[ABOVE: Isotropic Micro-Finished Part photography by Mark Riley, BV Products]
Dave Davidson is with SME Manufacturing and an advisor to the Machining/Material Removal Technical Community; he can be reached at firstname.lastname@example.org. Jack Clark, also an SME member, is with the Colorado State University’s Department of Mechanical Engineering and with Surface Analytics, LLC; he can be reached at email@example.com.
TAKE-AWAY: Barrel, vibratory, centrifugal and spindle finishing can improve part performance and service life while minimizing hand-deburring and hand-finishing.
Related Topics (Index Terms):
- Mechanical Finishing
- Mass Finishing
- Isotropic Surfaces
- Wear and Fatigue Resistance
When presented with edge and surface finishing problems, many manufacturers continue to reach for solutions that rely on out-of-date, time-consuming and labor-intensive methods. It is still not unusual to see precision parts and critical hardware being manually handled, and edge and surface finishing operations being performed with abrasive hand tools or manually controlled power tools that use coated abrasives or abrasive filaments.
This situation often arises from insufficient planning and a lack of understanding of what will be required to render the manufactured part or component acceptable for the end user. At the root of the problem is a manufacturing and design engineering culture that considers its work done when the part comes off the machining center or the fabricating machine. Too often, part edge and surface condition is simply someone else’s problem.
This is a situation that deserves and is getting an increasing amount of scrutiny. It is a subject repeatedly discussed at the “Deburring, Edge-Finish, and Surface Conditioning Technical Group” sponsored by SME Manufacturing’s Machining/Material Removal Technical Community in Dearborn, Michigan.
The costs of neglecting to consider deburring and surface conditioning in production planning and engineering can be substantial. Frequently overlooked, however, are the potentially serious problems that can develop from the ad hoc and interim solutions that are selected to deal with what now has become—in some instances—a manufacturing crisis.
The manufacturers who resort to hand or manual finishing do not do so because of its cost on a per piece basis (it is by far the most costly method of handling the problem) but, often, it is the most obvious solution and the easiest and the quickest to implement. The reason this problem persists is that there is an imperfect understanding of the serious hidden cost this manual and uncontrolled approach imposes. This is not to say that some deburring and finishing problems don’t require some manual intervention; some do. In many cases, however, manual methods are selected because they are an easy and quick fix.
The first casualty of this manual approach is the investment the manufacturer has made, often in the millions, for precise and computer controlled manufacturing equipment. The idea behind this investment was to have the ability to produce parts that are uniformly and carefully manufactured to exacting specifications and tolerances. At this point, in too many cases, the parts are then handed off to manual deburring and finishing procedures that will guarantee no two parts will ever be alike.
Figure 1 and 2: It is axiomatic that almost all surfaces produced by common machining and fabrication methods are positively skewed. These positively skewed surfaces have an undesirable effect on the bearing load of surfaces, negatively impacting the performance of parts involved in applications where there is substantial surface-to-surface contact. Specialized high energy finishing procedures can truncate these surface profile peaks and achieve negatively skewed surfaces (Fig 2.) that are plateaued, presenting a much higher surface bearing contact area. (Photo Courtesy of Jack Clark, Surface Analytics).
Moreover, the increased complexity and precision requirements of mechanical products have reinforced the need for accurately producing and controlling the edge and surface finish of manufactured parts. Variations in the surface texture can influence a variety of performance characteristics. The surface finish can affect the ability of the part to resist wear and fatigue; to assist or destroy effective lubrication; to increase or decrease friction and/or abrasion with cooperating parts, and to resist corrosion. As these characteristics become critical under certain operating conditions, the surface finish can dictate the performance, integrity and service life of the component.
The role of mass finishing processes (barrel, vibratory and centrifugal finishing) as a method for removal of burrs, developing edge contour and smoothing and polishing parts has been well established and documented for many years. These processes have been used in a wide variety of part applications to promote safer part handling (by attenuation of sharp part edges) improve the fit and function of parts when assembled, and produce smooth, even micro-finished surfaces to meet either functional or aesthetic criteria or specifications. Processes for developing specific edge and/or surface profile conditions on parts in bulk are used in industries as diverse as the jewelry, dental and medical implant industries on up through the automotive and aerospace industries.
Industry has always been looking to improve surface condition to enhance part performance, and this technology has become much better understood in recent years. Processes are routinely used to improve the life of parts and tools subject to failure from fatigue, and to improve their performance. These improvements are mainly achieved by enhancing part surface texture in a number of different, and sometimes complementary, ways.
Positive vs. Negative Surface Skewness. The skew of surface profile symmetry can be an important surface attribute. Surfaces are typically characterized as being either negatively or positively skewed. This surface characteristic is referred to as Rsk (Rsk—skewness—the measure of surface symmetry about the mean line of a profilometer graph). Unfinished parts usually display a heavy concentration of surface peaks above this mean line (a positive skew). It is axiomatic that almost all surfaces produced by common machining and fabrication methods are positively skewed. These positively skewed surfaces have an undesirable effect on the bearing ratio of surfaces, negatively impacting the performance of parts involved in applications where there is substantial surface-to-surface contact. Specialized high energy finishing procedures can truncate these surface profile peaks and achieve negatively skewed surfaces that are plateaued or “planarized,” presenting a much higher surface bearing contact area. Evidence confirms that surface finishing procedures tailored to develop specific surface conditions with this in mind can have a dramatic impact on part life. In one example, the life of tooling used in aluminum can stamping operations was extended 1,000%, or more, by improved surface textures produced by high-energy mechanical surface treatment
Directionalized vs. Random (Isotropic) Surface Texture Patterns. Somewhat related to surface texture skewness is the directional nature of surface textures developed by typical machining and grinding methods. These machined surfaces are characterized by tool marks or grinding patterns that are aligned and directional in nature. It has been established that tool or part life and performance can be substantially enhanced if these types of surface textures can be altered into one that is more random in nature. Post-machining processes that use free or loose abrasive materials in a high energy context can alter the machined surface texture substantially, not only reducing surface peaks, but generating a surface in which the positioning (or lay) of the peaks and valleys has been altered appreciably. These “isotropic” surface effects have been demonstrated to improve part wear and fracture resistance, bearing ratio and improve fatigue resistance. These performance related effects are even more pronounced with high energy finishing methods such as centrifugal barrel finishing and spindle finishing. See Figures 3 and 4.
Figure 3: Before Centrifugal Isotropic Finishing (Photo Courtesy of Jack Clark, Surface Analytics).
Figure 4: After Centrifugal Isotropic Finishing (Photo Courtesy of Jack Clark, Surface Analytics).
Residual Tensile Stress vs. Residual Compressive Stress. Many machining and grinding processes tend to develop residual tensile stresses in the surface area of parts. These residual tensile stresses make parts susceptible to premature fracture and failure when repeatedly stressed. Certain high-energy mass finishing processes can be implemented to modify this surface stress condition, and replace it with uniform residual compressive stresses. Many manufacturers have discovered that as mass finishing processes have been adopted and put into service, the parts involved have developed a working track record with unanticipated developments (their parts are better) and not just in the sense that they no longer have burrs or 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 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 typically are non-selective in nature. Edge and surface features of the part are processed identically and simultaneously. These methods also produce isotropic surfaces with negative or neutral surface profile skews. Additionally, they consistently develop beneficial compressive stress equilibrium. 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.
High-speed dry process spindle finishing. Dr. 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. Using this spindle oriented deburr and finish method, it is possible to produce compressive stresses in the MPa = 300 – 600 range that formed to a surface layer of metal to a depth of 20 – 40 μm. Spin pit tests on turbine disk components processed with the method showed an improved cycle life of 13090 ± 450 cycles when compared to the test results for conventionally hand deburred disks of 5685 ± 335 cycles, a potential service life increase of 2 – 2.25 times, while reducing the dispersion 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 significant improvement over tests performed on conventionally ground plates that started to fail after: (1.1 – 1.5) × 104 cycles.
High Energy Centrifugal Finishing is also widely used in the manufacture of medical, surgical and dental devices to develop very highly refined surfaces. Shown here is a dental device made up both of acrylic and cast chrome alloy materials that have been machine polished. (Photo courtesy of Mass Finishing Inc.)
Mass media finishing techniques (barrel, vibratory, and especially high energy centrifugal and spindle finish) can be used to improve part performance and service life, and these processes can be tailored or modified to amplify this effect. Although the ability of these processes to drive down deburring and surface finishing costs, when compared with 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 finish processing needs to be better understood and deserves closer study and documentation. Industry and public needs would be well served by a research consortium of partners at the industry, university, and governmental agency levels to better understand the role surface textures and stress equilibrium play in enhanced component life and performance.
The authors wish to acknowledge the technical assistance of the following: Dr. Michael Massarsky, Turbo-Finish Corp., Barre, Massachusetts; Thomas Mathisen, Mass Finishing Inc., Howard Lake, Minnesota, and Katie MacKay at MacKay Manufacturing, Spokane, Washington.
ADDITIONAL TECHNICAL MATERIAL:
Below are some process video footage demonstrations of high-speed centrifugal isotropic finishing. These automated edge and surface finishing methods are capable of producing very refined low micro-inch surfaces that can improve functional part performance and service life.
Isotropic Micro-Finishing Part Photography by Mark Riley, BV Products
Centrifugal barrel finishing
Centrifugal barrel finishing (CBF) is a high-energy finishing method, which has come into widespread acceptance in the last 25-30 years. Although not nearly as universal in application as vibratory finishing, a long list of important CBF applications have been developed in the last few decades.
Similar in some respects to barrel finishing, in that a drum-type container is partially filled with media and set in motion to create a sliding action of the contents, CBF is different from other finishing methods in some significant ways. Among these are the high pressures developed in terms of media contact with parts, the unique sliding action induced by rotational and centrifugal forces, and accelerated abrading or finishing action. As is true with other high energy processes, because time cycles are much abbreviated, surface finishes can be developed in minutes, which might tie up conventional equipment for many hours.
The principle behind CBF is relatively straightforward. Opposing barrels or drums are positioned circumferentially on a turret. (Most systems have either two or four barrels mounted on the turret; some manufacturers favor a vertical and others a horizontal orientation for the turret.) As the turret rotates at high speed, the barrels are counter-rotated, creating very high G-forces or pressures, as well as considerable media sliding action within the drums. Pressures as high as 50 Gs have been claimed for some equipment. The more standard equipment types range in size from 1 ft3 (30 L) to 10 ft3, although much larger equipment has been built for some applications.
Media used in these types of processes tend to be a great deal smaller than the common sizes chosen for barrel and vibratory processes. The smaller media, in such a high-pressure environment, are capable of performing much more work than would be the case in lower energy equipment. They also enhance access to all areas of the part and contribute to the ability of the equipment to develop very fine finishes. In addition to the ability to produce meaningful surface finish effects rapidly, and to produce fine finishes, CBF has the ability to impart compressive stress into critical parts that require extended metal fatigue resistance. Small and more delicate parts can also be processed with confidence, as the unique sliding action of the process seems to hold parts in position relative to each other, and there is generally little difficulty experienced with part impingement. Dry process media can be used in certain types of equipment and is useful for light deburring, polishing, and producing very refined isotropic super-finishes.
Below: SME Webinar Presentation on Centrifugal Isotropic Finishing by Dave Davidson (SME Tech Advisor) and Jack Clark (Surface Analytics.com)
Further reading: Internet resources
(1) “Isotropic Mass Finishing for Surface Integrity and Part Performance”, Article From: Products Finishing, Jack Clark, from Surface Analytics, LLC and David Davidson, from SME Deburr/Finish Technical Group, Posted on: 1/1/2015, [Barrel, vibratory, centrifugal and spindle finish can improve part performance and service life.] http://www.pfonline.com/articles/isotropic-mass-finishing-for-surface-integrity-and-part-performance
(2) “Turbo-Charged Abrasive Machining Offers Uniformity, Consistency” Article From: Products Finishing, by: Dr. Michael Massarsky, President from Turbo-Finish Corporation, and David A. Davidson, from SME Deburr/Finish Technical Group. Posted on: 6/1/2012. [Method can deburr, produce edge contour effects rapidly] http://www.pfonline.com/articles/turbo-charged-abrasive-machining-offers-uniformity-consistency
(3) “Turbo-Abrasive Machining and Finishing”. MANUFACTURING ENGINEERING – Aerospace Supplement, by: Dr. Michael Massarsky, President from Turbo-Finish Corporation, and David A. Davidson, from SME Deburr/Finish Technical Group. [Method first developed for the aerospace industry can improve surface integrity and part performance] http://www.slideshare.net/dryfinish/turboabrasive-machining-me-aerospace-supplement-reprint
(4) “The Role of Surface Finish in Improving Part Performnce”, MANUFACTURING ENGINEERING, by Jack Clark, Surface Analytics.com and David A. Davidson, from SME Deburr/Finish Technical Group.
(5) “Free Abrasives Flow for Automated Finishing”, MANUFACTURING ENGINEERING, ,by: Dr. Michael Massarsky, President from Turbo-Finish Corporation, and David A. Davidson, from SME Deburr/Finish Technical Group. [Exciting new methods of surface finishing that go beyond deburring to specific isotropic surface finishes that can increase service life] http://www.slideshare.net/dryfinish/october-2013-f2-deburring-1
(6) Turbo-Abrasive Machining Demonstration Video: https://www.youtube.com/watch?v=jYxqCxMIHNo
(7) SME Spokane, WA Factory Floor video, Centrifugal Finishing in the Precision Machine Shop: Demonstration) https://www.youtube.com/watch?v=dUdKjaysTYM
AUTHOR BIOGRAPHY – David A. Davidson, [firstname.lastname@example.org]
Mr. Davidson is a deburring/surface finishing specialist and consultant. He has contributed technical articles to Metal Finishing and other technical and trade publications and is the author of the Mass Finishing section in the current Metal Finishing Guidebook and Directory. He has also written and lectured extensively for the Society of Manufacturing Engineers, Society of Plastics Engineers, American Electroplaters and Surface Finishers Association and the Mass Finishing Job Shops Association. Mr. Davidson’s specialty is finishing process and finishing product development.