Is there any rule of thumb for how many parts to put in a vibratory machine? M.N.
The short answer is “as many as you can put in without the parts damaging each other.” There is a more complete answer, depending on several conditions. Is it a bowl or a tub? What media, if any, is being used? Are you filling the machine to its capacity? Do you have internal unloading? What is the reason you are mass finishing – for example, to debur, or burnish? Do you have enough volume of parts that you can fill the machine to capacity, load after load?
A well-designed vibratory machine is most efficient when loaded to capacity. (I say well-designed because there are some light duty machines that hardly have enough energy to turn over a full mass of parts and media – especially heavy media.) If you don’t have enough parts to make a normal load, you benefit from putting in more media to have the machine full. Deeper mass means shorter time cycles, so you get paid back for using the extra media. A lot has been written about the proper ratio of parts to media, with the optimum supposedly about 1:4. That rule of thumb goes out the window in many situations, and as long as there is enough media to do the work on the parts, and to keep them from damaging each other, the higher the part to media ratio, the lower your cost per part – unless the parts are damaged. With an internal unloader, you have to have enough media to carry the last few parts onto the screen deck.
It is common to find machines running less than half full. In that condition, the vibratory action is rough, and parts are damaged by bouncing hard against each other and against the media on the bottom. The number of parts per load is restricted, and processing costs are way out of line. The optimum level of the mass is very close to the top of the machine. With internal unloading it is even more important to have a full machine.
So, given all those considerations, you can put in as many parts as possible, up to the point that causes part-on-part damage.
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