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Mass finishing refers to any deburring, edge breaking, or surface conditioning process in which the workpieces, although confined in a chamber, are not fixtured. As the term is applied in manufacturing today, it includes tumbling barrels, vibratory finishers, centrifugal barrels, and centrifugal disc machines. Other processes, such as tumble-blast finishing, magnetic finishing, drag and spin finishing also fit loosely under the mass finishing umbrella.
Successful equipment changes inthe past decade include more automatic versions of typical batch machines, and much larger machines for very large parts such as aircraft wing spars, and jet engine components. A tub style batch vibrator holding more than eleven tons of plastic media was built for a special application. And, in 2008, vibratory finishing was combined with blast finishing in a single pass, helical style, multi-pass bowl machine. This machine is aptly named SpiraBlast, and, the process, SpiraBlasting. It can even wash and dry the parts before they are discharged.
The basic processes used in mass finishing have not changed dramatically, although some terms such as “isotropic” have been added to the mass finishing vocabulary to describe the omni-directional finishes obtained. In recent years, the phenomenon of surface stress relieving during mass finishing, other than shot blast peening, has been successfully applied.
Ceramic media technology has greatly improved with the development of high density (120 to 150 lbs/cu ft), tough media with and without abrasives.
Tumbling barrels most often are batch machines, but some are available for in-line, single pass processing, or automated batch finishing. Tumblers still have a useful place in manufacturing, particularly for part-on-part processing of small parts—such as flat washers and fasteners—and for deburring, polishing, and washing. Many of the smaller tumblers reside in jewelry manufacturing and lapidary shops, while large, automated systems are used for deburring and washing in high volume production of stampings, forgings, or similar parts.
Centrifugal tumblers are the most difficult to automate because of their design. Some improvements have developed over the years, but these are usually elaborate media handling systems with independent separating screens and overhead conveyors, bucket elevators, elevated hoppers, and other paraphernalia designed to make the operator’s job less strenuous, and usually more complicated. Centrifugal tumblers are the fastest of the mass finishing processes, and have earned themselves a niche when severe radii or stock removal are required. They also are excellent burnishing machines. The high speed of these machines often makes up for their labor intensity.
Vibratory finishing is the most popular choice of these processes. Much of its success is attributed to ease of automation. In the case of the round machines, the advent of internal separation of parts and media significantly raised the standard for manufacturing convenience.
Centrifugal disc finishing is moving into second place as the specified mass finishing method for new installations. Its speed offsets some of the disadvantages of being limited to batch operations, although creative handling system designs are making the process increasingly acceptable. The higher operating costs continue to discourage some prospective users.
How do you know which process is best suited for your application? First, become familiar with the basics of each approach. Here is a very brief description of each:
Tumbling Barrels. These are either open or closed work chambers. Media, water, compound, and parts are the usual ingredients. They provide a rolling and tumbling action of the workload. The speed of rotation is regulated to give peripheral velocities ranging from 12 –40 ft/min. Lower speeds are for polishing, and higher speeds are for more aggressive deburring and edge breaking. The action on the parts is restricted to areas that are easily contacted in the linear direction of the media, or part-to-part action. This restricts it to exterior surfaces and edges. Because parts fall on one another, there is more denting of the surfaces than with other processes. Tumbler sizes range from tabletop machines with less than a quart capacity, to very large machines that process more than 1,000 lb of parts in a single load.
Continuous design tumblers process large volumes of parts with in-line production. Also, automated batch tumblers offer longer time cycles for large batches of parts. Most tumbling barrels are capable of operating with, or without, media.
Tumbling equipment is also well suited to dry tumbling for deburring and cob drying. Tumblers can be equipped for cryogenic deflashing of rubber parts, usually using dry ice as media.
Centrifugal Tumblers. These remind you of a Ferris wheel because small baskets are mounted on the periphery of a large revolving wheel. The wheel turns fast, developing high centrifugal forces in the baskets. The small baskets rotate in the opposite direction to the large wheel, causing their contents to rub together under very high pressure. In most machines, the large wheel rotates vertically; less popular horizontal models are also available. The force of the pressure is 15 to 20 times the force of gravity. This gives finishing similar to that of a conventional tumbler, only very much faster. Another advantage is that finishes can be very smooth because of the high compressive loads applied to the surface. Some handling systems are offered to facilitate loading and unloading of the individual baskets. This is, however, a batch operation.
Vibratory Finishing. The basics are a work chamber containing workpieces, media, and compound solution. The media and/or solution are eliminated in some cases. The chamber is vibrated in the range of 900 to 3600 vibrations/min, with an amplitude ranging from one or two millimeters to as much as ten or fifteen millimeters. Generally, the amplitude and frequency are used in inverse proportions, higher frequency machines having lower amplitudes. The action of the ingredients results in deburring, edge breaking, and polishing as the workpieces and/or media interact. One advantage over the other three processes is that concave and interior surfaces are subjected to some action. A major advantage is that handling of parts and other ingredients is easily automated.
The parts can be handled in batches, or continuously fed into through-feed machines. Toroidal (round) machines are available with internal unloading devices that operate manually, or automatically.
Tub style machines can handle very large parts. The largest tub style machine built to date has 425 cu ft capacity, and is vibrated by ten eccentric shafts, five on each side and powered by a single 60 HP motor.
The original, continuous, through-feed vibrators are long troughs, or tubes, with parts and media rolling repeatedly as they travel the straight length of the tub. The parts and media are separated at the discharge end, and media is conveyed back to the beginning. These single, in-line, through feed machines are very popular. An alternative to in-line designs is available in the round machines that can process the parts in a single pass. These systems have fewer components because separation occurs internally and the media never leaves the process bowl. They have a lower power requirement and lower maintenance. Each style should be considered when short (under 30 min) time cycles suffice.
The need for longer process times in a single pass is being answered with some creative designs. Two of these use the round concept with circular channels. One of these winds the channel around the center post in increasing diameters while in a descending helical path. Parts and media travel downward to an external, though integrated, media separator. The media is conveyed back to the beginning. The other uses stacked channels of the same radius and the mass travels upwards in a helical path to be discharged onto an integrated separation deck. Media is gravity fed down to the lower, starting level. Another innovative design uses straight tubes stacked on each side of the machine. The tubes slope downwards, moving the mass to the lower end. Then, the mass enters a U-shaped channel and is transported to the top of the tube on the opposite side of the machine. This continues until parts and media have traveled the length of all the tubes. Typically, three tubes are on each side of the machine. The advantage of this design is that tubes of any length or diameter can be employed to meet the production requirement. All three of these so-called “multi-pass” machines use very little floor space considering the length of the process channel. All offer a minimum of part-to-part contact, giving them an advantage with very delicate parts. A common disadvantage is that less work is performed in a given process time. Of the multi-pass designs, the most popular has been the torroidal, helical design with circular channels of increasing diameters wrapped around the center post.
Automated batch machines, both tubs and bowls, provide varied process times and flexibility for a variety of applications. The straight through and single pass round machines generally have time cycles under 30 min. The multi-pass machines offer time cycles as long as two hr. Vibratory finishers range in sizes from table top models holding less than a half gallon, to through-feed and batch machines with greater than 400 cu ft capacities.
Although there are many designs for automating vibratory finishers, hand loading and unloading of parts is still very popular. This is sometimes the best way to protect delicate parts, and it can be economically sound. A single operator working amongst several vibrators can keep them all employed while he manually loads and unloads the parts. In one finishing room, 17 round, 10-cu ft machines are manually loaded and unloaded by a single operator.
Centrifugal Disc Finishers. These are batch processing machines, as are the centrifugal barrels. The work chamber is a vertical cylinder; it can have either straight, or curved, stationary sides. The action occurs when the disc shaped bottom of the chamber is rotated in the range of 150 to 300 RPM.
Centrifugal force pushes the workload outward while the spinning bottom plate pushes it forward. When reaching the side of the chamber the mass rises, until everything falls back toward the bottom to repeat the trip. It looks like a deep whirlpool, or very much like a food blender at work. The sliding and impinging action of parts and media on each other create the deburring and polishing action. This process is particularly suited to smaller parts and can even be done part-on-part, with or without a liquid. Production rates are attractive due to shorter time cycles than those achieved in vibratory or tumble finishing. Machine capacities are in the 1 to 10 cu ft range. Innovative handling systems offset many of the disadvantages of batch operations. See the illustration of tandem machines with automated parts separation and media return, with a common cob dryer.
SpiraBlasting. Because this process uses shot blasting in the first phase, it can deburr parts much faster, and cheaper, than any of its cousins in the “tumble-finishing” field. The media can be any typical dry media from steel shot to polycarbonate wire. The finish can then be refined in the vibratory finishing phase, washed, dried and automatically discharged. Applications for this dual process are obviously limited, but when it is right for the job, it is the unique choice.
If you start your selection by having parts processed in vibratory finishing this will establish a base line time cycle, as well as providing an example of the finish you might expect. If time cycles and finish are acceptable, you will have many options within the vibratory finishing arena. If the process cycle is really too long—let’s say more than 4 hr—one of your options is to consider chemically accelerated processing in a vibrator. A more likely option will be centrifugal disc processing (when vibratory cycles are in the range of 2 to 4 hr) and centrifugal barrel finishing if you need more severe edge breaking, or if vibratory time cycles exceed 4 hr. This is not to start a debate about these general parameters, but just to give you the basic idea. There are some circumstances, however, in which conventional, or automated tumbling barrels are a good choice. Before moving from a vibratory selection to one of the other processes, get complete advice on all the possible variations of vibratory finishing. The other mass finishing options, except for simple tumblers, will likely be far more costly, both in initial investment and floor space, and in the ongoing cost of labor, maintenance, and materials.
After deciding on a mass finishing process, you can evaluate the many designs offered to best provide the process in your environment. The improvements in the design of vibratory finishers and in handling systems for centrifugal disc and barrel finishers give you many options. You can now fit the higher energy processes into a production line, or into a cell concept of manufacturing. The limitation for parts handling in a centrifugal disc and centrifugal barrel processes is that parts must be run in individual batches, whereas with vibratory or tumbling processes parts can be run in batches, or continuously run through the chamber. In addition, don’t forget to consider manual handling as discussed earlier.
Many questions come up as to sizing of through-feed machines, both vibratory and tumble styles. Two process variables must be known to determine the size of machine you need. The variables here are the time cycle necessary, and the ratio of parts to total volume. All through-feed machines have an approximate time cycle, so match your requirement on that basis, first. Divide 60 by the number of minutes in the time cycle, and you have the number of machine loads processed in one hour. The machine volume times the loads per hour gives you the gross volume of parts and media per hour. This volume times the ratio of parts to total volume gives the volume of parts per hour. Do not be misled by the overall size of the equipment—productivity of these machines is NOT in direct proportion to machine size.
As already mentioned, some of these processes can be run without liquids and some without media. For example, corncob is a possible media type, and it is always run dry. While cob is most often used as the drying media for already processed parts, it can actually provide some deburring and polishing. Part-on-part processing should be understood by anyone finishing small parts. Part-on-part processing is possible in any of the four basic equipment styles, and it can be used for deburring, polishing, or washing.
The vast majority of mass finishing operations involve liquids and media. The selection of these ingredients has significant bearing on the success of the operation, as well as on the design of equipment you purchase. For example, if the compound can clean and rust inhibit in one operation, you won’t need additional equipment for washing or rust inhibiting. And, when specifying a washer, you may only need one chamber, or at least the solution can cascade from the last stage to the first, resulting in cleaner parts and lower operating costs.
Media. Media serves the purpose of separating parts from each other and interacting with each individual part to do the required finishing. Media is made from a variety of materials such as ceramic, plastic, carbon or stainless steel, wood, leather, corn cob, nut shells, river rock—you get the idea. The choice depends on the desired result. The trick is to reach all the areas of the parts to be contacted, do the desired finishing job, not damage the parts, and not get lodged in passageways or blind holes.
The most frequently used material is ceramic. Extruding the wet clay through a die shaped as a star, circle, or triangle is the common manufacturing method. The extruded length of clay is then cut, or chopped, into the desired length resulting in stars, cylinders, and triangles of specific lengths. The angle of the cut is another variable, and manufacturers offer 0-, 22-, 30-, 45- and 60-degree angle cuts for various purposes. Not all ceramic media is extruded. Some is slip cast and some is pressed into a die. The clay, or slurry, will contain a quantity of abrasive although polishing media often has no abrasive content. The abrasive is a fine grain, usually about 80 – 200 mesh, and made from fused aluminum oxide, silicon carbide, emery, quartz, or some other abrasive. Each manufacturer of ceramic media offers many grades ranging from heavy polishing to heavy cutting compositions, and with bulk weight in the range of 80 –150 lbs/cu ft. Large machines and high-energy machines tend to fracture the media. This has led to the development of special ceramic media compositions that are not as friable as previous products. These tougher compositions have also proven valuable by reducing the fracturing of media that often occurs when finishing very large, heavy parts.
Plastic media is molded, or slip cast, and contains a similar range of abrasives to those used in ceramic media. Because of the forming process, some shapes are unique to plastic media. Plastic media is less dense than ceramic, ranging from 55–75 lbs/cu ft. The advantage to plastic media is that larger sizes can be used without damaging most parts, thereby reducing the incidents of lodging in passageways and blind holes. In some cases, the finish imparted by plastic media is preferable to that of other media products. Plastic generally cuts slower and wears out faster than ceramic.
Carbon steel and stainless steel media is popular for burnishing, polishing, and washing. It is very heavy, weighing up to 300 lbs/cu ft, resulting in high compressive loads and smooth finishes. It is sometimes used for light deburring and edge breaking, often without lodging where other products might lodge. Steels have the advantage of very low attrition rates, although replacement due to losses and carryout will usually amount to 5–10% a year. Filling a machine with steel media is very expensive, and not all mass finishing machines can handle the weight. Another disadvantage is that carbon steel media is subject to corrosion unless properly treated with the compounds. In the category of metallic media, you can consider using nails, screws, brads, tacks, and even your own scrap punchings, or other forms. Some stamping houses and powdered metal parts manufacturers make their own metallic media.
Compounds are the chemicals added to the water in mass finishing. They are used for a multitude of reasons such as to condition the water against hard water deposits, clean the parts, brighten the parts, rust inhibit, degrease, descale, and polish. They can also be in a category known as “chemical accelerators”, designed to greatly improve the speed of surface conditioning through rapid oxidation of the part surface concurrent with the abrasive action of vibratory finishing. We will discuss chemical accelerators following the basic information about more conventional compounds.
Most compounds for mass finishing are liquids that are mixed with tap water in concentrations from about 1% to 10%. Powder compounds are available but are not popular except when used to include abrasives for special purpose applications such as part-on-part finishing. Some hybrids in the form of thick liquids or pastes are also offered as carriers of special abrasives for cutting or polishing operations.
OSHA and EPA regulations concerning operator safety and environmental compatibility have been on the increase. Compounds are banned if they contain any nitrite in combination with certain amines. Limits have been imposed on mercury content in industrial waste streams, so if you use flow-through compounding you should have your compounds checked for mercury level. Many European countries have banned secondary amines, such as diethanolamine (DEA), from metalworking and cleaning compounds.
It is also good to avoid pHs under 4.0 or over 11.0 as they can irritate the skin and nostrils of the operators. Strong primary amines and hydroxides should be avoided when they result in high pHs. Any hazardous ingredients above 1% of the compound, or carcinogens above 0.1%, must be listed on the MSDS. If you have any questions about the ingredients or cautions, contact your supplier’s technical representative. It is good to talk directly with the person responsible for writing the MSDS.
Compound solutions are used in one of three ways: batch loading, such as with many tumbling barrels; flow-through in which a pre-mixed solution is passed through the mass and out to a sanitary drain; and, recirculation through a settling tank, filter, or other treatment, and back through the machine. In the U.S., flow-through compounding is the most common method, and it is arguably the most economical. In Europe, most machines use closed-loop systems, recirculating the compound to avoid any waste effluent. There are many factors to consider in closed loop systems, and the road to this utopia is littered with expensive disasters. In any case, compounds used in closed loop systems must be formulated to control the growth of bacteria, mold, and fungus.
Compound mixing and proportioning systems will benefit any mass finishing system that uses a chemical solution. They come in three basic types: Venturi systems, electric pulse systems, and water motor systems. Each type can provide a range of concentrations and will automatically draw concentrate from the supply drum, mixing it with incoming tap water. The venturi system uses water flow through an expansion chamber to create the suction for drawing concentrate. The concentrate is metered through an orifice selected to give the proper concentration. The electric pulse system uses a diaphragm or piston pump to draw compound from the drum and then forcefully inject it into the water stream going to the process. The water motor is an injection type using incoming water pressure to drive a motor that operates the injector mechanism. There are advantage and disadvantages to each style. The only caution here is that injector types send a stream of raw tap water between injections, and the proportion may be dependent on total flow rate. There are also automatic systems for feeding powder compounds to the system.
Chemical accelerators are used somewhat differently than conventional compounds. They work by rapidly oxidizing the surface of the parts during the vibratory process. The media action only works on the highest peaks on the surface, quickly leveling those peaks as the oxidized layer is removed. The surface is thereby smoothed at an accelerated rate. How smooth the surface will become is related to the time cycle and the media selected. If only rapid removal of a rough surface is desired, such as following a course grinding operation, the media can be a conventional ceramic cutting media. If smoother finishing is desired, the media can be a burnishing ceramic media, or even stainless steel media. There are several options available, and the process can be designed to meet a wide variety of objectives.
Chemical accelerators are necessarily aggressive to the basic metals from which the parts are made. This obviously requires that the process be stopped quickly when the desired results are obtained. So, chemical accelerated finishing differs from most other finishing in that it is at least a two-step operation, and the second step, that of neutralizing the oxidizers must be promptly and accurately accomplished. Generally, there is a final step of burnishing, thoroughly cleaning, and/or rust inhibiting the parts.
Chemical accelerated finishing is less forgiving than other mass finishing processes. The quantity of chemical used is directly related to the surface area of parts being finished. This means that parts loading in the machine is very critical. If it varies, the operator must be trained to calculate the chemical requirement for the variation of surface area being processed. Knowing when the process is done, and knowing if it must be repeated, requires visual observation of the mass as well as pH measurements of the fluid. Also, the neutralizing step must be precise and timely, followed by rust inhibiting that is just as time sensitive. More highly trained and skilled operators are a necessity. Management must understand and follow OSHA and EPA guidelines for wastewater disposal.
Just how much faster is chemical acceleration? That can only be answered on a case-by-case basis. Before settling on this more exotic variation of mass finishing, be absolutely sure that you have maximized the performance of your more conventional methods. As the old saying goes, “the grass may look greener on the other side of the fence, but it still has to be mowed.” Make sure your mower is up to the task.
This overview of mass finishing should help you ask the right questions when selecting equipment and processes. Remember to carefully analyze the variables of each method before making your final selection.
The author has visited hundreds of plants in numerous countries to suggest improvements and cost saving methods of mass finishing. It is rare that such a visit does not result in 30%, or greater, cost savings for the users of such equipment. The following comments are offered in the expectation that they will help you achieve the lowest possible mass finishing costs.
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