MECHANICAL FINISHING ¥~r'Microfinishing and Surface Texturesby David A. DavidsonPEGCO Process Laboratories, Bartlett, N.H., E-mail.pegco@worldpath.net

The role of mass finishing processes as amethod for removal of burrs, developing edgecontour, and smoothing and polishing parts

has been well established and documented for manyyears. These processes have been used in a widevariety of part applications to promote safer parthandling (by attenuation of sharp part edges);improve the fit and function of parts when assem-bled; and produce smooth, even microfinished sur-faces to meet either functional or aesthetic criteriaor specifications. Processes for developing specificedge and/or surface profile conditions on parts inbulk are used in industries as diverse as the jewel-ry, dental, and medical implant industries on upthrough the automotive and aerospace industries(see Figs. 1 and 2).

Figure 1. (Before) This micrograph was taken with an electronmiscroscope at 50OX magnification. It shows the surface of araw, unfinished "as cast" turbine blade. The rough initial surfacefinish as measured by profilometer was in the 75-90 Ra (min.)range. As is typical of most cast, ground, turned, milled, EDM,and forged surfaces this surface shows a positive Rsk (Rsk -skewness - the measure of surface symmetry about the meanline of a profilometer graph. Unfinished parts usually display aheavy concentration of surface peaks above this mean line,generally considered to be an undesirable surface finish charac-teristic from a functional viewpoint.)

Less well known and less clearly understood isthe role specialized variants of these types ofprocesses can play in extending the service lifeand performance of critical support componentsor tools in demanding manufacturing or opera-tional applications.

Industry has always been looking to improve sur-

Figure 2. (AfterlThis SEM micrograph (500X magnification) wastaken after processing the same turbine blade in a multistepprocedure utilizing orbital pressure methods with both grindingand polishing free abrasive materials in sequence. The surfaceprofile has been reduced from the original 75-90 Ra (min.) to a5-9 Ra (min.) range. Additionally, there has been a plateauing ofthe surface and the resultant smoother surface manifests a neg-ative skew (Rsk) instead of a positive skew. This type of surfaceis considered to be very "functional" in both the fluid and aero-dynamic sense. The smooth, less turbulent flow created by thistype of surface is preferred in many aerodynamic applications.Another important consideration is that surface and subsurfacefractures seem to have been removed. Observations withbackscatter emission gave no indication of residual fractures.

face condition to enhance part performance, andthis technology has become much better under-stood in recent years. Processes are routinely uti-lized to specifically improve life of parts and toolssubject to failure from fatigue and to improve theirperformance. These improvements are mainlyachieved by enhancing part surface texture in anumber of different and sometimes complementaryways.

To understand how microsurface topographyimprovement can impact part performance, someunderstanding ofhow part surfaces developed fromcommon machining, grinding, and other methodscan negatively influence part function over time. Anumber of factors are involved.

POSITIVE VE SUS NEGATIVE SURFACE SKEWNESSThe skew of surface profile symmetry can be animportant surface attribute. Surfaces are typicallycharacterized as being either negatively or posi-tively skewed. This surface characteristic isreferred to as Rsk (Rsk - skewness - the measure

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Figure 3.These profilometer graphs illustrate in 2-D the differencebetween an as-cast positively skewed surface and the same typ-ical surface processed with a sequence of high-energy loosemedia operations to produce an improved surface topology thatis potentially very useful for mission critical components thatrequire improved wear, fracture, or fatigue resistance.

of surface symmetry about the mean line of a pro-filometer graph). Unfinished parts usually displaya heavy concentration of surface peaks above thismean line, (a positive skew) (see Fig. 3). It isaxiomatic that almost all surfaces produced bycommon machining and fabrication methods arepositively skewed. These positively skewed sur-faces have an undesirable effect on the bearingratio of surfaces, negatively impacting the per-formance of parts involved in applications wherethere is substantial surface-to-surface contact.Specialized high-energy finishing procedures cantruncate these surface profile peaks and achievenegatively skewed surfaces that are plateaued,presenting a much higher surface bearing contactarea. Anecdotal evidence confirms that surface fin-ishing procedures tailored to develop specific sur-face conditions with this in mind can have a dra-matic impact on part life. In one example the lifeof tooling used in aluminum can stamping opera-tions was extended 1,000% or more by improvedsurface textures produced by mechanical surfacetreatment.

DIRECTIONALIZED VERSUS RANDOM (ISOTROPIC)SURFACE TEXTURE PATTERNSSomewhat related to surface texture skewness inimportance is the directional nature of surface tex-tures developed by typical machining and grindingmethods. These machined surfaces are character-ized by tool marks or grinding patterns that arealigned and directional in nature. It has been estab-lished that tool or part life and performance can besubstantially enhanced if these types of surface tex-

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Figure 4. 3-~ microsurface topographical maps are coming intoincreasing use to better quantify surfaces as they relate to partservice life and performance. This surface is one that has beenprocessed to blend-in parallel rows of surface peaks left behindfrom fine grinding operations. The resultant surface is one thatis more isotropic or random in nature. This type of surface canbe an important surface attribute to parts that are subjected torepeated stress or strain and parts that undergo high force load-ing of opposing surfaces.

tures can be altered into one that is more random innature. Postmachining processes that utilize free orloose abrasive materials in a high-energy contextcan alter the machined surface texture substantial-ly,not only reducing surface peaks, but generating asurface in which the positioning of the peaks hasbeen altered appreciably (see Fig. 4).These "isotrop-ic" surface effects have been demonstrated toimprove part wear and fracture resistance, bearingratio, and improve fatigue resistance.

RESIDUAL TENSILE STRESS VS. RESIDUALCOMPRESSIVE STRESSMany machining and grinding processes tend todevelop residual tensile stresses in the surface areaof parts. These residual tensile stresses make partssusceptible to premature fracture and failure whenrepeatedly stressed. Certain high-energy mass fin-ishing processes can be implemented to modify thissurface stress condition and replace it with uniformresidual compressive stresses.

NEW PROCESSESPEGCO Super-Hone is a group of proprietaryprocesses that improves edge condition and surfacephysical properties on tooling. The processes involve

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Figure 5. The need for some edge preparation on cutting toolsin many applications has been well documented. Most of theprocesses currently utilized (including manual ones) concen-trate on modifying the edge to produce an edge land tostrengthen the edge. Although these types of processesimprove cutting edge geometries they do not typically addresssurface profile topography issues at the cutting edge such asprofile skewness, isotropiciy, load bearing ratio, and residualstress correction. Specialized high-energy methods can and do.

complete envelopment and processing of all edgeand surface areas of the tool and are nonselective innature. These processes develop surface compres-sion and stress relief on all surfaces, as well as uni-formly microhoning and polishing all edge areas,regardless of tool or edge geometry (see Fig. 5).

Although the processes promote changes that aresignificant and crucial to improved tool perform-ance, the changes in the physical cutting edge are sosubtle that dimensional integrity of the tool is main-tained. These alterations of the cutting edge surfacearea and supporting structures contribute to animprovement in mechanical strength and can avertpremature wear or fracture of the tool.

The processes also have a tendency to improveoverall symmetry of the cutting features. This isespecially contrasted with manual honing methodsin which abrasive filament wheels, abrasive sticks,or even coated abrasive paper are utilized to pre-condition edges by hand methods. Surface profileimprovement is isotropic in nature, minimizing thefatigue and fracture problems associated with uni-directional surface patterns in surfaces created byproduction grinding methods. Smoother, moreisotropic surfaces provide greater lubricity for

improved chip flow, often facilitating higher feedand speed rates with processed tools.

This edge conditioning procedure is an outgrowthof research to improve the tribological functionalityof wear surfaces, with a view to extending usefulservice life and preventing premature failure of crit-ical components that are highly stressed orstrained. It has been found that surface treatmentprocesses that can develop some measure of com-pressive stress and an isotropic microfinish to edgesand surfaces can improve load bearing ratios andother wear-resistant factors far more than improve-ment in one of these areas alone.

When utilized to develop tool surface and edgeimprovements these processes can dramaticallyimprove tool performance and maximize productivi-ty in a number of areas: greater tool life and dura-bility, increased machine feed rates, increased toolspeed rates, improved surface finish on parts,reduced tool change-over and downtime cycles, andimproved tool performance meeting high tolerancerequirements.

SUMMARYMany parts or tools that are subject to fatigue,fracture, or wear can gain substantial improve-ments in life and performance from alterations totheir overall surface texture. Improvements inoverall smoothness, load bearing ratio, surface pro-file skewness, and isotropicity can in manyinstances improve life and performance and cutoperational costs dramatically. Manufacturers thathave not subjected their parts to an analysis todetermine the potential benefits ofthis kind of pro-cessing may be making parts "that are not all thatthey can be."

BIBLIOGRAPHYDavidson, D.A., Metal Finishing 2002 Guidebook and

Directory, 100(lA):104-117; 2002Massarsky, M.L. and D.A. Davidson, Abrasives, OctINov;

1999Davidson, D.A., "High Energy Dry Process Polishing,"

SME Technical Paper MR90-389, Dearborn, Mich.,Society of Manufacturing Engineers; 1990

Massarsky, M.L. and D.A. Davidson, "Turbo-AbrasiveMachining Theory and Application," SME TechnicalPaper MR95-271, Dearborn, Mich., Society ofManufacturing Engineers; 1995

"Surface Metrology Guide," http://www.predev.com/smg.Milan Mich., Precision Devices Inc.; 2001

"PEGCO Finishing Links," http://www.nvo.comlkpegco/use-fullinks, Bartlett, N.H., PEGCO Process Laboratories;2001

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