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. Those are the subjects of this article. Other processes such as tumble-blast finishing, magnetic finishing, drag and spin finishing may also fit under the mass finishing umbrella, but this article does not cover them.

The evolution of mass finishing migrated through the four basic processes in this order: Tumbling barrels that date back to biblical times; centrifugal tumbling barrels that sped up the process; vibratory finishers; and, finally, centrifugal disc finishers. Within each style of equipment, some automation of the media separation and/or the parts traffic through the machines is available.

The successful 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. The 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 relief has been measured.

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. 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 the mass finishing 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. Relatively high operating costs continue to discourage some prospective users.

Process Descriptions
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 are either open or closed work chambers. They provide a rolling and tumbling action of the workload. The speed of rotation is regulated to give peripheral velocities ranging from 12–40 feet/minute. Lower speeds are for polishing, and higher speeds are for more aggressive deburring and edge breaking.

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.

Figure 1. Automated batch tumbler. Courtesy of Ransohoff

Tumbler sizes range from tabletop machines with less than a quart capacity, to very large machines that process more than 1,000 lbs 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. Figure 1 shows an automatic batch tumbler. This machine processes either with or without media.

Media, water, compound, and parts are the usual ingredients. The 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 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. In most machines, the large wheel rotates vertically; less popular horizontal models are also available. The small baskets rotate in the opposite direction to the large wheel, causing their contents to rub together under very high pressure—15 to 20 times the force of gravity. This gives finishing similar to that of a conventional tumbler but 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. Centrifugal tumbling is, however, a batch operation.

Vibratory Finishing uses 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 900–3,600 vibrations a minute, with amplitude ranging from 1–2 mm to as much as 10–15 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 of vibratory finishing 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.

Figure 2. Large tub vibrator holds 22,500 lbs media. Courtesy of Ultramatic.

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 and powered by a single 60-hP motor. Figure 2 shows a large vibratory finishing machine.

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-minute) 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.

Figure 3. Multi-Pass Design. Courtesy of VibraFinish.

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. Figure 3 is an illustration of the latter design.

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.

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 minutes. The multi-pass machines offer time cycles as long as two hours. 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.

Figure 4. Tandem centrugal discs. Courtesy of Rossler.

Centrifugal disc finishers are batch processing machines, as are the centrifugal barrels. The work chamber is a vertical cylinder, although it may have curved sides. The sides are stationary. 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 and forward. When reaching the side of the chamber the mass is pushed up, until everything falls back toward the bottom to repeat the trip. The sliding and impinging action of parts and media on each other create the deburring and polishing action.

Centrifuged disc finishing is particularly suited to smaller parts and can even be done part-on-part, with or without a liquid. Production rates can be attractive due to shorter time cycles than those achieved with 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. Figure 4 shows tandem machines with automated parts separation and media return, with a common cob dryer.

Process Selection
Start your selection by having parts processed in vibratory finishing will establish a base line cycle time and provide an example of the finish. 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 four 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 2–4 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. Moreover, there are some circumstances 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 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. In recent years, equipment manufacturers have been very innovative, particularly in the design of vibratory machines and in material handling systems for centrifugal disc machines. Either of these processes can be automated and integrated 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 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. Two process variables must be known to determine the size of machine you need. Those variables are cycle time and the ratio of parts to total volume. Cycle time should be the primary consideration. Divide 60 by cycle time (in minutes), and you have the number of machine loads processed in one hour.

Machine volume times 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.

Compounds and Media
As already mentioned, some of these processes can be run without liquids and some without media. For example, corncob is a frequent 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 is often overlooked as a major cost saving method and 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. This will give you an overview of the products offered, and some selection criteria to help you arrive at an informed decision for purchasing the best process.

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. 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 to 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 80–135 pounds per cu ft.

Large machines and high-energy machines tend to fracture ceramic media. This has led to the development of special compositions that are not as friable as previous products.

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 per cu ft.

The advantage of 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.

Steel and stainless steel media is popular for burnishing and polishing. It is very heavy, weighing up to 300 lbs per 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 10%.

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 manufacturers of powder make their own metallic media.

Compounds for mass finishing are usually liquids that are mixed with tap water in concentrations from about 1–10%. 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.

The performance qualities of compounds may include water conditioning, cleaning, metal brightening, rust inhibiting, degreasing, descaling, and polishing. Compounds used in closed-loop compound systems should also include biocides and bio-stabilizers to prevent bacteria, mold, and fungus from growing in the reservoirs.

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 diethanolamine (DEA) from metalworking and cleaning compounds. It is also good to avoid pH under 4.0 or over 11.0, as they can irritate the skin and nostrils. Strong primary amines and hydroxides should be avoided when they result in high pH. Any hazardous ingredients above 1% of the compound, or carcinogens above 0.1%, must be listed on the Material Safety Data Sheets (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.

Figure 5. Compound system. Courtesy of Markee.

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 United States, 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.

Compound mixing and proportioning systems (Figure 5) will benefit any mass finishing system that uses a chemical solution. They come in three basic types. Those are 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 solution.

Electric pulse systems use 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 system using incoming water pressure to drive a motor that operates the injector mechanism.

There are advantages 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. Figure 5 shows a compound dispensing system that uses the venturi proportioner to keep a reservoir filled with mixed solution ready for single or multiple machines. There are also automatic systems for feeding powder compounds to the system.

This overview of mass finishing should help you ask the right questions when selecting equipment. Remember to exhaust the possible variables of each method before selecting any one over the others. PFD