Surface Finishing Reaches New Heights Mass media finishing techniques can improve aircraft part performance and service fife. By D a v i d A. D a v i d s o n , D e b u r r i n g / S u r f a c e F i n i s h Spec ia l i s t , D e b u r r i n g , E d g e F i n i s h i n g S u r f a c e C o n d i t i o n i n g G r o u p

M ass finishing processes have long been wide- ly adopted throughout industry as a pre- ferred method for producing advanced edge

and surface finish effects on many types of machined and fabricated components. American industry is at the forefront for aggressively deploy- ing these methods to improve their edge and surface finishing operations.

In his "Deburring and Edge Finishing Hand-book" (1999 edition)," Laroux Gillespie developed a com- parat ive table which pointed out that in some mechanical finishing equip- men t categories, such as rotary barrels, vibratory finishing, and centrifugal barrel finishing equipment, American industry leads the world in terms of the number of equipment installations. Despite this, all too often situations still exist where archaic, even primitive hand or manual finishing methods are used to produce edge and surface finishing effects.

This is not to say that some industrial part appli- cations are not going to require a manual deburring approach--some do. In many cases, however, hand or manual methods are still being utilized because auto- mated or mechanized methods have not been consid- ered or adequately investigated.

Commenting on an often observed dichotomy in precision manufacturing operations in an essay titled "Boeing Issues an Invitation," Rodney Grover of the Society of Manufacturing Engineers (SME) refer- enced a situation that is still all too common. Many manufacturers, after spending vast sums on CNC machining equipment to produce parts to very pre- cise tolerances and specifications, consistently, in the

This large aluminum component was previously deburred with hand tools. Implementing a vibratory finishing process with a tub-shaped chamber reduced processing time from hours to minutes and reduced direct manual deburring labor to nil. More importantly, surface finish and edge contour effects have been produced on all criti- cal areas of the part, with a part and feature consistency and uniformity not possible with manually directed or sin- gle point-of-contact abrasive methods. Photo courtesy of Robert M. Kramer, Kramer Industries.

end, hand off these expensive parts to a deburring and finishing department that utilizes hand methods, with all the inconsistency, non-uniformity, rework and worker injury potential that implies. Even when manual methods cannot be completely eliminated, mass media finish techniques can and should be used to produce an edge and surface finish continuity that cannot be duplicated with manual or single-point-of- contact methods.

Developing an overall edge and surface finish con- tinuity and equilibrium can have a significant effect

on the performance and service life of critical compo- nents as well.

In the past, mass finishing methods have been thought to be limited to uniformly pro- cessing large numbers of small to moderately sized components to precise edge and surface finish specifica- tions. Increasingly, this type of processing is being investigat- ed by manufacturers of large and very large components to drive down t h e high costs associated with utilizing hand tools or hand-held power tools to abrasively modify part edges and surfaces.

Machinery capable of pro- cessing very large compo- nents can now be built. Equipment with chamber

capacities as large as 200 cubic feet have been designed to accommodate individual parts.

In some cases, the parts are fixtured within the pro- cessing chamber to amplify processing effects on specified areas, or prevent edge damage on extreme- ly heavy parts. In other cases or circumstances, parts are suspended in the media mass for more equalized surfacing and stress equilibrium effects.

Complex rotating parts, such as power generation turbine disks as large as four feet in diameter, have

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This large aircraft engine tur- bine disk has been processed with the Turbo-Finish method. This dry abrasive finishing method has been successful in bringing mass media finish economies to large, complex, rotationally oriented parts. In addition to the uniform and consistent edge contours developed, the method also produces highly sophisticated isotropic surface finishes by radically altering the character of the as-machined or as- ground surface finish. Photo courtesy of Michael L. Massarsky, Turbo-Finish Corp.

These titanium test coupons show a before and after exam- ple of mass finishing process- es being used to blend in milling cutter paths. Transforming the positively skewed surface profiles of machined parts into parts with isotropic and negatively skewed surface characteristics can be an important element in any program where surface improvements are being developed to improve wear resistance and metal fatigue resistance on critical parts.

been edge-contoured and surface condi- t ioned with spindle- fixtured processes such as the Turbo- Finish method.

Mass media finish- ing processes have gained widespread acceptance in many industr ies primarily as a technology for reducing the costs of producing edge and surface finishes. This is particularly t rue when manual debur- ring and finishing pro- cedures can be mini- mized or eliminated.

Many manufactur- ers have discovered that as mass finishing processes have been adopted, put into serv- ice, and the parts involved have devel- oped a working track record, an unantici- pated development has taken place.

Their parts are bet- te r - -and not just in the sense that they no longer have burrs, sharp edges, or that they have smoother surfaces. Depending on the application, they last longer in service, are less prone to metal fatigue fail- ure, exhibit better tri-

bological properties (less friction and better wear resistance), and from a quality assurance perspective are much more predictably consistent and uniform.

The question that comes up is, "Why do commonly used mass media finishing techniques produce this effect?" There are several reasons. The methods typical- ly are nonselective in nature. Edge and surface features of the part are processed identically and simultaneous- ly. These methods also produce isotropic surfaces with negative or neutral surface profile skews.

Additionally, they consistently develop beneficial

This shafted gear utilized in heli- copter turbine applications has been processed in centrifugal bar- rel finishing equipment to pro- duce specific isotopric finishes with high-load-bearing ratios to improve gear tooth life and over- all performance efficiency.

compressive stress equilibriums. These alterations in surface characteristics often improve part perform- ance, service life, and functionality in ways not clearly understood when the processes were adopted.

In many applica- tions, the uniformity and equilibrium of the edge and surface effects obtained have produced quality and

performance advantages for critical parts that can far outweigh the substantial cost-reduction benefits that were the driving force behind the initial process implementation. This assertion has been affirmed by both practical production experience and validation by experiment in laboratory settings.

David Gane and his colleagues at Boeing have been studying the effects of using a combination of fix- tured-part vibratory deburring and vibratory bur- nishing (referred to by them as "vibro-peening" or "vibro-strengthening") processes to produce sophisti- cated edge and surface finish values and beneficial compressive stress to enhance metal fatigue resist- ance. In life-cycle fatigue testing on t i tanium test coupons it was determined that the vibro-debur- r ing/burnishing method produced metal fatigue resistance that was comparable to high intensity peening that measured 17A with Almen strip meas- urements. The striking difference between the two methods, however, is that the vibratory burnishing

Centrifugal barrel machines such as these can produce excep- tional edge and surface finishes in very short cycle times. Accelerated process effects can be developed because of the high-speed interaction between abrasive media and part sur- faces, and because media interaction with parts is characterized by high pressure by virtue of the high centrifugal forces devel- oped in the processes.

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This large power generation tur- bine blade was made utilizing 6- axis machining technology. Centrifugal barrel finishing tech- nology was used to clear and blend in the milling cutter paths and then develop very refined and burnished isotropic surfaces in the foil area.

method produced the effect while retaining an overall surface roughness average of 1 pm (Ra), while surface finish values on the test coupon that had been processed with the 17A high-intensity peening had climbed to values between 5 to 7 ~m (Ra). The conclu- sion the authors reached in the study was that the practical- ity and economic feasi-

bility of the vibro-deburring and burnishing method increased with part size and complexity.

Michael Massarsky of the Turbo-Finish Corp. was able to supply comparative measurements on parts processed by his method for edge and surface finish improvement. Utilizing this spindle oriented deburr and finish method, it is possible to produce compres- sive stresses in the MPa = 300 to 600 range that formed to a surface layer of metal to a depth of 20 to 40 ~m.

Spin pit tests on turbine disk components processed with the method showed an improved

The mass finishing method is usually thought of in terms of facil- itating the surface finishing of large numbers of smaller parts. As can be seen from this photo, very large structural components such as this titanium airframe bulkhead can also be processed. When coupled with both fixtured and sequential finish tech- niques, these kinds of processes can not only be used to replace costly manual deburr operations, but also produce significant compressive stress and work-hardening effects that can dramati- cally increase metal fatigure resistance properties. Studies have shown that as part size grows, the more economical and practical vibratory deburring and vibratory peening/burnishing processes become as potential replacements for hard deburring and con- ventional shot peening process combinations. Photo courtesy of Giant Finishing Inc.

cycle life of 13,090 +450 cycles when compared to the test results for conventionally hand- deburred disks of 5,685 _+335 cycles, a potential service life increase of 2 to 2.25 times, while reducing the dis- persion range of cycles at which actual failure occurred. Vibratory tests on steel test coupons were also performed to determine improvements in metal fatigue resistance. The plate specimens were tested with vibratory amplitude of 0.52 mm and load stress of 90 MPa. The destruction of specimens that had surface finishes developed by the Turbo-Finish method took place after (3-3.75)'104 cycles, a signif- icant improvement over tests performed on conven- tional ground plates that started to fail after (1.1-1.5)'104 cycles (see Figure 2).

In his handbook, Gillespie makes a very astute observation. "Typical burrs are not the result of poor planning or poor engineering. They are a natural result of machining and blanking processes. Large burrs, however, may be the result of poor planning." A similar axiom could be said to exist regarding sur- face finishes. "Rough, non-isotropic surface finishes with undesirable stress conditions are not the result of poor planning or poor engineering. They are a nat- ural result of almost all common machining, grind- ing, fabrication and abrasive methods. These results can be exacerbated by abusive machining and grind- ing, and improved or reversed with mass media fin- ishing techniques."

Mass media finishing techniques improve part performance and service life, and these processes can be tailored or modified to amplify this effect (see Figure 3). Although the ability of these processes to drive down deburring and surface finishing costs when compared to manual procedures is well known and documented, their ability to dramatically affect part performance and service life are not. This facet of edge and surface finishing deserves closer scruti- ny. This is also true with larger and more complex parts--only more so.

REFERENCES 1.Gane, David H., et al., "Evaluation of Vibro strengthen-

ing for FatigueEnhancement of Titanium StructuralComponents on Commercial Aircraft," Ti- 2003 Science and Technology; Proceedings of the lOth World Conference on Titanium, Hamburg, Germany, edited by G. Lutejering and J Albrecht.Wiley-Vch, Vol 2., pp 1053-1058; July 13-18, 2003.

2.Massarsky, M.L., and D.A. Davidson, "Turbo-Abrasive Machining," CODEF Proceedings, 7th International Deburring Conference, Berkeley, Calif.: CODEF [Consortium on Deburring and Edge Finishing], University of California at Berkeley; June 2004.

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This graph was developed from the results of spin pit testing performed on turbine disks to determine the effects of edge/surface finishing methods on metal fatigue resistance. Significantly improved results in terms of service life were obtained by theTAM finish method.TAM produced a feature-to- feature stress equilibrium that could not be matched by meth- ods that involved manual methods. Similar effects have also been noted when other mass media finish methods replace manual deburring.

3.Massarsky, M.L., and D.A. Davidson, "Turbo-Abrasive Machining--A New Technology for Metal and Non- Metal Part Finishing," The Finishing Line, Vol. 18. No. 4, Dearborn Mich. :Association of Finishing Processes, Society of Manufacturing Engineers; Oct. 30, 2002.

Shown here is an automotive engine blockready for mass media finishing. High performance automotive engine applications are one of the most rapidly expanding areas for mass media finish- ing technology, Many involved in competitive automotive racing have adopted mass media finishing techniques as a way of reducing friction, improving wear resistance and increasing horsepower. The techniques used to produce these effects take mass finishing methods to the next level, in terms of producing specialized surfaces that will materially affect the performance of the component. Much of the information available about these effects is highly anecdotal in nature. However, the fact that these surface finish methods are being so widely adopted in such a competitive and results-oriented environment may encourage other manufacturers to explore what surface finish improvement can do for their critical part performance.

4. Massarsky, M.L., and D.A. Davidson, "Turbo-Abrasive Machining and Turbo-Polishing in the Continuous Flow Manufacturing Environment," SME Technical Paper MR99-264, Conference Proceedings: 3rd International Machining and Grinding Conference, Cincinnati, Oct 4-7, 1999, Dearborn, Mich.: Society of Manufacturing Engineers; 1999.

5. Gillespie, LaRoux, Deburring and Edge Finishing Handbook, Dearborn, Mich., Society of Manufacturing Engineers, 1999.

6. Davidson, D.A., "Mass Finishing Processes," Metal Finishing Guidebook Directory, 100(1A):104-117; 2002.

7. Davidson, D.A., "Micro-Finishing and Surface Textures," Metal Finishing, 100(7):10-12; July, 2002.

8. Massarsky, M.L. and D. A. Davidson, "Turbo-Abrasive Machining and Turbo-Polishing in the Continuous Flow Manufacturing Environment," SME Technical Paper MR99-264, Conference Proceedings: 3rd International Machining and Grinding" Conference, Cincinnati; Oct 4-7, 1999, Dearborn, Mich.: Society of Manufacturing Engineers; 1999.

9. Rossman, Edward F., [Boeing], "Collected Thoughts On High Speed Machining of Titanium," SME Technical Paper, Dearborn, Mich.: Society of Manufacturing Engineers; 2004.

10. Grover, Rodney, "Boeing Issues an Invitation," Dearborn, Mich.: Society of Manufacturing Engineers; 2004; http://www.sme.org.

A C K N O W L E D G E M E N T S The author wishes to acknowledge the technical assistance of the following members of the newly formed Society of Manufacturing Engineers DESC Technical Group [Deburring, Edge-Finish, Surface Conditioning]: Michael Massarsky, Turbo-Finish Corp.; David H. Gane, Boeing; Edward F. Rossman, Ph. D., Boeing; Jack Clark, ZYGO Corp.; LaRoux Gillespie, PE, CmfgE, Honeywell; and Rodney Grover, Society of Manufacturing Engineers. Many of these colleagues will be present at a technical session concerning deburring and surface finishing methods for aircraft frame components sponsored by the Society of Manufactur ing Engineers at WESTEC on April 6 in Los Angeles.

Dave Davidson is a deburring and surface finishing specialist, consultant, and the 2005 chair for the Deburring, Edge Finishing Surface Conditioning (DESC) Group. He can be reached (e-mail) ddavid- son@deburringsolutions.com FITf

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