We make bronze castings that have two flat ground surfaces and several drilled and tapped holes. The castings weight about 6.5 lb each. We vibratory finish to debur, using 1-inch plastic cone media. We went to plastic media because the 1 1/8 x 5/16 inch ceramic triangles we used before were denting the flat ground surfaces. Now, the problem is that parts bump into each other and create larger dents than we had before. Our machine is a 10 cu ft bowl type machine. Is there anything we can do to prevent denting and still use the vibratory process, preferably with the same machine?
You have a good chance of accomplishing this. The two most common causes of dented parts are parts banging together, and media getting caught between the liner and the parts. A less common reason is very large, hard media hammering the parts. Your ceramic media is approaching the size that can damage the parts. In all cases, one of the solutions to the problem is to adjust the machine settings for heavy parts. (This is something you can do with a bowl machine that you cannot do with a tub style. This explains my bias in favor of bowl designs. And, not all bowl machines are easily adjustable—some have nearly inaccessible bottom weights.)
There is another answer, available to both bowl and tub styles, and that is fixturing. This method was discussed in this column in April 2000. I won't repeat all that information now. If you have a tub style machine, you might as well refer to that article, because you don't have the machine setting options we are going to discuss.
There are top weights and bottom weights on your bowl machine. The angle between these weights can be adjusted. Set that angle to 45 degrees, with the bottom weight leading the top weights as the motor rotates. This is called the "lead angle." (Be sure before starting this experiment that the motor is rotating in the direction shown in the manual.) This setting will give a very smooth roll, and only moderate travel around the bowl. Next, increase the bottom weights so that the total weight on the bottom is three to four times as much as the total top weight. In doing this, you may first want to reduce the amount of top weight so that four times that amount does not exceed the capacity for weight on the bottom. Different brands have different arrangements for distributing the weight, but get as close to these weight ratios as you can. This abundance of weight on the bottom provides more lift in the bowl, helping to float the parts, thus minimizing contact pressure against the bottom.
Now, run the machine with one part in it. Even with the light plastic media there may be enough buoyancy to keep the part from the bottom. You will also notice that there is very little forward travel. This will reduce part-on-part damage. Put more parts in the machine and see the difference.
Next, change back to the ceramic media and observe the improved buoyancy. My guess is that you will favor the ceramic media. Your ceramic may, however, be too large and still hammer the parts. If that is the case, look to smaller sizes, and perhaps a different shape, such as a star.
If this works better, but parts are still damaging each other, you can consider fewer parts per load, or bowl dividers. After that, you can look into steel media, and finally, fixture processing.
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