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Fundamentals of Vibratory Deburring, Finishing and Isotropic Micro-Finishing

For additional information contact Contributing Editor: David A. Davidson | Deburring/Finishing Technologist

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 on manufactured components. This is particularly true when manual deburring and finishing procedures can be minimized or eliminated. These types of processes can also help manufacturers gain a significant competitive advantage by improving part functional performance and service life and by adding a quality aesthetic to parts impossible to duplicate by competitors who don’t utilize this type of processing.

There are a number of different mass finishing processes in common use throughout the industry. Among these are barrel, centrifugal, vibratory and spindle finishing. Vibratory systems have become the predominant method. Much of this has to do with advantages inherent to the method in terms of ease of use, automation and material handling. When first developed in the 1950’s these systems were typically rather modest in size, and were used for deburring and finish processing of smaller components. Being able to process larger batch lots of modestly sized parts was important to competitiveness. The face of American manufacturing has changed considerably since those days. Smaller, simply designed parts in large numbers are now almost always sourced offshore. Much of American manufacturing and machining is now concentrated on making larger, higher value and more complex parts. As part size has grown, so has the size and processing capability of vibratory systems. Many larger and complex parts commonly finished in vibratory equipment today, would not have been considered as viable candidates for this type of automated mechanical finishing in the past. 100 cubic foot., and even 200 cubic f00t capacity vibratory systems capable of handling very large and heavy components are utilized in a number of different industries for large part edge and surface finishing.

Vibratory Finishing is an important component of a group of industrial processes referred to as mass finishing or mass media finishing. Mass finishing is a term used to describe a group of abrasive industrial processes by which large lots of parts or components made from metal or other materials can be economically processed in bulk to achieve one or several of a variety of surface effects. These include deburring, descaling, surface smoothing, edge-break, radius formation, removal of surface contaminants from heat treat and other processes, pre-plate and pre-paint or coating surface preparation, blending in surface irregularities from machining or fabricating operations, producing reflective surfaces with nonabrasive burnishing media, refining surfaces, and developing super-finish or micro-finish equivalent surface profiles. All mass finishing processes utilize a loose or free abrasive material referred to as media within a container or chamber of some sort. Energy is imparted to the abrasive media mass by a variety of means to impart motion to it and to cause it to rub or wear away at part surfaces.

Nearly all manufactured parts or components require some measure of surface refinement prior to final assembly, or the final finish or coating required to make the parts acceptable to the consumer or end-user. Most manufacturing companies who employ mass finishing techniques do so because of the economic advantages to be obtained, especially when compared with manual deburring and surface finishing techniques. Mass finishing processes often reduce or eliminate many procedures that are labor intensive and require extensive part handling. This is especially important in meeting increasingly stringent quality control standards, as most mass finishing processes generate surface effects with part-to-part and lot-to-lot uniformity that cannot be replicated with processes in which parts are individually handled. It has become a manufacturing engineering axiom that part reject and rework rates will plummet if a mass finishing approach can be implemented to meet surface and edge finish requirements.

Although each of the mass finishing process types carries with it a unique set of process strengths and weaknesses, all of them are sufficiently versatile to be able to process a wide variety of part types successfully. A plethora of abrasive media types, sizes, and shapes makes it possible, in many cases, to achieve very different results within the same equipment, ranging from heavy grinding and radiusing to final finishing. Components from almost every conceivable type of material have been surface conditioned using mass finishing techniques including ferrous and nonferrous metals, plastics, composition materials, ceramics, and even wood. Vibratory finishing is still the most predominant mass finishing method in the United States, and it falls into two broad categories in terms of the equipment being used: round bowl and tub designs.

Round-Bowl Vibratory Systems Round-bowl equipment normally has a processing chamber that resembles the bottom half of a doughnut. Although up to 20% slower than tub-style machines, and having occasionally more unwieldy media changeover routines, the advantages in automation and material handling for these machines have often given them an edge in any processing cost per part analysis. The vibratory motion generator on these machines is customarily a vertical shaft mounted in the center-post area of the bowl. Adjustments related to the eccentric weights on this shaft will affect the rolling motion of the media, as well as the forward spiral motion of the media in the bowl chamber. This spiral motion is one of the machine’s more salient advantages, as it promotes an even distribution and segregation of parts in the mass, thus lessening the chance of part-on-part contact (In some critical part applications, dividers are used to create processing pockets or segments to eliminate any chance of part-on-part contact.)

Like tub machines, equipment size varies from small bench models, whose capacity are measured in quarts or gallons, to very large equipment in excess of 100 cubic foot capacities. Successful processing requires appropriate media and compound selection, correct amplitude and frequency adjustments of the motion generator, and precisely determined water flow rate and compound metering rates. Unlike barrel systems, whose water levels are determined once at the beginning of the cycle, vibratory systems have a constant input and throughput of water into the system (both flow-through and recirculation systems are employed, although flow-through is generally much preferred in terms of maintaining part cleanliness.).

Water levels are critical to process success. Too much water will impede the vibratory motion of the mass. Too little will permit a soils/sludge buildup on the media, reducing its cutting efficiency. Flow-through functions can be automated with appropriate controls and metering devices. For parts requiring relatively short cycle times, round-bowl machines can be configured to perform in a continuous mode, the parts being metered in and then making one pass around the bowl, and exiting via the internal separation deck. Some designs include a spiral bottom to enhance loading from the machine onto the separation deck, lessening the likelihood of part-on-part contact at the entrance to the separation deck. Ease of use and economy are the hallmarks of vibratory finishing methods, and have contributed to making this perhaps the most accepted deburring and surface conditioning method for finishing parts in bulk. The equipment performs well in either batch or continuous applications. Standard applications usually can be run most economically in round-bowl-type equipment. Larger parts may require more specialized tub-type equipment, large volumes of parts, which can be processed in relatively short cycles, can make use of continuous tub or bowl equipment, or even multipath equipment. The latter can offer parts transfer from one operation to a secondary-type operation within the confines of the same machine, but different chambers. Vibratory action itself often will preclude the ability to develop super finishes or microfinishes unless specialized chemically accelerated methods are adopted.

Vibratory equipment ranges in size from 1 cubic foot [30 L] capacity up to 200 cubic feet [6000 L]. Tub vibrators are considered to have more aggressive media action than round-bowl machines, and they are capable of processing very large, bulky parts (as large as 6 ft by 6 ft) or potentially awkward part shapes (parts 40-ft long and longer). The vibratory motion generators consist of rotating shafts with sets of eccentric weights attached either at the bottom of the U-shaped tub or one of the sidewalls. This equipment is usually loaded from the top of the chamber, and usually unloaded through a discharge door located on a side panel. Parts and media can be screened on an external separation deck. This arrangement allows for relatively quick load/unload or media changeover cycles when compared with other equipment. Tub-shaped or tubular-shaped vibrators are commonly utilized for continuous high volume applications where the time cycle required to process the parts is relatively short. Media return conveyors and feed hoppers are used to meter the correct ratio of media and parts to the loading area of the machine, while media and parts are separated on a continuous basis by a screen deck located at the unload or discharge area of the machine. Tub-type machinery is also used extensively for batch applications and can be easily sub-compartmentalized for parts that require total segregation from each other.

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.

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.

Vibratory Isotropic Micro-Finishing

Isotropic Micro-Finishing is used to produce a high quality surface finish to components, typically used on parts where high contact stresses are present. The improvements in surface finish gained by this superfinishing process can reduce wear and stress concentrations and improve overall finish while extending component life.

Wherever metals come into contact with each other contact stresses and friction occur. Both these conditions regulate and reduce the performance and compromise the design of the component. Micro-finishing is a means of regaining those losses by producing a superfine finish where it is most needed – at the point of contact.

The benefits are greatest with high contact stress applications and high fatigue life requirements. It is particularly beneficial in mating gear applications where it is proven to reduce contact stresses but also to reduce individual tooth bending which is key to maintaining good fatigue life with a resultant reduced operating temperature.

While it is known that reduced operating temperatures are an indication of increased performance the secondary benefit is that there is less heat to dissipate and as such the specific cooling cross section or cooling flow rate can be reduced all of which complement efficiency and performance.

The Process This process is radically different from conventional machining. The following is a brief overview of the chemically accelerated vibratory finishing process using high density, non- abrasive ceramic media.

Equipment: vibratory machines The process is carried out in specially designed vibratory finishing bowls and or tub/trough machines. These durable machines have been around for more than 60 years. Vibratory machines are available in sizes from 15 L to 1000 L working capacity. This means gears and other components can be finished ranging in size from less than six mm in diameter to more than two meters in diameter and quantities from one to hundreds at a time.

Consumables: high density, non-abrasive ceramic media The process utilizes high density, non-abrasive ceramic media in the vibratory finishing machine. It is considered non-abrasive since it does not contain discrete abrasive particles and alone is unable to abrade material from the hardened surface of the components being processed. The media is selected from a range of shapes and sizes best suited for maintaining the geometry of the parts. The selection of the correct media is a critical part of the process.

Process chemistry The unique and significant feature of the process is the surface leveling/smoothing mechanism utilized to achieve the surface finish. A reactive chemistry is used in the vibratory machine in conjunction with the media. When introduced to the vibratory machine this chemistry produces a stable, soft conversion coating across the asperities (peaks and valleys) of the components. The rubbing motion across the components developed by the machine and media effectively wipes soft conversion coating off the ‘peaks’ of the parts surfaces, thereby removing a micro-layer of metal. After this continual process is complete, the conversion coating is wiped off one final time using a neutral soap to produce a mirror-like surface. This process does not affect the integrity of the parts either structurally or dimensionally and any very sensitive part areas can be effectively masked if required prior to processing with VIM.

Performance Benefits • Reduced friction • Increased part durability • Improved corrosion resistance • Reduced wear • Reduced lubrication requirements and cost • Improved oil retention • Reduced contact and bending fatigue • Improved pitting resistance • Reduced vibration and noise attenuation • Reduced applied torque requirements • Improved surface finish uniformity (part-to- part, feature-to-feature and lot-to-lot) • High-quality, micro-finished surfaces

Reduced Friction Benefits • Increased fuel economy • Reduced contact fatigue • Increased power density • Lower operating temperature • Extended mean time between maintenance overhauls • Reduced maintenance costs • Eliminated break-in • Extended component life • Reduced metal debris • Reduced part failures • Minimized overheating

Many cooperating parts including gears and gear sets in a variety of industries remain subject to fatigue, fracture and wear. Such parts can gain substantial improvements in life and performance, from alterations to their overall surface texture. Improvements in overall smoothness, load-bearing ratio, surface profile skewness and isotropicity can, in many instances, improve life and performance and cut operational costs dramatically. Manufacturers that have not subjected their parts to an analysis to determine the potential benefits of this kind of isotropic processing may be making parts that are not all that they can be.

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REFERENCES

  1. Thompson, Samuel R., “Selection of Vibratory Equipment”, SME Technical Paper MR81-381, Dearborn, MI: Society of Manufacturing Engineers, 1981

  2. Thompson, S. R., Comparison of Rectangular Tub and Inline Continuous Vibratory Equipment”, SME Technical Paper MR83-679, Dearborn, MI: Society of Manufacturing Engineers, 1983

  3. Kittredge, John B., “Selection of Mass Finishing Equipment”, SME Technical Paper MR87-153, Dearborn, MI: Society of Manufacturing Engineers, 1987

  4. Gillespie, LaRoux, Mass Finishing Handbook, pp. 237-247, New York, NY: Industrial Press, 2007

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