than internally. One pos-sible reason may be that the highest stresses areusually found at the surface (e.g., bending and tor-sion) and the surface is vulnerable to stress raisers,such as machining notches, scratches, and pits.Surface finish affects the strength of a part subject-ed to fatigue loading because most machining oper-ations leave a notch pattern and fatigue cracks usu-ally originate in a notch.”

Corrosion and coating impact: Poor surfacefinish introduces millions of new points for crevicecorrosion on the surface. Also, a rough surface can

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Figure 5: Impeller-like parts can be processed with CentrifugalBarrel Finishing (CBF); Turbo-Finish (TAM), and Abrasive FlowMachining (AFM) methods to produce uniform edge contours,but part performance is enhanced by the isotropic and plateaudsurfaces created in the foil area of the part.

Figure 4: Aircraft engine vane segments can be deburred,radiused, and are polished with a number of different methods.These components processed with centrifugal barrel finishing(CBF), which has developed needed edge and surface finisheswhile developing high-quality surfaces with useful stress andisotropic characteristics.

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make it difficult to get goodresults with non-destructivetesting methods like die pene-trants—especially when theroughness is in a pattern (suchas produced by flycutting ormilling). Rougher surfaces orsharper exterior edges canscratch coated or painted sur-faces during assembly and mightallow hidden corrosion to spreadunderneath what might tem-porarily appear as good finishes.

The physics, electrochemistry,etc., are well documented aboutapplying a coating to a sharpedge. When using any type ofelectrically catalyzed process(anodizing, electrocoating, elec-trostatic spray painting, etc.)current density fluctuations pre-vent the build-up of a uniformcoating thickness. Variations incoating thickness have manynegative aspects, such as vari-able friction at joint surfaces,areas for localized corrosion, pit-ting, galvanic cells, etc. Corrosionfatigue and stress corrosioncracking are obvious concerns.

Joint friction and preloads:Also, with riveted structure, fric-tion (due to the clamping force ofthe fasteners) between fayingsurfaces in a joint serves a cou-ple important functions. First,the friction provides a bit of“shear preload”—the joint can

take a certain amount of shearwithout loading the fasteners orsheet in bearing. The greaterthe friction, the more resistantthe joint will be to working looseand smoking rivets. This ties innicely to the second function: highfrequency (engine) vibrationsthroughout the structure aredamped or dissipated throughjoint friction. The greater the fric-tion, the greater the high-frequen-cy-fatigue resistance of a mechan-ically fastened joint.

If a burr is sitting between thefastened sheets preventing goodcontact of the faying surfaces,much of this friction is lost. Ahigher surface roughness willlead to higher friction forces toovercome when torquing a bolt.This means that less preload (Fi)will be developed, with a corre-sponding decrease in load atwhich gapping occurs [Fi/(1-C)],which increases chances forleaks (stuff coming out, or stuffgoing in), and also leads to worsefatigue performance (higheralternating tensile stresses). Ahigher surface roughness mayalso lead to preload relaxation—exacerbating all of the above.

As one reader noted, “This isthe classic ‘shanking and sheetgapping’ syndrome, caused byburrs and ‘liberated burrs’[chips].” Rough surfaces provide

less surface area of contact, giv-ing rise to higher and very local-ized contact stresses. If flavoredwith a little salt mixed in andthrow in some corrosion, thiscould be a disaster.

Good seating: A fastener holewith a good, sharp, burred cornerwill have obvious problems withseating when met with a fasten-er that has a radiused junctionbetween head and shank. Poorbonding of structures, in light-ning strikes, can cause cata-strophic local structural failure.

Static discharge: Sharp out-side corners on structure act aselectrical charge concentrators,and can be a static discharge haz-ard. For the same reason, sharpcorners can cause undesirableresults in electroplating opera-tions. One reader asks, “If an over-ly rough surface causes corrosion,could this joint develop a staticcharge?” If there are two conduc-tive metal surfaces separated by adielectric (oxide) and you add somemovement or vibration—presto—static charge because of rough sur-faces (as opposed to burrs).

Issues between movingparts: Mating faces must befinely machined (or finished) to:

AIRLINE/AIRCRAFT PARTS FINISHING

Figure 6: Centrifugal bar-rel finishing was used tochange the character ofsurfaces on this titaniumtest coupon. Centrifugal,vibratory, and AFM meth-ods are being used tochange surface character-istics that can affect partperformance. The uppercoupon is typical of asmachined (milling cutter-path or ground) surfaceswith a positively skewedsurface has been alteredto exhibit a plateauedsurface with attenuatedor blended peaks, shownin the lower test coupon.

Figure 7: Centrifugal barrel machinepreparing to process aircraft vane seg-ments, deburring vane edges and alsosmoothing and polishing the foil surfaceareas simultaneously. (Photo courtesyof Tom Mathisen, MFI.)

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• Avoid friction;• avoid heat due to friction. Excessive heat may

change the properties of the material surface, withunpredictable consequences;

• have better lubrication. The active film in a finemachined surface will be more efficient becausethere will be more surface in contact with thelubricant. This will permit better heat transferfrom the part to the lubricant (there is a limit tohow fine a finish a surface should have. The auto-motive industry intentionally adds some surfacepatterns to hold the oil in internal combustionengines;

• excessive roughness may develop high materialwear, leading to high play, and high replace fre-quencies of the parts;

• roughness produces friction as stated above.Friction can lead to electricity (tribo-electriceffect). Electricity can lead to corrosion.Electrical issues: As noted above, friction

between rough surfaces will create electrical energy.That energy can create an accelerated galvanic-cor-rosion anode or cathode site, if all (most) other sur-faces are coated or insulated. Burrs are sources ofstatic discharge.

Burrs and surface roughness will both interferewith good, uniform surface contact between fayingsurfaces in a mechanical joint. This increases theelectrical resistance of the joint and, if severe, cancause problems with electrical bonding of structure;interfering with effective grounding of electricalequipment and/or antennae, and become a minia-ture plasma cutter in the event of a lightning strike.

Current density due to sharp edges and burrs cancut through protective coatings on mating surfacesand radii, providing a minute area of “clean metal”electrical path to drive corrosion dramatically worsethan if no protective coating were there to beginwith due to the extremely high resultant currentdensity. The hole-punching force of high currentdensity results in stress risers to enhance SCC andcorrosion fatigue. For aircraft assemblies, sharpedges become spark over points whenever voltage isapplied (static, lightning strikes, etc.)

Hydraulic and gas leaks: Higher values of sur-face roughness (and burrs) increase leakage rateunder/around gaskets and seals. Nipping gaskets,seals, and O-rings on sharp edges during installation,or scouring them on rougher surfaces during opera-tion of rotating equipment, can accelerate leakage.

Sometimes a surface that is finished too well canhinder sealing. O-rings need something to hold on to—if your surface finish is too fine and the compres-sion on the O-ring is too light, the O-ring is likely to

fail. In one industry, engineers specify 63ra for mostsurfaces that will contact a secondary sealing ele-ment. (They do, however, require flatness and sur-face finish to an extreme on other parts—millionthsof an inch for mechanical seal faces). There aretimes when a sharp edge is needed. Labyrinth sealsin gas turbines spring to mind, as do squealer tipson compressor blades.

Peening issues: Excessive surface roughness cansometimes be an indication of over-peening, whichnegates the beneficial aspects of compressive resid-ual stress. Aluminum and magnesium are especial-ly prone to over-peening, which results in manylocalized areas of increased stress. Problems withfracture (stress intensity) and fatigue (crack nucle-ation sites) are then possible/probable. Joint prob-lems can arise from excessive surface roughness,and over-peening is yet another method for creatingsurface roughness.

Shot peening, mass finishing, surface polishing,deburring, and rounding off all add a sustained com-pressive stress into the material. This stress willcounteract the tensile stress caused by a crack andhelp to contain its propagation.

EDGE AND SURFACE CONDITIONS THATINFLUENCE PART PERFORMANCETo understand how edge and surface quality canimpact part performance, some understanding ofhow part surfaces developed from common machin-ing, grinding, and other methods can negativelyinfluence part function over time. A number of fac-tors are involved:

Positive vs. negative surface skewness: Theskew of surface profile symmetry can be an importantsurface attribute. Surfaces are typically characterizedas being either negatively or positively skewed. Thissurface characteristic is referred to as Rsk (Rsk–skew-ness–the measure of surface symmetry about the meanline of a profilometer graph). Unfinished parts usuallydisplay a heavy concentration of surface peaks abovethis mean line (a positive skew).

It is axiomatic that almost all surfaces producedby common machining and fabrication methods arepositively skewed. These positively skewed surfaceshave an undesirable effect on the bearing ratio ofsurfaces, negatively impacting the performance ofparts involved in applications where there is sub-stantial surface-to-surface contact. Specialized high-energy finishing procedures can truncate these sur-face profile peaks and achieve negatively skewedsurfaces that are plateaued, presenting a muchhigher surface bearing contact area. Anecdotal evi-dence confirms that surface finishing procedures

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tailored to develop specificsurface conditions with thisin mind can have a dramaticimpact on part life. In oneexample, the life of toolingused in aluminum canstamping operations wasextended 1,000% or more byimproved surface texturesproduced by mechanical sur-face treatment.

Directionalized vs. ran-dom (isotropic) surfacetexture patterns: Somewhatrelated to surface textureskewness in importance is thedirectional nature of surfacetextures developed by typicalmachining and grindingmethods. These machinedsurfaces are characterized bytool marks or grinding pat-terns that are aligned anddirectional in nature. It hasbeen established that tool orpart life and performance canbe substantially enhanced ifthese types of surface tex-tures can be altered into onethat is more random innature. Post-machiningprocesses that utilize free orloose abrasive materials in ahigh-energy context can alter themachined surface texture sub-stantially, not only reducing sur-face peaks, but generating a sur-face in which the positioning ofthe peaks has been altered appre-ciably. These “isotropic” surfaceeffects have been demonstrated toimprove part wear and fractureresistance, bearing ratio andimprove fatigue resistance.

Residual tensile stress vs.residual compressive stress.Many machining and grindingprocesses tend to develop resid-ual tensile stresses in the surfacearea of parts. These residual ten-sile stresses make parts suscepti-ble to premature fracture andfailure when repeatedly stressed.Certain high-energy mass finish-

ing processes can be implement-ed to modify this surface stresscondition, and replace it withuniform residual compressivestresses. Although, there aremany mechanical surface treat-ments that will improve edge andsurface finish quality. A numberof processes are now specifiedspecifically because of their repu-tation as performance-enhancingprocesses, some of these are dis-cussed below.

Abrasive flow machining(AFM) is a process that, underpressure, extrudes a semisolidabrasive media that conforms tothe shape of the surface or pas-sage that is being processed.Polishing, deburring, and edgeradiusing are accomplished any-

where that the media can beforced to flow. The abrasive flowpolished surface has no smearedmetal, and the radii generatedon any 90 degree edges are trueradii. The elimination of stressrisers, damaged metal layers,and the generation of roundedges are used to help extendcomponent life.

Rotating parts can especiallybenefit from the AFM process.Fans, blisks, blades, disks, andspacers can all benefit from thissurface and edge conditioning.Highly polished surfaces alsotend to pick up less coke and car-bon. This is especially importanton fuel systems components.Blades and vanes located in boththe cool and warm sections of the

Figure 8: The Turbo-finishmethod, more commonlyused for process rotating air-craft engine parts, not onlydeburrs and produces edgecontour, but also developscompressive stress equilibri-um and isotropic surface val-ues that can be critical topart life in service. (CourtesyDr. Michael Massarsky,Turbo-Finish Corporation.)

Figure 9: Large aircraft frameparts can be deburred, simi-lar machinery can also beused with steel media toproduce important residualcompressive stress in aircraftframe components, animportant consideration forlarge titanium componentssuch as the one illustratedhere. (Photo courtesy ofSamuel R.Thompson.)

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engine can also benefit from highly polished sur-faces in less turbulent air flow across their surfaces.The abrasive flow process imparts compressiveresidual stress. Although it will never replace shotpeening, it is used to extend part life on componentsthat, by configuration, cannot be shot peened.

The process is used to enhance holes and slotsprior to eddy current inspection. Many componentsthat are being inspected as part of an engine over-haul are hand polished (butterflied) prior to inspec-tion. The hand process is inconsistent and time con-suming. AFM can be managed so that only the cokeand carbon are removed, greatly optimizing theinspection process. Small holes on fuel system com-ponents and turbine blades and vanes can be flowtuned to ±1%. By more efficiently tuning cooling air,hot section components will last longer and requireless air. Fuel-delivery components benefit from moreuniformity in both spray shape and flow rate. AFMis used as the final machining and sizing operation.The AFM process can be used to control stockremoval to ±0.0001 of an inch.

CENTRIFUGAL BARREL FINISHINGCentrifugal barrel finishing (CBF) is a high-energyfinishing method (see Figure 4) that has come intowidespread acceptance in the last 25 to 30 years.

Although not nearly as universal in application asvibratory finishing, many important CBF applica-tions have been developed in the last few decades.These kinds of processes are utilized widely withinthe aerospace and aircraft engine industriesbecause of their ability to produce high-qualityisotropic surface finishes rapidly on parts, such asturbine blades and vane segments and developinguseful compressive stress values simultaneously.

Two or four barrels are mounted at the periphery ofa large turret. Each barrel is loaded with media,parts, and water to approximately 50% to 90% full.During operation, rotation of the large turret createsa centrifugal force on the media and parts inside eachbarrel. This force compacts the load into a tight mass,causing the media and parts to slide against eachother, removing burrs and creating superior finishes.This action also reduces the cycle time needed to com-plete the finishing of the parts by up to a factor of 30over conventional vibratory and barrel equipment.

Turbo-finish and performance issues: This

technology has been demonstratedto successfully impart compressivestresses into critical areas of rotat-ing parts in a fashion that isunique. The method is also capableof producing surface conditions at

these critical edge areas that contribute to increasedservice life and functionality of parts that are severe-ly stressed in service. Among these are: (1) the cre-ation of isotropic surfaces; (2) the replacement of pos-itively skewed surface profiles with negative orneutral skews; and (3) the development of an overallstress equilibrium in parts with a complex feature set.

As previously mentioned, all common machiningand manual finishing methods produce unevenstress hot-spots in machined parts. This occursbecause of the rapid rise and fall of temperature onmetal surfaces at the tool or wheel point of contact.TAM not only produces beneficial compressivestresses, but also in many cases, where all surfacesand features are effected identically and simultane-ously, can promote a stress equilibrium or uniformi-ty throughout the entire part. Thus, TAM could belooked at as a corrective after process for criticalparts that suffer from these machining-related sur-face integrity issues.

The synergy involved in developing these kinds ofeffects can add a potential value to service life, per-formance, and functionality of parts that far exceedsthe value of the improvements to fit, function, andaesthetics commonly associated with other mechan-ical or mass finishing processes. Unlike single-point-of-contact machining technologies, the technology isrelatively simple to control once process parametersfor a given part have been developed and, thus, hasthe attributes of reliability and repeatability of sim-pler mechanical (vs. digital feedback) technologies.However, it accomplishes uniform results on verycomplex parts that often cannot be achieved reliablyby other much more complex processes.

The technology involves developing a fluidized bedof media in which the part to be processed is par-tially immersed while being rotated. A wide varietyof differing results may be achieved by varying theprocess parameters (media, process time, rotationalspeed, etc.). Process results can be closely controlledand are programmable, and are totally repeatable,providing unequaled process quality control. Theprocess is dry, and involves no chemicals or environ-mentally unfriendly materials.

VIBRATORY FINISHING LARGE AEROSPACECOMPONENTSVibratory equipment can be designed to accommodate

Figure 10: Metallic media, such as the steel media shapesillustrated here, have long been known to develop com-pressive stress in barrel and vibratory finishing opera-tions while burnishing, and cleaning part surfaces. Thiscapability is now being used in larger sized equipment tostrengthen large airframe components. Photo courtesyof Abbott Ball Co.

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aircraft components of extraordinary size. Large com-ponents, such as aircraft engine cases and airframes,can be finished with this method, not only cutting theextensive costs related to manual deburring butimproving the uniformity and quality of edge and sur-face finish quality. Additionally, these processes candevelop not only useful compressive stress but pro-vide something very much like a stress equilibriumenhancement throughout the part, as all part featurescan be processed identically. Modified methods origi-nally developed in the former Soviet Union withmetallic media can also be used to intensify thiseffect, and has even been used to restore useful serv-ice life to stressed or strained parts in overhaul cycles.

SUMMARYMany parts that are subject to fatigue, fracture, orwear can gain substantial improvements in life andperformance from alterations to their overall sur-face texture. Improvements in overall smoothness,load bearing ratio, surface profile skewness andisotropicity can, in many instances, improve life andperformance and cut operational costs.

REFERENCESGillespie, LaRoux, “Mass Finishing Handbook,” Society of

Manufacturing Engineers, (New York, IndustrialPress) p. 61; 2007.

Gillespie, LaRoux, “Compiled Problems Caused by Burrsand Sharp Edges,” (Spokane, Wash: Society ofManufacturing Engineers, Deburring, Edge-Finishand Surface Conditioning Technical Group, Spokane),Newsletter, Vol 2, No. I, January 8, 2006, [Davidson,D.A., ed.]; 2006.

Davidson, D.A., “Mass Finishing Processes,” 2005 Metal

Finishing Guidebook and Directory, 103(6A):78–89; 2005.Massarsky, M. L., Davidson, D. A., “Turbo-Abrasive

Machining and Turbo-Polishing in the ContinuousFlow Manufacturing Environment”, SME TechnicalPaper MR99-264, Conference Proceeding: 3rdInternational Machining and Grinding Conference,Cincinnati, Oct 4–7, 1999, Dearborn, Mich.: Society ofManufacturing Engineers, 1999.

Gane, David H., Rumyantsev, H.T., Diep, Bakow, L.“Evaluation of Vibrostrengthening for FatigueEnhancement of Titanium Structural Components onCommercial Aircraft.” Ti-2003 Science andTechnology; Proceedings of the 10th World Conferenceon Titanium, Hamburg Germany, 13–18 July 2003,Edited by G. Lutejering and J Albrecht. Wiley-VCH,Vol. 2. pp 1053–1058.

Massarsky, M. L., Davidson, D. A., “Turbo-AbrasiveMachining,” CODEF PROCEEDINGS, 7th InternationalDeburring Conference, Berkeley, Calif.: CODEF[Consortium on Deburring and Edge Finishing],University of California at Berkeley; June 2004.

ACKNOWLEDGEMENTSThe author wishes to acknowledge the technicalassistance of the following members of the newlyformed Society of Manufacturing Engineers DESCTechnical Group [Deburring, Edge-Finish, SurfaceConditioning]. Dr. Michael Massarsky, Turbo-FinishCorporation; David H. Gane, Boeing; Edward F.Rossman Ph. D., Boeing; Jack Clark, ZYGOCorporation; LaRoux Gillespie, PE, CmfgE, RodneyGrover, Society of Manufacturing Engineers.

For more information, please contact the author at(e-mail) ddavidson@mgnh.dyndns.org. mf

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