Vibratory Finishing
Although vibratory finishing techniques are similar to barrel finishing and usually involve the same components (water, compound and media), the equipment is different. The vibratory finishing process creates smoother surfaces and can abrade inside deep cavities or tubular parts. A vibratory finishing process can finish a variety of parts (e.g. fragile or extra-large) which the barrel tumbler is incapable of finishing. Vibratory tumblers are safer for delicate parts or large parts that could be damaged in a barrel tumbler, since the part only moves a fraction of an inch per stroke, as opposed to sliding the full diameter of a barrel tumbler. Vibratory machines are easier to load and unload, will abrade or polish in less time, keep parts cleaner and maintain a better color. They are also more easily automated or semi-automated, can process more parts than a barrel tumbler of the same size, can recirculate water and compound, cause less media wear in proportion to the amount of work performed and permit fast inspection of parts. Listed below are a variety of factors that make for a successful vibratory finishing process.
Media: Use the largest possible media for fast abrasive action and best circulation of parts. Of course, the problems of lodging, separation and damaging parts must be included in media selection. Small stones (3/8″ or less) should not be run at speeds lower than 1600 RPM. Lower speeds will separate parts from media. Plastic media abrades faster when a minimum amount of water is employed and produces a better finish as the quantity of water is increased.
Compounds: Plastic media usually requires only a cleaning compound. Ceramic media requires occasional runs with abrasive compound to remove glaze. When using a heavy abrasive compound, do not recirculate water.
Water: Use the minimum amount of water possible to achieve the best cutting action. As the water flow increases, the cutting action is nil. At this point, the media will burnish (shine) the parts.
If two different types of metals are being processed in the same tub, a heavy flow of water will prevent contamination. The selection of cleaning chemistry will also help facilitate multi-metal procession results. Small media generally will hold large amounts of water after it has been running for some time, reducing circulation. (When this occurs, turn off the machine and the pump for a few minutes to permit excess water to drain off. Restart without the pump.) Abrasive action increases as water use decreases. However, surface finishes become worse. Large amounts of water are generally necessary for polishing.
Frequency and Amplitude: Bulky or fragile parts should run at fast speeds, but low amplitudes. This is also true if a low part to media ratio is employed. Fast speeds and small amplitudes produce the best surface finishes and should be used for polishing operations or internal deburring. Increasing the amplitude of the vibrator improves circulation of the parts and creates a more abrasive action to speed the cutting rate.
Vibratory Load: Vibratory finishing systems work best when they are 75-90% full. A volume ratio of three parts media to one part metal will prevent injury to critical parts. A “rough” job can be run on parts without using media. Thin flat pieces can tend to stick together. Adding very small media will help keep them separated.
Speed: Very few vibrators have a variable speed control. The usual frequency is 1700 vibrations per second. Once the speed drops much below this, the parts can sink to the bottom.
Media are designed for many things:
Cut
Media which cut can remove burrs and can smooth surfaces. As a carrier of abrasive grain, the large medium pieces effectively increase the impact force of the abrasive on the metal part to be cut, thereby improving the efficiency of the abrasive. Cutting media develop dull, matte surfaces.
Luster
Some grades of medium are designed to promote luster on the surface of metal parts. These products are generally non-abrasive or have a very low degree of abrasiveness. They deburr by peening, rather than actually removing the burr. Media selection, therefore, will control the degree of surface luster, making the part bright and shiny or developing a very matte, dull surface characterized by a completely random scratch pattern, or anything in between.
Part separation
A very important function of the medium is to separate parts during the deburring, cutting, surface improving or burnishing operations. The media: parts volume ratio is normally used to control the amount of part-on-part contact which will occur in a vibratory or tumble finishing operation. At low ratios, considerable part-on-part contact occurs, while at higher ratios part-on-part contact is limited.
Surface scrubbing
Media have the unique ability to scrub surfaces and physically assist compounds in their cleaning function. Both abrasive and non-abrasive media are effective in this. They can remove organic soils, scale, and other inorganic residues. Media come in a wide range of materials in order to fulfill various needs.
Aluminum media
Aluminum media are typically cast parts and are available in a wide variety of shapes and sizes. Aluminum scrubs parts and can work in conjunction with cleaning compounds to clean parts. Since aluminum is fairly nonabrasive it tends to remove surface impurities without affecting the part’s surface qualities. Its cost is typically higher than other cast media. Wear rates are lower than ceramic but higher than steel media.
Preformed ceramic media
Ceramic media are manufactured by mixing clay-like materials and water with abrasives, forming the mud into shapes, drying the shapes, and firing them at high temperatures to vitrify the binder. Many of these binders are porcelain-like in nature. Variability in these products occur both with the type of binder used, firing temperatures, the amount, size and type of abrasive grains they contain, and their uniformity of firing. This type of media today is the general workhorse of mass finishing systems and is the type of medium generally used, because of its availability in a variety of shapes and sizes, low cost, and low wear rate.
Preformed resin-bonded media
Plastic or resin-bonded media utilize a wider range of abrasive types and sizes than preformed ceramics. The most popular grades are those using quartz as an abrasive. Aluminum oxide, silicon carbide and other abrasives are also used. Usually, low-cost polyester resins are employed as the binder and the various shapes are produced by casting. Resin bonded media is good for preparing a metal surface for plating.
Steel
Case hardened, stress-relieved steel preformed shapes are available in a variety of sizes and configurations. Balls, balls with flat spots, ovoids (footballs), diagonally cut wire similar to angle-cut cylinders, ball cones and cones (both of which are different from the general concept of cones) and pins are the most commonly used. Steel media weigh approximately 300 pounds per cubic foot and are expensive for initial installation, but, because of their minimal attrition rate and extreme cleanliness, are being more widely used for light deburring applications and cleaning. Compounds are available to keep steel burnishing media clean and bright for extended periods.
Synthetic random-shaped media
The most popular synthetic random media is fused aluminum oxide, which is available in a number of grades. The more loosely bound, coarse-grained materials are characterized by fast cut and high depreciation rates. Because of the dark color of fused aluminum oxide, the soil generated by this material is excessive in many applications. Fine-grained fused aluminum oxide is generally employed for burnishing and in this respect is unexcelled in many applications with the possible exception of steel. Where some light cutting is required, fine-grained aluminum oxide can develop a better luster on stainless steels and other hard surfaces than can be achieved with steel burnishing media.
Natural random-shaped media
River rock, granite, quartz, limestone, emery and other naturally occurring abrasive materials are also used in vibratory and tumble finishing applications. In general, these media are not very efficient in vibratory equipment because of their high attrition rates.
Cob meal, walnut-shell flour, and related materials
These are used for drying applications because of the natural ability of these materials to absorb water from metal surfaces. These can also be blended with abrasives and used for fine-polishing applications in vibratory, barrel, or spindle finishing equipment.
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