We have an eight-cu ft, tub-style vibratory finisher for burnishing stampings prior to decorative plating. The process has worked well for more than 10 years, but earlier this year we discovered that the liner was wearing through on the ends and at the bottom. This was the original rubber liner. Our compound supplier recommended that we re-line with polyurethane for longer life. This was done, and since returning the machine to service, we are having problems getting good burnishing. The mass doesn’t roll well, if at all, and sometimes it actually reverses direction. The sales representative said we may be mixing the compound too rich, but changing concentration did not solve the problem, and the steel media is beginning to corrode at the lower concentration. We contacted the lining shop, and they said that if they had known it was a steel-ball burnishing application, they would have recommended a different liner. What now? Thanks for any advice or help you can provide. C.B.
Ball burnishing vibrators are built differently than regular deburring vibrators. Steel media weighs about 300 lbs/cu ft, and it takes a lot of well directed energy to get it rolling—this is particularly true with steel balls. This extreme weight load requires higher horsepower, heavier springs and usually a highly reinforced, stress-relieved weldment. In addition, because many steel burnishing media are spherical, the bowl lining is usually softer. This allows the balls to sink in slightly and to be propelled more aggressively into the rolling motion inside the bowl. With a hard liner, there will be too much slippage between the media and the liner. When a polyurethane liner is used for ball burnishing, it is made from a softer composition, usually around 65A Durometer—deburring liners are up to 95A.
Be very careful about using lower compound concentrations because rust inhibiting is extremely important with carbon steel media. Make a cup of solution at the proposed concentration and soak some of the burnishing media in it for a few days to test the corrosion inhibiting.
It will be very expensive to replace your lining with a softer one, although this is the best answer. Your supplier may not be willing to make this correction. Some other possibilities are worth trying. A less lubricious compound, perhaps in combination with a ceramic burnishing media, may meet your requirement although the deep luster of ball burnishing may not be matched.
Another possibility is to rough up the surface of the liner with a disc sander or wire wheel. This may work for a month or two and have to be repeated. Changing from balls to ballcones, or any non-spherical media, may also solve the problem. This answer, however, will be almost as expensive as a new lining.
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