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Mass-finishing fundamentals.

Is there and "old Joe" still filing away in some dark corner of your shop? You're not alone. The average US metalworking manufacturer invests less than 2 percent of available capital for deburring and surface-finishing equipment. Yet, these operations typically represent more than 10 percent of a part's manufacturing cost. In fact, it's estimated that the cost for manual deburring exceeds $2 billion annually.

The problem is that hand finishing, and in particular manual deburring, adversely impacts workpiece production two ways: part quality varies and manufacturing cost increases. Mass-finishing processes can help on both counts.

For example, a consumer-products company that makes snowmobiles, lawn mowers, and garden tractors once did all of its deburring by hand at an annual direct-labor cost of $9000. When mass-finishing equipment was added, this fell to $2130. Regarding quality improvement, jet engines run more efficiently because mass finishing consistently produces better surfaces on stators and turbine blades (see box). And some diesel engines and automotive components are now 20 percent more efficient because mass finishing generates properly radiused edges and better surface finishes.

"Some manufacturing engineers think mass finishing is finishing massive parts. I don't," says J Bernard Hignett, vice president at the Harper Co, East Hartford, CT. "I define it, and so does SME, as loading parts into a container filled with a massive media and then introducing motion so the media rub against the components." Refer to Figure I for a comparison of mass-finishing process. Tumbling around

"Tumbling is a mass-finishing process used as far back as the Middle Ages," points out John Rampe, president of Rampe Mfg, Cleveland, OH. "At the time, it involved rolling barrels around a courtyard. The barrels, filled with metal parts and abrasive stones, were sometimes pushed for days until desired radii and surface finishes were achieved."

Even though other mass-finishing techniques have made advances, tumbling (or barrel finishing) is still widely used today. In the process, parts slide against abrasive media along with a compound/water solution. (Note: Some parts finish best when run dry or part-on-part.)

Conventional barrels are multi-sided and are mounted horizontally at a 45-degree angle. "For the most efficient finishing," advises Rampe, "a barrel should be loaded with 50 percent to 60 percent of its total capacity. As the vessel rotates, the multi-sided design prevents the load from sliding until gravity causes the parts and media to tumble down. Finishing action occurs when parts move down the front of the load. Only 10 to 20 percent of a load's volume is being finished at any given moment; the rest is stationary, waiting for the barrel to rotate."

Cycle time for barrel finishing generally ranges from less than 1 hr to 12 hr or more, which doesn't compare favorably with its chief competitor, vibratory finishing. Normally, whatever can be done by tumbling can be finished four times faster by a vibratory machine. Vibratory finishing

The concept of vibratory finishing was developed in Germany during WW II and brought to this country shortly thereafter. The equipment comes in two basic designs--bowl machines and tub machines. Of the two, many more vibratory bowls are in use.

Vibratory tubs are best for finishing large or long parts, and also are used extensively for in-line continuous processing of parts.

"The firt vibratory machines used in the US were the tub machines," reflects Rampe. "They had a single eccentric shaft mounted on the underside of the tub. The elliptical vibratory motion caused the media and parts to work against each other.

"With the advent of the dual-shaft vibratory machine (a shaft mounted on each side of the centerline of the mass), the orbit became circular; and parts and media moved in a continuous slurry action. Because of the round tub and circular orbit, the forces generated were equal throughout the vessel, tereby giving a very aggressive finishing action at only an 1/8" amplitude.

"The low amplitude didn't hinder aggressive finishing," he continues, "but did guarantee minimal impacting of parts against the media and each other. High impacting, by the way, is a main source of impingement or nicks in parts."

According to Rampe, the dual-shaft tub machine, because it can develop strong forces at low amplitude, is ideal for processing parts with steel media. Steel media (weighing 300 lb/cu ft) requires great energy to develop good finishing action. If this energy is developed through a high vibratory amplitude, undesirable part impingement results.

The finishing action of a vibratory bowl is similar to the tub vibrator. An eccentric shaft mounted vertically in the center of the bowl column develops a vibratory motion, causing media and parts to roll in a smooth, continuous motion. This action also produces a third dimension of motion, where media move radially around the bowl's circumference.

Vibratory finishing can remove 0.001" to 0.002" of metal during a finishing cycle, abrading the same amount of material on all surfaces.

Besides faster finishing cycles, a major difference between vibratory finishing and tumbling concerns ability to abrade recessed surfaces. "Even though a part running in a barrel has media jammed in every cavity, there is no internal finishing action," says Rampe. "However, with a vibratory machine, media and parts are in constant motion, working against each other externally as well as internally." High-energy finishing

"Unquestionably, most current activity in mass finishing is with the standalone vibratory units," says Harper's Hignett. "Nevertheless, an increasing proportion is moving to high-energy mass-finishing processes, such as centrifugal-disk and centrifugal-barrel finishing." See Figures 2 and 3.

"Centrifugal disk uses a ball-shaped container with a disk at the base spinning at high speed," he points out. "This forces everything in contact with the disk outwards with considerable force; anything above is pushed out with slightly reduced force. Media and parts in contact with the vessel's wall subsequently are pushed up. Pressure of the media against a part is substantially greater than can be achieved by any vibratory process.

"Centrifugal-disk finishing is more than 10 times faster than vibratory finishing," Hignett emphasizes. "Also, the process better controls generation of radii, and there is less part-on-part impingement. Unfortunately, a portion of the action in a vibratory machine isn't abrasive rubbing, but rather tapping. Such impinging can adversely affect surface finish.

"Sometimes, though, impingement doesn't matter. When this is the case, and if cycle time is less than one hour, vibratory finishing is usually more economical. But, as finer finishes are specified by designers, and cycle times increase, high-energy finishing will become preferred. Expect high-energy mass finishing to gain market share over the next 10 years." Media and compounds

Talking about mass finishing without looking at media is like examining a razor without considering the blades. Media carry out the desired work (i.e., deburring, polishing, burnishing, descaling, etc.). They also suspend and separate parts in the work chamber.

Rampe comments, "Media can range from loose, natural, random-shaped abrasives to preformed aluminum oxide or silicon carbide. The bonding agents holding everything together can be either a light resin or a hard, heavy ceramic.

"The right medium for a finishing process gives results at the most economical price. In some cases, river rock (at $5/ton) is most economical: in others, a ceramic, fast-cut, preform medium (at $1/lb) is best." (Refer to Tooling & Production's May '81 issue for details on media selection.)

Hignett adds, "In vibratory equipment, media density (weight) impacts how aggressively material is removed from workpieces. On the other hand, with high-energy mass finishing, media aggressiveness is controlled by varying input energy, not by changing the material."

"Success or failure of the quality of a mass-finishing operation greatly depends on the compound used in conjunction with the media," believes Rampe. "Water can be used alone, but a compound in solution usually is more economical and produces a high-quality part. A compound's principal function is keeping both the parts and media clean, so cutting is faster and the medium lasts longer."

Secondary functions include that a compound act as a lubricant and coolant, and provide rust protection during and after a finishing cycle.

Rampe concludes, "What can be processed dry can be done better wet. Adding a compound may slightly increase your processing costs, but it dramatically improves part quality."

For more information on mass finishing from Harper Co, circle E52. For more from Rampe Mfg, circle E53.
COPYRIGHT 1984 Nelson Publishing
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Author:Coleman, John R.
Publication:Tooling & Production
Date:May 1, 1984
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