Its the finish that counts

MECHANICAL FINISHING

‘It’s the Finish that Counts’Edge and surface finish can be important factors in drivingpart performance of metal parts.By Jack Clark, Applications Engineering Manager, Zygo Corp., Middlefield, Conn.; MichaelMassarsky, President, Turbo-Finish Corp., Barre, Mass.; and David A. Davidson, Society ofManufacturing Engineers, 2006 Chair: Deburring Technical Committee, Spokane, Wash.

Prior to the early 1970s, there was little to noawareness of the relationship between part func-tion and the process that created a piece part.

Many areas of automotive power train manufacturingwas starting to realize that component failure wascausing increases in scrap and warranty costs eventhough the used metallurgy, fit, and form were speci-fied and controlled properly. There was no unifiedunderstanding of the root cause of premature wear,leakage, noise, unusually high operating temperatures,and catastrophic failure, or even if they were related.

DEFINITIONSA conventionally produced surface (turned, milled,ground, EDM) is typically Gaussian in nature, that is,the peak and valley distribution is pretty much equalin height. This type of surface can be very unstable andunpredictable when wear and load bearing are consid-ered. The images in Figure 1 demonstrate this type ofsurface.

There are many ways to produce plateaued surfaces.They are varied in approach but all have the ability tocontrol the surface peak characteristics separately forthe valley characteristics. This is summarized in Figure2, showing that the peak and valley distributions can be

controlled to allow substantial bearing load capabilities(broad, flat areas) with well-defined lubrication, debriscollecting valleys.

ENGINE CYLINDER BORESHoning cylinder bores produce an evenly distributed(Gaussian) surface of peaks and valleys. Lubrication(lube) retention is dependent on the valley portion of thesurface, allowing even distribution over the entire sweptarea of the piston and rings. Contrary to that, the peakportion of the surface was not supporting the needs ofother bore requirements, such as reducing friction, seal-ing to the piston ring, and reduction of wear. As extend-ed warrantee life (now over 100,000 miles in diesel appli-cations), emission requirements, and higher combustiontemperatures evolved, the need for more control of thesurfaces became evident.

Plateau honing was first used as a final cylinder borefinishing technique in the 1970s. Later (1980s), the dieselindustry was instrumental in incorporating this tech-nique to extend engine life and improve the sealing of thepiston ring to the bore. Today, the gasoline engine manu-facturers are finding that they must migrate to these boreconditioning practices to meet emission requirementsand, again, extend predictable engine life.

24 www.metalfinishing.com

Figure 1: Typical positively skewed surface common to mostgrinding and machining operations. (Courtesy of ZYGO Corp.)

Figure 2: Plateaued surface as typical of surface conditioning pro-duced by a variety of mass finishing methods.

The solution has been in respecifying bore materialsand implementing new honing techniques. The oldstandard of cast iron was a material that did not sup-port finely controlled machining processes.Consistently, there were issues with tears and folds inthe surfaces, poor plateau, and frictional characteris-tics. Manufacturers have migrated to various ferrousand non-ferrous materials that better support tem-perature variations, high unit surface loading, fric-tional requirements, and dimensional distortions typ-ical of today’s advanced engine designs.

The constant that must be maintained to ensurepart function of the sealing system is the specificationand consistency of the surfaces involved. There arevery specialized ISO standard parameters that weredesigned to describe plateaued finished surfaces.These are the bearing ratio parameters (Rpk, Rk, andRvk) and, more recently, the probability parameters(Rpq, Rmq, and Rvq). They monitor the peak and val-ley regimes of the surface, independently allowing theproduction team to know that the process and, there-fore, the piece part, will function.

GEAR CONTACT SURFACESGears of all types must be designed to maintain theirintegrity under very high local surface loads (seeFigure 3). The industry has always battled what mate-rial, what form, what surface will survive the varyingdesigns and environments. Again, the surface mustdistribute the lube well and, at the same time, have thecapability to withstand very high unit area load.

Generally, the plateauing process will produce a surfacefree of local anomalies that can increase the local unitload and initiate a failure site. The mechanisms for sur-face failure are many and, sometimes, are difficult toidentify. It has been found that when attention is paid tothe integrity of the contacting gear surfaces by reportingthe correct parameters during production, gear life isextended, noise and operating temperatures are reduced,and wear is minimized.

THE IMPORTANCE OF SURFACE FINISHDESIGN FOR FUNCTIONThe mechanism that allows for improved “functionali-ty” for all surfaces is basic to surface performance —toaccept the loads imposed and resist wear. Traditionalprocesses that generate form and control fit do not nec-essarily dictate whether that part or assembly willfunction. Over the last three decades, it has been real-ized that there is another contributor to part perform-ance—surface finish.

If a manufacturer does not account for surface finishcharacteristics like lube retention, micro burr removal,identification of torn and folded material, directionality,

and load-bearing capability, the performance of compo-nents in the system cannot be predicted.

Through progressive process development, evolvingmeasures (new ISO standard parameters), and amature understanding of the “function” of surfaces,manufacturers can design parts that become assem-blies that are in systems performing over predictable,extended lives. This is the key to reduced warrantycosts, reduced scrap, lower production costs, and satis-fied customers.

Case study—Mechanical surface finish methodoriginally justified for cost and direct laborreduction produces important performancerelated edge/surface effects.

Turbo-abrasive machining [TAM] is an emerging edgeand surface finish technology for processing gas turbinerotating hardware. This method is used primarily fordeburring and edge contour development on featureedges, especially those located on the peripheral or cir-cumferential area of turbine and compressor discs. Theprocess is capable of producing consistent and uniformedge and surface effects on these critical areas.

Currently, much of this work is performed manually orwith single point-of-contact methodologies with relative-ly high reject/rework rates. Although much of the eco-nomic justification for adopting TAM technologies is cen-tered around the substantial automation of direct laborand the attendant cost reductions, an even more impor-tant consideration is the potential for improvements inpart performance and functionality. The process has anumber of characteristics that are potentially importantfor long-term service utility of components in opera-tion. Among these are stress equilibrium—as all edgefeatures are processed identically and simultaneously,


May 2006 25

Figure 3: Very sophisticated surfaces are being developed toimprove contact fatigue in gears.Vibratory, centrifugal and spe-cialized spindle systems can be used to improve bearing loadand fatigue life properties. The gear shown above wasprocessed in a multi-sequence centrifugal operation to bringcontact surfaces of the gear below 10Ra. (Courtesy of debur-ringsolutions.com.)



stresses on critical edge areas offeatures, such as broach slot, havea uniformity not possible toachieve with either manual or sin-gle point-of-contact processing;and isotropic surface generation—parts are also improved bychanges developed on the overallsurface of parts.

Unlike conventional machiningor grinding methods, TAM proce-dures develop non-linear surfacepatterns that are far less subject

to potential surface crack initia-tion and propagation and have theadded advantage of promoting sur-faces with higher levels of contactrigidity for seal joint contactenhancement.

Additionally, the procedurechanges the nature of surfaceskews. Traditional or conventionalmachined surfaces are character-ized by positively skewed profiles inwhich surface peaks are the pre-dominant surface feature.

This is a surface characteristicconsequence common to most allstandard machining and grindingmethods. Abusive machining andgrinding exacerbates this natural,but undesirable, surface character-istic. In contrast, TAM surfaces arecharacterized by negative or neu-tral skews in which the surface tex-ture pattern is far more randomand multi-directional than thoseproduced by machine cutting toolsor grinding wheels.

This more homogenous surfacepattern has an additional function-al advantage in that load bearingand wear resistance qualities ofsurfaces are improved as a directresult.

TAM is a “cool” process that ismore non-selective than tradition-al processing, there is little tem-perature phase shift involved asedges and surfaces are altered.Two basic mechanisms areinvolved in this machiningprocess: Envelopment of rotationalparts within a fluidized bed of rel-atively small abrasive granularmedia, fixtured parts are rotatedat sufficient speed to develop effec-tive intensity interaction betweenpart surfaces and edges, and theabrasive particles suspended with-in the fludized bed area.

Case studies: Edge and sur-face finishing for performanceand fatigue failure resistance:The technical literature is repletewith examples of case studies indi-cating substantial fatigue resist-ance can be developed with massfinishing methods such as barrelfinishing.

One study published by Iron Agemagazine back in 1959 documentedstudies on the compressive stressesto be developed with various typesof abrasive and non-abrasive mediain tumbling barrels. Some processesare especially well known for thischaracteristic.

Steel media or ball burnishingprocesses using vibratory tech-

Circle 037 on reader information card or go to www.thru.to/webconnect


May 2006 27


niques are known not only for pro-ducing aesthetically pleasing sur-faces, but also enhancing service lifeof components because of the bulk-density of the media itself (300lb./ft3 as compared to 80 to 100lb./cu. ft3 with typical abrasiveceramic media types).

Some promising research hasbeen done by major aerospacecompanies that indicate large airframe components can beprocessed to enhance fatigue life,and that fatigue life of componentsstressed or weakened in servicecan be improved during overhaulcycles as well.

Hignett, in writing a technicalpaper for SME in 1979, spelled outa number of applications wherecentrifugal barrel finishing wasspecified specifically because ofthe part performance attributesthe process was able to develop.Among these were:• Components for roller chain

and silent chain, includingthe slide plates, bushings,rollers and pins. The centrifugalprocess was able to deburr,descale, edge finish, surface fin-ish, and impart compressivestress on the parts simultaneous-ly. As the loose abrasive mediabeing used in these systems areapplied against part surfaces withrelatively high pressure, verysmall media were utilized to com-pletely and uniformly deburr andradius through holes on the sideplates without rolling the burrsover. Other finishing methodscould not accomplish this, and, asa result, sharp edges would beexposed when burrs were “torn”off in assembly, creating a stressraiser that would contribute topremature failure of chains due tofatigue cracks initiating at thesharp edge. Like many other sim-ilar applications, the success ofthis process was dependent on theprocess’s ability to produce “holis-tic” changes in the parts them-

selves on a number of differentlevels. These changes not onlycontributed to the dimensionaluniformity of the parts, permit-ting more automated assembly,but also changed the nature ofpart edge and surface characteris-tics and stress conditions in a waythat not only facilitated extended

service life, but actually made theservice life more predictable.

• Dental partials and ortho-dontic bands, brackets, andwire parts: In recent years,high-pressure centrifugal finish-ing has become a standardmethod for producing needededge and surface finish quality

Railroad Place • PO Box 330100 • West Hartford, CT 06133 • Fax: 860-233-1069

www.abbottball.com

C O M P A N Y

Call: 860-236-5901

Abbott Steel Media improves deburring, deflashing, descaling and burnishing on metal and plastic parts:

Finishes Faster, Lasts Longer. Exceptionally Long Life.

Doesn’t Wear or Lodge. Eliminates costly resizing, removal and replacement.

Runs Clean. Won’t create soils. Works with biodegradable solvents.

Saves Steps.Cleans and finishes at once. Removes die lines.

Send finished and unfinished samples to our lab for a free test.





for these parts, reducing finishing costs by as muchas 70% over manual methods.Given the fact that the end destination for the parts

are inside the end user’s mouth, uniformly consistentedge contour and very refined low micro-inch surfacefinishes without dimensional distortion are required.Like many demanding surface condition treatmentapplications, several different cycles using succes-sively finer abrasive materials may be required toachieve the needed result. As can be imagined, theimproved fatigue and wear resistance properties areproduct attributes that contribute to customer satis-faction considerably.

• Carbide and HSS tooling: A number of differentmass finishing operations are utilized by the cuttingtool industry to develop edge and surface finishes onthe tooling that contribute to improved cutting per-formance and tool longevity. Vibratory equipment,especially equipment equipped with high-frequencyelectro-magnetic drives, is utilized to produce uniformedge-hone or edge preparation on carbide inserts toimprove cutting performance and tool life.High-pressure finishing methods, some of them

employing part fixtures, are also used to producespecialized edge and surface finish effects on bothcarbide and HSS tooling. Among the methodsemployed are centrifugal barrel processing and sev-eral different variants of “drag finishing.” Someprocesses produce edge and surface effects by pres-sure and compression, utilizing relatively high bulk-density non-abrasive loose media in “wet-processes.”Other, more fixture-centric methods apply low bulk-density dry media with micro-fine abrasives against

tooling edges and surfaces at very high rates of pres-sure and flow.Significant improvements in tool performance and

longevity are claimed for both “new” tools and“resharps” that have edge and surface conditions mod-ified with these types of methods. Parts currently beingprocessed include end-mills, drill-bits, spade blades,broaches, hobs, and even circular saw blades for powertools. Several manufacturers claim tool life increases of200 to 300% for tools processed by these methods, inmany cases obviating the need for coatings.

• Reed and flapper valves: These reed componentsare sometimes flexed more than 12,000 cycles perminute in normal service. Centrifugal barrel finish-ing promotes extended fatigue life on these compo-nents by developing both generous edge-contours tolimit stress concentration points, as well as simulta-neously developing low-profile surfaces with highcompressive stress properties.

One case study cited a reduction in the number offatigue failures developing at the predicted half-life ofthe part of over 95% when surface finished with cen-trifugal methods.

• Stainless steel coil spring fatigue resistance: Anextensive comparative testing program was conduct-ed to evaluate high-pressure (centrifugal barrel)methods as a means to improve fatigue life on coilsprings. In one test, the coil springs were tested tofailure by depressing them from 1.104-inch lengths to0.730 inch, developing a stress change from 0 to50,000 psi.

The coil springs processed with conventional surfacefinish and shot peening methods all failed between


May 2006 29


160,000 and 360,000 cycles. Parts processed in a single,high-pressure centrifugal barrel method for 20 minutesfailed between 360,000 and 520,000 cycles, a typical per-formance increase of 60%.All centrifugally finished parts that failed below 400,000cycles did so because of clearly visible surface defects orinclusions. These parts could have been easily removedfrom the production stream by visual inspection becauseof the prominence of the defect, given the refined surfacefinish. Such defects were masked because of the highlytextured nature of the peened surfaces, making visual oroptical elimination near impossible.

Tests performed on another set of production springswere even more striking, with all springs processed instandard or conventional processes, failing before 600,000cycles, and none of the same springs processed in high-pressure centrifugal methods failing before the 800,000cycle limit of the test.

THE IMPORTANCE OF EDGE AND SURFACECONDITION QUALITYDeburring and surface conditioning has often beentreated as the “poor relation” or “step-child” of manu-facturing engineering. Many parts and componentshave been shown to have sub-optimal performancebecause of a lack of edge- and surface-quality-consciousdesign. Edge and surface finish can be an importantfactor in driving ultimate part performance and func-tionality. In what has become a standard reference onthe subject, (“Deburring and Edge FinishingHandbook”) author LaRoux Gillespie enumerated 25different potential problem areas that might resultfrom insufficient care being given to edge and surfacefinishing process selection. They are:• cut hands in assembly or disassembly;• interference fits (from burrs) in assemblies;• jammed mechanisms (from burrs);• scratched or scored mating surfaces (which allow

seals to leak);• friction increases or changes (disallowed in some

assemblies);• increased wear on moving or stressed parts;• electrical short circuits (from loose burrs);• cut wires from sharp edges and sharp burrs;• unacceptable high-voltage breakdown of dielectric;• irregular electrical and magnetic fields (from burrs);• detuning of microwave systems (from burrs);• metal contamination in unique aerospace assem-

blies;• clogged filters and ports (from loose burr accumula-

tion);• cut rubber seals and O-rings;• excessive stress concentrations;• plating buildup at edges;

• paint buildup at edges (from electrostatic spray overburrs);

• paint thinout over sharp edges (from liquid paints);• edge craters, fractures, or crumbling (from initially

irregular edges);• turbulence and nonlaminar flow;• reduced sheet metal formability;• inaccurate dimensional measurements;• microwave heating at edges;• reduced fatigue limits;• reduced volumetric efficiency of air compressors;• reduced cleaning ability in clean room applications;• reduced photoresist adherence at edges;• and to the list we would add less aesthetic appeal.

To summarize, are we certain that we clearly under-stand how edge and surface finish quality might con-tribute to how manufactured parts and componentswill function, perform and last in service? Tools arereadily available. Will we use them? To an increasingdegree, to borrow a well-worn phrase used by one fin-ishing compound manufacturer, “It is the finish thatcounts.”For more information, contact Dave Davidson at (e-mail) [email protected]. mf
