Mass Finishing: What is it All About?

Article From: Products Finishing, from The Markee Corp.

Posted on: 2/21/2011

The choices in equipment, media and compounds for your applications.

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Discharge end of a FloThrough vibratory finisher. Photo courtesy of UltraMatic.

Extended-time cycle bowl machine. Photo courtesy of Rosler.

This tub-style batch machine is designed for very long parts. Photo courtesy of Rosler.

Compact cob dryer. Photo courtesy of UltraMatic.

Ceramic media

Ceramic media is available in many shapes. Photo courtesy of UM Abrasives.

These plastic media illustrate several of the shapes not available with ceramics. Photo courtesy of VibraFinish.

A bowl machine with removable center column. Photo courtesy of SWECO.

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, and drag and spin finishing also fit loosely under the mass finishing umbrella.

First, an Overview
Successful equipment changes during the 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 11 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 (120–150 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. 

So Which Process is Best?
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 to 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 from 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 3,600 vibrations/min, with an amplitude ranging from 1 or 2 mm to as much as 10 or 15 mm. 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 10 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 2 hours. Vibratory finishers range in sizes from tabletop 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.

Making a Selection
If you start your selection by having parts processed in vibratory finishing, this will establish a baseline time cycle as well as provide 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 hours—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 hours) and centrifugal barrel finishing if you need more severe edge breaking or if vibratory time cycles exceed 4 hours. 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 centrifugal disc and centrifugal barrel processes is that parts must be run in individual batches, whereas with vibratory or tumbling processes parts can be either 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 multiplied by the loads per hour gives you the gross volume of parts and media per hour. This volume multiplied by 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, corn cob 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. Imagine the increased efficiency of a chamber full of parts and no media, and you realize that this process is worth learning. In one case, two 10-ft bowl vibrators replaced four centrifugal disc machines with a substantial savings in operating cost and floor space.

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 inhibit rust in one operation, you won’t need additional equipment for washing or inhibiting rust. 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 to 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.

The other major category of media is generally referred to as plastic media, and there are two types: resin-bonded polyester plastic and urea methanal. The first is generally referred to as “resin bond plastic” and the latter as “synthetic” media. Both are lumped together as “plastic" in general references. It is useful, however, to know which one is intended.

Plastics are made by the slip-casting method, and ceramics (most of them) are extruded. Slip casting can be compared with making ice cubes in a plastic tray. The liquid state is poured, or slipped, into cavities in a flexible mold. After the product is cured, the mold is flexed and the pieces fall out. As described earlier, ceramics are usually extruded products with certain limitations on their geometry. Most shapes can be made by either process. There are, however, some shapes unique to each method. For example, plastic can be made in bowtie and five-sided pyramid shapes while ceramics can be made in angle-cut cylinders. Cones are most easily produced by slip casting. Many years ago, slip-cast ceramic cones and tetrahedrons were being promoted, but they may now be extinct.

The two types of plastic media each have their advantages. The largest manufacturer of plastic media makes about equal quantities of resin-bonded and synthetic. Resin bond can yield a somewhat smoother surface, resulting in very bright plating and very good anodizing. The synthetics produce a shinier finish on stainless steel and other metals that will not be plated. Also, very thin or delicate parts are treated more gently in small synthetic media with its 50- to 55-lb/cu fu bulk density.

Both types of plastic media can be made with added abrasives using the same abrasive technology. Most commonly used abrasives are silica, zircon and variations of aluminum oxides. The size of the abrasive grains and the quantity used in the media are variables available to meet a wide variety of applications.

Resin bonds produce a foamy residue that can dry quickly on parts and become  very difficult to remove. This foam also floats from many hours in the waste-settling tanks, whereas synthetic foam settles within minutes and there is much less foam volume. With either media, it is very helpful to use non-foaming compounds containing rinse aids.

Three areas exist in which plastics are really better than ceramic: When very large media pieces are necessary to prevent lodging but ceramic pieces would damage the parts; when very light and delicate parts would be damaged by the weight of the media; and when very smooth, nick-free surfaces are required, such as for pre-plate or pre-anodizing finishes, or for a shiny final finish. In applications that require large media to prevent lodging, it is likely that large ceramic media will put nicks and dents on ground surfaces and edges, and this condition is often avoided by using large plastic media. This is particularly true with softer metals, but can also apply to cast iron, steel and other hard alloys. Two examples of parts requiring a fine finish are medical devices and highly decorative motorcycle parts.

Another category of media is carbon steel and stainless steel media. These  are popular for light deburring, burnishing, polishing and washing. They are very heavy, weighing as much as 300 lbs/cu ft, resulting in high compressive loads and smooth finishes. They are sometimes used for light deburring and edge breaking, often without lodging where other products might lodge. In the first 10 or 15 min of processing in steel media, sharp edges and light burrs may be broken off or hammered smooth, challenging the performance of abrasive media. Steels also have the advantage of very low attrition rates, although replacement due to losses and carryout will usually amount to 5–10 percent 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.

All About Compounds
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 percent. 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.

Compounds used for deburring should have a very low lubricity and should keep abrasive media clean and sharp. Compounds for burnishing and brightening parts should be very lubricious to protect and shine the part surfaces. You can feel the compound quality by pressing and rubbing it between your fingers. 

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 lower than 4.0 or higher than 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 higher than 1 percent of the compound or carcinogens higher than 0.1 percent 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
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.

Cost-Saving Tips
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 percent or greater cost savings for the users of such equipment. 

The following suggestions are offered in the expectation that they will help you achieve the lowest possible mass finishing costs:

 

  • Clearly define the degree of deburring and/or surface finishing you actually need to satisfy your customer. It is not uncommon to remove a part halfway through the normal cycle only to find that it is already acceptable. 
  • Measure the flow rate of the compound. In flow-through systems the unnecessary cost of high flow rates is easy to calculate. We frequently find the flow rate to be two or three times as much as needed. In closed systems, a high flow rate may not waste compound, but it will result in longer time cycles due to excess cushioning of the workpieces, and glazing of the media.
  • Fill the machines to capacity whenever possible. We see two- and three-shift operations at times when all the work could have been run in one shift had the machines been running at capacity.
  • Use the correct media (size, geometry, weight, abrasive content) for your parts. The wrong media can easily double the time cycles. It is a good practice to get two or more opinions on media selection from experienced vendors. Do not overlook opportunities to run part-on-part.
  • Use compounds that provide adequate corrosion protection to avoid the extra cost and time of applying rust inhibitors after finishing.
  • If you are deburring with a “slippery” compound, consider a lower-lubricity compound. You may then be able to use a longer-life media and still have deburring and time cycles. Benefits include lower flow rates, longer media life and cleaner parts.


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