Nicking and a Slow Production Rate
We have two problems: First, there is some nicking on the parts, and we have always wanted to reduce that effect; second, we are not satisfied with the production rate of 40 parts/hr per machine. Any suggestions?
Q. We make aluminum die cast pieces that are 4 inches wide, 14 inches long, and ½ inch thick. We burnish and deburr these pieces by tumbling them in stainless steel ball cone media. The tumbling barrels are 42 inches in diameter and 50 inches long and have two compartments. We load the barrels to a level of 60% and then add 20 work pieces in each compartment. We fill the barrels with a premixed solution until all the parts are covered and then rotate at 8 rpm. The burnish effect is excellent, attaining good color and a smooth surface in 40 min.
We have two problems: First, there is some nicking on the parts, and we have always wanted to reduce that effect; second, we are not satisfied with the production rate of 40 parts/hr per machine. It takes eight tumbling barrels to net our 300 parts/hr requirement. We have tried vibratory finishing, but the luster is not acceptable. Fixtured spin finishing in dry media was successful; the capital investment, however, was considerably more than we can justify. Any suggestions? H.D.
A. My first thought is to consider fixture processing in the existing equipment. This is done by attaching holding devices to the inside of the doors. The parts will then protrude into the interior of the barrel and, instead of sliding down the face of the media during tumbling, the media will pour across the parts almost continuously as the barrel rotates. Higher speed can be used, because part-on-part impingement is of no concern. Cycle time may be reduced by 25–35%. You may be able to fixture more than twenty pieces inside each door; this will greatly increase production. A lower volume of compound can be used. You will have to experiment with media levels based on the fixture design. It is usually necessary to change direction of the barrel rotation about half way through the process to get uniform finishing on all sides.
If this works for you, the only equipment expense will be to buy some extra doors and build the fixtures. If your motors are not reversible, you can turn the doors around for the second half of the process. If you don’t have variable speed you can just settle for the speed as it is, although cycle time may not be optimized.
My second thought is that you should consider a little less luster and go with vibratory finishing. Many companies spend excessive dollars on part appearances that are generally sacrificed shortly after the part is put in service. You did not state the final use of these parts, but be sure you really need that high luster.
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Surface finish types for commercially supplied stainless steel sheet are detailed in various standards. ASTM A480-12 and EN10088-2 are two; BS 1449-2 (1983) is still available, although no longer active. These standards are very similar in that they define eight grades of surface finish for stainless steel. Grade 7 is “buff polished,” while the highest polish—the so-called mirror polish—is designated Grade 8