Much has been written about media selection. Usually, there is a caveat that you must be able to separate the parts from the media but not much advice on how to do it. That separation process is the subject of this month's column. The following are possible techniques: hand separation; magnetic separation; screen separation; and the possibility of not using media at all.
When the parts and media are about the same size, hand picking may be the only option. With tumbling barrels-where this whole mass finishing thing began-hand separation generally requires dumping the load onto a screen, or maybe just onto the floor, and picking the parts out of the media. Open tumblers can be handpicked without dumping them, but this is seldom practical.
Vibratory finishing definitely allows more efficient handpicking than a tumbling barrel. While the mass is in motion, the operator can actually reach below the surface of the media and retrieve parts. Handpicking is done with both tub type and round finishing machines. The round design is generally favored if handpicking is to be used.
Is it efficient? Here is an example. A company is pre-plate finishing steel parts about the size and shape of a golf club head. Plating will highlight any nicking of the parts, and the parts will be scrap. In a 10-ft round vibrator, properly adjusted for the operation, as many as 400 of these parts can be finished. But, getting them out any way other than handpicking will result in nicks on the parts. The operator can remove all 400 pieces and individually place them in trays in about 15 min.
Handpicking of very large pieces is also the method of choice, not just because the parts might be damaged on a screen deck, but also because they may be too heavy to put across a screen deck. Overhead hoists with special apparatus for picking large parts may assist the operator.
If the parts are ferrous, a hand-held magnet can help catch small parts. This can be used with centrifugal disc, open barrel and vibratory machines. Magnetic separation can also be automated. In one method, the load is discharged onto a non-magnetic surface and passed under a rotating drum magnet that picks out the parts and places them on a conveyor. In another method, a drum magnet is rotated above the vibrating parts and media, pulling out parts and transferring them to a conveyor. Some media may be caught with the parts, necessitating some hand sorting, or the addition of a second magnetic hand-off. Magnetic separation can also be used for sorting ferrous and nonferrous parts. It is good to degauss the parts before the next operation.
Screen separation is by far the most common method. The basic requirement is that the parts and media be substantially different in size. Ideally, the media is smaller than the parts and falls through a screen while parts stay on top. This situation allows easy internal separation in bowl machines, or across mechanical screen decks in external systems. Reverse separation is when the parts fall through and the media stays on the screen. In some cases, two or more screens are employed to first separate parts and media, and then to separate undersized media to reduce lodging problems. Reverse separation as well as multiple screen separation can be designed for both the internal and external systems. And, these can be combined with magnetic separation if necessary.
The best case is processing without media, which is possible with some smaller parts. No separation is required.
When selecting mass finishing as a method, planning for separation of the parts and media is important.