Contributing Editor: Dave Davidson, Deburring/Finishing Technologist 509.563.9859 | ddavidson@deburring-tech-group.com | dryfinish.wixsite.com/iso-finish
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 contact Dave Davidson at ddavidson@deburring-tech-group.com I can also be reached at 509.563.9859. Information about equipment for bringing Centrifugal Iso-Finishing capability to your facility is also available…
Part Fixturing and Surface Finishing
Included in these mass finishing methods are traditional barrel tumbling, vibratory and centrifugal finishing. A closely related set of processes would be fixture-centric processing such as the spin-finish, drag-finish, spindle-finish and the turbo-finish methods. The fixture methods produce results by imparting motion to parts that are fixtured (by either dragging, rotating or developing a planetary motion) and are immersed in loose abrasive or polishing media. The force with which part edges and surfaces interact with loose media can be considerably higher than that developed by mass media processes, where parts are placed randomly within the media mass, and are dependent on the loose media motion to achieve the surfacing results. Fixturing parts in more conventional barrel or vibratory methods is also not uncommon. This is done for a variety of reasons, including the need to prevent any part-on-part contact, but also to increase the amount of force flow of media against part surfaces, to accelerate cycle times and produce more pronounced surface finish effects.
Above: Demonstration of sequential processing with Centrifugal Iso-Finishing. Note how the
barrel has been sub-divided into separate compartments, in may cases this compartmentalization
can replace the fixturing required by other methods.
Part applications for fixture finishing in conventional equipment vary widely, to cite some examples: Some manufacturers of brass musical instruments (trumpets, french horns, trombones) fixture brass instrument assemblies in the barrel or vibratory chambers and flow soft polishing granulate media through the assemblies to replace multiple buffing operations. Similarly, some manufacturers of medical and surgical implant devices fixture the devices in high-energy centrifugal barrels, and produce very refined surfaces on cobalt chrome and titanium substrates by processing the devices through a sequence of successively finer loose abrasive operations. Fixtured processing in vibratory equipment can accommodate even very large structural parts. This is an important application for large structural aerospace parts. The method can be used to reduce the need for costly manual deburring and finishing methods on airframe components. More importantly, with the proper sequence of abrasive and non-abrasive operations, it can be used to develop very significant compressive stress and work hardening characteristics to the parts, enhancing their wear and fatigue failure resistance dramatically
Old Dog – New Tricks, Sequential Processing
One trait that many of today’s more sophisticated mass finishing operations share is a reliance on multiple-step sequential processing. In this type of processing, very rough surfaces can be brought to a highly polished or micro-finished state. This is done by initially processing the parts with a coarse abrasive material, and then following up with a sequence of finer abrasives. (See the video below)
Above: Video case study of Centrifugal Iso-Finishing used for both deburring and super iso-finishing precision components in the contract machining environment.
One time-honored method for producing very refined surfaces is dry barrel processing. This technology was originally developed and heavily utilized in the northeastern United States as early as the 1930’s; similar methods were developed concurrently in Europe.
The method was developed primarily to mitigate the high labor costs associated with hand-buffing large numbers of consumer-oriented articles such as eyewear and jewelry. This technique was widely accepted as a standard method for producing very refined consumer acceptable product finishes that had previously been the sole province of those buffing methods and is still utilized for these types of applications. This sequential principle has been adapted for use in other types of equipment for other part finishing applications. Where reflective surfaces are desired on parts being finished in vibratory equipment, it is not unusual now to see secondary vibratory processes with burnishing media or dry process polishing media develop those surfaces. Many processes have been developed for centrifugal disk and centrifugal barrels where three or more steps are utilized in order to bring part surfaces to very low micro-inch surface profiles or to develop very reflective surfaces for cosmetic reasons.
Mass Finishing Processes and Compressive Stress Effects. Even simple tumbling can develop residual stresses that can provide some functional improvements to service life in certain components. High-energy mass finishing methods can magnify this effect many times. In the early 1990’s some researchers pioneered the use of electron microscope (SEM) analysis to determine or quantify surface finishes as they relate to possible life extension and component functionality.
This early work showed that it was possible to improve the functionality and service life of many different types of components by a two-fold improvement in metal surface profile and integrity.
Processes such as peening are commonly used for metal surface integrity improvement to mitigate crack propagation points and improve service life by improving wear and metal fatigue resistance. It was found that high energy loose media sequential finishing could develop not only compressive stresses but very level, or negatively skewed plateaued surfaces, that provided a great deal more bearing load surface to parts which interacted with other part surfaces.
This graph compares the compressive stress developed with conventional shot peening with that developed with centrifugal iso-finishing methods. The data support the conclusion that centrifugal finishing can improve part service life by improving fatigue failure resistance as well as wear resistance for many types of cooperating parts or parts stressed in service. Diagram courtesy or Ralph Labelson, at Richwood Industries.
In one application, stamping dies used for forming aluminum can tops were given a useful life of approximately ten times that was anticipated of parts that had not been surface finished with this method. Another application cited by Richard Gilliam in a technical paper describing centrifugal barrel processing noted that extensive cycling tests conducted by a spring manufacturer. “This ability to improve resistance to fatigue failure is graphically demonstrated by the results of some tests made by a manufacturer of stainless steel coil springs. A group of springs was taken from a standard production run. Half of the sample was finished in the manufacturer’s usual manner of barreling followed by shot peening, while the other half was CBF-treated for 20 minutes. The springs were then tested to failure by compressing them to a stress change from 0 to approximately 50,000 psi. The results showed that all the springs finished by the conventional method failed between 160,000 and 360,000 cycles. The springs that had been processed by CBF failed at between 360,000 and 520,000 cycles, an average improvement of 60%.”
In addition to the development of useful compressive stress values, high-energy surface finishing methods such as centrifugal iso-finishing also contribute to improved part performance and service life by developing isotropic surfaces (surfaces that are random in nature rather than being characterized by linear and parallel machining and grinding lines. Additionally, they change the positively skewed surfaces left by most manufacturing methods (machining, grinding, casting etc.) to negatively skewed surfaces that have much better load bearing characteristics and much more wear resistant. (SEM photos courtesy of Jack Clark, Surface Ananlytics, LLC)
In one application, stamping dies used for forming aluminum can tops were given a useful life of approximately ten times that was anticipated of parts that had not been surface finished with this method. Another application cited by Richard Gilliam in a technical paper describing centrifugal barrel processing noted that extensive cycling tests conducted by a spring manufacturer. “This ability to improve resistance to fatigue failure is graphically demonstrated by the results of some tests made by a manufacturer of stainless steel coil springs. A group of springs was taken from a standard production run. Half of the sample was finished in the manufacturer’s usual manner of barreling followed by shot peening, while the other half was CBF-treated for 20 minutes. The springs were then tested to failure by compressing them to a stress change from 0 to approximately 50,000 psi. The results showed that all the springs finished by the conventional method failed between 160,000 and 360,000 cycles. The springs that had been processed by CBF failed at between 360,000 and 520,000 cycles, an average improvement of 60%.”
Substantial compressive stress effects can also be generated in lower energy types of equipment with very dense metal media. In commenting on this, John Rogers, process laboratory manager for the Abbott Ball Co., Inc. noted some points made in a company publication: “Steel media is smooth and heavy. It is not abrasive in action. Rather, the media’s weight and strength increase the smoothness and pressure of its finishing action. Workpieces keep their tolerances intact, gain compressive stress, and achieve the ultra-clean, microscopically smooth surface.
Substantial compressive stress effects can also be generated in lower energy types of equipment with very dense metal media. In commenting on this, John Rogers, process laboratory manager for the Abbott Ball Co., Inc. noted some points made in a company publication: “Steel media is smooth and heavy. It is not abrasive in action. Rather, the media’s weight and strength increase the smoothness and pressure of its finishing action. Workpieces keep their tolerances intact, gain compressive stress, and achieve the ultra-clean, microscopically smooth surface.
Various steel media shapes. Steel media used widely in both barrel and vibratory finishing systems. At 300 lbs./cu. ft. this heavy material changes surfaces with compression instead of abrasion. In high-energy finishing systems, materials such as porcelain or sintered bauxite are used to produce similar and more pronounced surface finish effects
The wide selection of media shapes and sizes allows full control over the type and amount of contact obtained between the media and the part. Steel media is heavy, weighing approximately 300 pounds per cubic foot. The media mass forms a dense cushion that produces rapid finishes, yet does not harm fragile parts.
As steel media impinges on a part, its surface is work-hardened. The working action imparts compressive stress as a beneficial by-product of the finishing process. In many instances, the process can replace steel shot peening as a work-hardening step. Parts processed with steel media have longer cycle lives and greater resistance to wear as a result of this compressive stress action.
Turbo-Finish Machines and Turbo-Abrasive Machining
Dr. Michael Massarsky, the inventor of the Turbo-Finish process, initially developed the process for improving edge and surface finish methods for rotating parts in the aircraft engine industry. The process replaces much of the manual deburring formerly required on these types of parts. TAM machines could be likened to free abrasive turning centers. They utilize fluidized bed technology to suspend abrasive materials in a specially designed chamber. Parts interface with the abrasive material on a continuous basis by having part surfaces exposed and interacted with the abrasive bed by high speed rotational or oscillational movement. This combination of abrasive envelopment and high-speed rotational contact can produce important functional surface conditioning effects and deburring and radius formation very rapidly. Unlike buff, brush, belt and polish methods, or even robotic deburring, abrasive operations on rotating components are performed on all features of the part simultaneously. This produces a feature-to-feature and part-to-part uniformity that is extraordinary.
Video above: Aerospace turbine component being deburred and surface finished with Turbo-Finish, a high-speed dry spindle finishing and deburring method
TAM processes share characteristics common to both machining and mechanical finishing processes. A much higher degree of control is possible than is the case with conventional finishing processes. TAM processes can utilize very sophisticated computer control technologies to create processes which are custom tailored to the needs of specific parts. Like machining processes, the energy to produce the cutting or abrasive action that develops the desired surface effect arises primarily from the rotational energy of the part itself. Unlike both machining processes and manual deburring processes, with their single point of contact, TAM processes perform abrasive machining or grinding on all features of the part by abrasive media envelopment.
High speed dry spindle finishing
Turbo-Finish and Turbo-Abrasive Machining. Parts are deburred and surfaced in time cycles of a few minutes, as opposed to the multiple hour cycles necessary when performing this work manually
Dry Spindle finishing or Turbo-Abrasive Machining [TAM] processes were developed originally to address deburring and surface conditioning problems on complex rotating components within the aerospace industry. Aerospace parts such as turbine and compressor discs, fan disks and impellers pose serious edge finishing problems. Manual methods used in edge finishing for these parts not only were costly and time-consuming but more to the point, human intervention, no matter how skillful at this final stage of manufacturing, is bound to introduce some measure of non-uniformity in both effects and stresses in critical areas of certain features on the part. TAM provides a method whereby final deburring, radius formation and blending in of machining irregularities could be performed in a single machining operation. This machining operation can accomplish in a few minutes what in many cases took hours to perform manually. It soon became obvious that the technology could address the edge-finishing needs of other types of rotationally oriented components such as gears, turbo-charger rotors, bearing cages, pump impellers, propellers, and many other rotational parts. Non-rotational parts can also be processed by fixturing them to the periphery of disk-like fixtures.
Another important feature of the process is its use of high intensity small abrasive particle contact to produce surface effects. This results in the ability to process intricate or complex part shapes easily. Although the abrasive material used for processing is similar in some respects to grinding and blasting materials, TAM produces an entirely different and unique surface condition. One of the reasons for this is the muiti-directional and rolling nature of abrasive particle contact with part surfaces. Unlike surface effects created with pressure or impact methods such as air or wheel blasting, TAM surfaces are characterized by a homogeneous, finely blended, abrasive pattern developed by the non-perpendicular nature of the abrasive attack.
Unlike wheel or belt grinding, surface finishes are generated without any perceptible temperature shift at the area of contact and the micro-textured random abrasive pattern is a much more attractive substrate for subsequent coating operations than linear wheel or belt grinding patterns.
TAM processing can be especially useful when part size, shape or complexity preclude the use of other mechanical finishing processes. TAM deburrs, and develops edge and surface finishing effects very rapidly and has unique metal improvement and compressive stress generation capabilities. Aqueous waste treatment and disposal costs are avoided by a completely dry abrasive operation. The process is primarily intended for external surface and edge preparation, although some simpler interior areas and channels can be processed as well. Complex geometric forms can be easily accessed. Repeatability and uniformity can be even further enhanced with PLC or computer-controlled processing, and with all features of the part receiving identical and simultaneous abrasive treatment, feature-to-feature, part-to-part and lot-to-lot uniformity on parts can be exceptional.
Isotropic Micro-Finishing and Super-Finishing The process of isotropic micro-finishing and superfinishing is used in applications such as Formula One, V8 Supercars, wind turbine transmissions, helicopter transmissions and other performance-critical part applications. These processes are especially useful to improve surfaces in any area where it is desirable to reduce friction and heat and increase efficiency and service life. Specialized chemicals and processes have been developed over the past decade in order to produce these important surface effects economically. (See the video below for an example of vibratory chemically assisted isotropic finishing)
Several high-performance engine component manufacturers are sending their products to specially equipped contract finishing service providers for final finish processing. Some of them offer it to their customers as an add-on, and others build the cost into the components’ selling price. Typical items processed include automotive gears, motorcycle gears, crown wheel and pinions, camshafts, oil pump internals, steering rack and pinion, and crankshafts. Not only does the Vibratory Isotropic Micro-Finishing [VIM] process significantly reduce wear in parts such as the ones listed above but it also enhances the durability and efficiency of metal components, resulting in cost savings and added value to parts and the manufacturer’s operational budget. □
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 contact Dave Davidson at dryfinish@gmail.com I can also be reached at 509.563.9859 Information about equipment for bringing Centrifugal Iso-Finishing capability to your facility is also available…
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