My customer is asking for a five to ten thousandths radius (0.005-0.010 inch) on the outside diameter of a round steel forging that is 2.4 inch diameter × 0.375 inch. The part is ground on both sides and on the OD, leaving a sharp edge all around. We also have to preserve, or improve, on a 30 RMS finish. We have been vibratory finishing the parts in medium cutting, three-eighths inch by three-eighths inch ceramic triangles. In one hour running time, we only get a two thousandths break, and we are within the microfinish requirement. We can’t justify a longer time cycle. How much edge break improvement can we expect by switching to one of the faster cutting media, and do you think we will still meet the finish requirement?
Solving your problem should not be difficult, but involves more than just a change in the abrasive capacity of the media. What you are really doing is filing the edges with a small tool, an abrasive triangle. Think of the size, shape, and cut you would select if you were doing the job with a hand file. You would not select a very short file because you would not want short, choppy strokes. A longer file would be more effective, and so it is with the media. A larger side dimension, such as seven-eighths inch will be more effective. The long stroke may even be more significant than the degree of cut.
However, the geometry of a three-eighths inch by three-eighths inch triangle is “blocky.” If you watch this media as it contacts the parts during processing, you will notice that it doesn’t slide well over the edges. It is more of a short hammering action. To get that filing motion, you need a more elongated shape. You will get good filing action with three-eighths inch by seven-eighths inch media. And, because it will run more smoothly, less hammering, you may improve the finish while increasing the edge break. With a longer media, you can also consider cylinders as well as triangles. Generally, I prefer the triangles because they tend to get into more areas than do cylinders, although you did not mention any part configuration that would favor either shape.
Once you decide on a larger size and a less blocky shape, you can consider the cutting ability of the media. You do get a rougher finish from faster cutting media, but in the smaller sizes, such as we are discussing here, you can probably go all the way to the fastest composition and still meet the 30 RMS requirement. Faster cutting media will, of course, wear out faster. You mentioned “medium” cutting media, and if this means it is about half way up the scale of cutting compositions on the market, you should get the radius you want within the one hour. Be sure, however, that your media is not glazed. That is another subject, covered in this column in October 1996. If you do not have that issue, and would like to receive a coy, contact us by phone at (513) 527-8800 or by email.