Developments in Mass Finishing Technology

Mass Finishing Fundamentals

Mass Finishing is a term used to describe a group of industrial processes by which large lots of manufactured parts can be processed in bulk economically to achieve a variety of surface effects. These economical processes, in contrast with hand deburring methods, develop these effects with a high degree of part-to-part and lot-to-lot uniformity and consistency.  These effects might include edge break, edge contour, surface smoothing and improvement, tool mark blending, burnishing, polishing, super-finishing and micro finishing.  In these types of processes, energy is imparted to an abrasive embedded or coated loose material known as media that is contained within the work chamber of a finishing machine.  Energy is then transferred from work chamber motion to the media and to the work-pieces or components placed in the media, by way of a random rubbing or scrubbing action to achieve some sort of edge or surface improvement and refinement.  The surface and edge effects produced are typically non-selective in nature, unless a part has been partially masked or fixtured.  While edge geometries can be modified (contoured) to some extent, it would be a mistake to consider these processes for substantial material removal operations that are best left to traditional grinding and machining methods.

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 are interacted 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.

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.  Each of the subsequent steps uses an abrasive material that has been calculated to clear and blend-in the abrasive pattern left in the surface by the preceding step. To use a common everyday analogy, almost everyone understands that to produce fine finishes in woodworking applications, it is necessary to use sanding operations with successively finer abrasive grits to produce cabinet or furniture quality surfaces..  The same principle holds true in mass finishing (or even hand-finishing) metal parts, when very smooth or polished surfaces are required.

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.

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 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.

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 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.

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.

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

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 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.

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.

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