Contributing Editor: Dave Davidson, Deburring/Finishing Technologist | 509.230.6821 | firstname.lastname@example.org | https://about.me/dave.davidson
If you have parts that need edge or surface finishing improvement and would like to have FREE sample part processing and a quotation developed for finishing the parts please contact Dave Davidson at email@example.com I can also be reached at 509.230.6821. Information about equipment for bringing Centrifugal Iso-Finishing capability to your facility is also available…
Some Case Studies — Edge and surface finishing for improved performance and fatigue failure resistance:
The technical literature is replete with examples of case studies indicating that substantial fatigue resistance can be developed with mass finishing methods such as barrel finishing. One study published by Iron Age magazine back in 1959 documented studies on the compressive stresses to be developed with various types of abrasive and non-abrasive media in tumbling barrels.
Some processes are especially well known for this characteristic. Steel media or ball burnishing processes using vibratory techniques are known not only for producing aesthetically pleasing surfaces but also enhancing the service life of components because of the bulk-density of the media itself (300 lb./ft3 as compared to 80 to 100 lb./cu. Ft3 with typical abrasive ceramic media types). Some promising research has been done by major aerospace companies that indicate large airframe components can be processed to enhance fatigue life, and that fatigue life of components stressed or weakened in service can be improved during overhaul cycles as well.
Hignett, in writing a technical paper for SME in 1979, spelled out a number of applications where high-intensity centrifugal barrel finishing was specified specifically because of the part performance and service life attributes the process was able to develop. Among these were:
- Components for roller chain and silent chain, including the slide plates, bushings, rollers and pins. The centrifugal process was able to deburr, descale, edge finish, surface finish, and impart compressive stress on the parts simultaneously. As the loose abrasive media being used in these systems are applied against part surfaces with relatively high pressure, very small media were utilized to completely and uniformly deburr and radius through holes on the side plates without rolling the burrs over. Other finishing methods could not accomplish this, and, as a result, sharp edges would be exposed when burrs were “torn” off in assembly, creating a stress raiser that would contribute to premature failure of chains due to fatigue cracks initiating at the sharp edge. Like many other similar applications, the success of this process was dependent on the process’s ability to produce “holistic” changes in the parts themselves on a number of different levels. These changes not only contributed to the dimensional uniformity of the parts, permitting more automated assembly, but also changed the nature of part edge and surface characteristics and stress conditions in a way that not only facilitated extended service life, but actually made the service life more predictable.
Above: Examples of typical part finishing applications.
Dental partials and orthodontic bands, brackets, and wire parts: In recent years, high-pressure centrifugal finishing has become a standard method for producing needed edge and surface finish quality for these parts, reducing finishing costs by as much as 70% over manual methods. Given the fact that the end destination for the parts is inside the end user’s mouth, uniformly consistent edge contour and very refined low micro-inch surface finishes without dimensional distortion are required. Like many demanding surface condition treatment applications, several different cycles using successively finer abrasive materials may be required to achieve the needed result. As can be imagined, the improved fatigue and wear resistance properties are product attributes that contribute to customer satisfaction considerably.
- Carbide and HSS tooling: A number of different mass finishing operations are utilized by the cutting tool industry to develop edge and surface finishes on the tooling that contribute to improved cutting performance and tool longevity. Vibratory equipment, especially equipment equipped with high-frequency electro-magnetic drives, is utilized to produce uniform edge-hone or edge preparation on carbide inserts to improve cutting performance and tool life. High-pressure finishing methods, some of them employing part fixtures, are also used to produce specialized edge and surface finish effects on both carbide and HSS tooling. Among the methods employed are centrifugal barrel processing and several different variants of “drag finishing.” Some processes produce edge and surface effects by pressure 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 pressure and flow. Significant improvements in tool performance and longevity are claimed for both “new” tools and “resharps” that have edge and surface conditions modified with these types of methods. Parts currently being processed include end-mills, drill-bits, spade blades, broaches, hobs, and even circular saw blades for power tools. Several manufacturers claim tool life increases of 200 to 300% for tools processed by these methods, in some cases obviating the need for coatings.
- Reed and flapper valves: These reed components are sometimes flexed more than 12,000 cycles per minute in normal service. Centrifugal barrel finishing promotes extended fatigue life on these components by developing both generous edge-contours to limit stress concentration points, as well as simultaneously developing low-profile surfaces with high compressive stress properties. One case study cited a reduction in the number of fatigue failures developing at the predicted half-life of the part of over 95% when surface finished with centrifugal methods.
- Stainless steel coil spring fatigue resistance: An extensive comparative testing program was conducted to evaluate high-pressure (centrifugal barrel) methods as a means to improve fatigue life on coil springs. In one test, the coil springs were tested to failure by depressing them from 1.104-inch lengths to 0.730 inch, developing a stress change from 0 to 50,000 psi. The coil springs processed with the conventional surface finish and shot peening methods all failed between 160,000 and 360,000 cycles. Parts processed in a single, high-pressure centrifugal barrel method for 20 minutes failed between 360,000 and 520,000 cycles, a typical performance increase of 60%. All centrifugally finished parts that failed below 400,000 cycles did so because of clearly visible surface defects or inclusions. These parts could have been easily removed from the production stream by visual inspection because of the prominence of the defect, given the refined surface finish. Such defects were masked because of the highly textured nature of the peened surfaces, making visual or optical elimination near impossible. Tests performed on another set of production springs were even more striking, with all springs processed in standard or conventional processes, failing before 600,000 cycles, and none of the same springs processed in high-pressure centrifugal methods failing before the 800,000 cycle limit of the test.
The Importance of Edge and Surface Condition Quality: Deburring and surface conditioning has often been treated as the “poor relation” or “step-child” of manufacturing engineering. Many parts and components have been shown to have sub-optimal performance because of a lack of edge and surface-quality conscious design. Edge and surface finish can be an important factor in driving ultimate part performance and functionality. In what has become a standard reference on the subject, (“Deburring and Edge Finishing Handbook”) author LaRoux Gillespie enumerated 25 different potential problem areas that might result from insufficient care being given to edge and surface finishing 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 assemblies;
• clogged filters and ports (from loose burr accumulation);
• cut rubber seals and O-rings;
• excessive stress concentrations;
• plating buildup at edges;
• paint buildup at edges (from electrostatic spray over burrs);
• paint thin out over sharp edges (from liquid paints);
• edge craters, fractures, or crumbling (from initially irregular edges);
• turbulence and non-laminar 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 understand how edge and surface finish quality might contribute to how manufactured parts and components will function, perform and last in service? Tools are readily available. Will we use them? To an increasing degree, to borrow a well-worn phrase used by one finishing compound manufacturer, “It is the finish that counts.”
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.
Centrifugal Isotropic 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.
Below: some examples of Centrifugal Iso-Finishing Equipment
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 the 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 used 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)
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.