by Dave Davidson, SME Manufacturing, Machining/Material Removal Tech Community, firstname.lastname@example.org
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Mass finishing processes have been widely adopted throughout industry as the optimum methodology for producing advanced edge and surface finish effects on many types of machined and fabricated components. America industry has long been in the forefront in aggressively deploying these methods to improve edge and surface finishing operations. In his Deburring and Edge Finishing Handbook, (1999 edition) Laroux Gillespie developed a comparative table which pointed out that in some mechanical finishing equipment 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. More than that, in some cases, American industry had installed as many finishing machine installations as the rest of the world’s manufacturing community — put together. 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 applications are not going to require a manual deburring approach – some do. In many cases, however, hand or manual methods are still being utilized because more automated or mechanized methods have not been considered or adequately investigated. Commenting on an often observed dichotomy in precision manufacturing operations, Rodney Grover of the Society of Manufacturing Engineers in essay entitled “Boeing Issues an Invitation” referenced a situation that is still all too common. That is that many manufacturers, after spending vast sums on CNC machining equipment to produce parts to very precise tolerances and specifications consistently, in the 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 simply cannot be duplicated with manual or single-point-of-contact methods. Developing an overall edge and surface finish continuity and equilibrium can have a significant effect on performance and service life of critical components.
In the past, mass finishing methods have been thought to be limited to uniformly processing large numbers of small to moderately sized components to precise edge and surface finish specifications. Increasingly, this type of processing is being investigated by manufacturers of large and very large components to drive down the high costs associated with utilizing hand tools or hand-held power tools to abrasively modify part edges and surfaces. Machinery capable of processing very large components is now being 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 processing chamber to amplify processing effects on specified areas or limit edge damage on extremely 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 been edge-contoured and surface conditioned with spindle-fixtured processes such as the Turbo-Finish method.
Mass media finishing processes have gained widespread acceptance in many industries primarily as a technology for reducing the costs of producing edge and surface finishes. This is particularly true when manual deburring and finishing procedures can be minimized or eliminated. Many manufacturers have discovered that as mass finishing processes have been adopted, put into service, and the parts involved have developed a working track record, an unanticipated development has taken place. Their parts are better—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 failure, exhibit better tribological properties (translation: 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. These methods produce isotropic surfaces with negative or neutral surface profile skews. Additionally, they consistently develop beneficial compressive stress equilibriums. These alterations in surface characteristics often improve part performance, service life and functionality in ways not clearly understood when the processes were adopted. In many applications, 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 fixtured-part vibratory deburring and vibratory burnishing (referred to by them as “Vibro-peening” or “Vibro-strengthening) processes to produce (1) sophisticated edge and surface finish values and (2) beneficial compressive stress to enhance metal fatigue resistance. In life cycle fatigue testing on titanium test coupons it was determined that the vibro-deburring/burnishing method produced metal fatigue resistance that was comparable to high intensity peening that measured 17A with Almen strip measurements. The striking difference between the two methods however, is that the vibratory burnishing method produced the effect while retaining an overall surface roughness average of 1 μm (Ra), while surface finish values on the test coupon that had been processed with the 17A high intensity peening had climbed to values between 5-7 μm (Ra). The interesting conclusion the authors reached in the study was that the practicality and economic feasibility of the vibro-deburring and burnishing method increased with part size and complexity
ABOVE: Isotropic Micro-Finishing Photography by Mark Riley, BV Products
Isotropic Finishing and Micro-Finishing with Centrifugal Finishing Equipment