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.
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.
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.
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. 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 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.
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.