Mass Finish that Looks like Custom
I recently viewed a custom motorcycle with highly polished stainless steel parts, including the large exhaust pieces. The owner said the parts were tumbled. I examined the parts for the telltale surface marks of mass finishing, but I could see nothing of the sort. All he would say was that it is a proprietary tumbling process used in a custom finishing shop, and that there was no hand finishing. Can you shed any light on this process?
Q. I recently viewed a custom motorcycle with highly polished stainless steel parts, including the large exhaust pieces. Being somewhat familiar with mass finishing—we use it for deburring stampings at work—I was interested in the method used to finish the larger pipes and muffler.
The owner said the parts were tumbled. I examined the parts for the telltale surface marks of mass finishing, but I could see nothing of the sort. Furthermore, there were no directional buffing or polishing marks. I questioned the owner a little more. I suspected that someone had invested hours of hand finishing. All he would say was that it is a proprietary tumbling process used in a custom finishing shop, and that there was no hand finishing. Can you shed any light on this process?
A. My guess is that the parts were processed in a type of tumbling barrel known as a slide hone. This is a very unusual machine, and it is difficult to accurately describe. Picture a ½-inch-thick steel disc about 60 inches in diameter. As many as 12 round or hexagonal tubes, about 10 inches in diameter and 2–4 ft in length, are welded on the disc. They are not, however, welded perpendicular to the face of the disc, but rather at 30° angles. The tube sides and bottom are usually lined with rubber.
The open end of the tubes will have removable doors—one solid, and one with perforations. The disc is mounted on a slow-speed driveshaft and tilted at 30° from vertical. As the disc rotates, each tube tips up and down as well as rolling over and over. Now, imagine the action of a part in one of these tubes. It slides from one end of the tube to the other and rolls over and over very slowly and smoothly. This slow, gentle tumbling barrel can handle long and delicate parts, one to a chamber, to produce a non-directional polished finish when accompanied by the proper burnishing solution and media. In some cases, more than one part is processed in each chamber—an example would be for fine finishing of stainless steel rod stock. The parts are removed by hand. The perforated door is then installed, and the media is rinsed and drained in preparation for the next process.
This is an expensive process, so the technique is practiced in very few finishing shops. But the results, as you reported, speak for themselves.
Here’s a primer on the types of finishes required for equipment used in sanitary applications.
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
Choice of equipment, media and compounds has a major impact on your finishing applications.