High-Energy Mass Finishing
A centrifugal-disc machine will run large quantities of smaller and tougher parts, give them a low-Ra finish in a relatively short cycle time, and automatically unload the parts and separate them from the media.
Q. We have large quantities of smaller, machined medical bone implants made of 316 stainless steel that require an 8-Ra finish prior to electro-polishing. We are considering purchasing a vibratory machine or a high-energy mass finishing machine to obtain this finish. What do you recommend?— J.S.
A. Our recommendation is a centrifugal-disc (CD) high-energy machine, which will run large quantities of smaller and tougher parts, give them a low-Ra finish in a relatively short cycle time (1-2 hrs,), and automatically unload the parts and separate them from the media.
The CD utilizes the energy of a constant- or variable-speed (100-200 rpm) rotating disc at the bottom of a bowl container. The rotating disc accelerates the media out to a stationary sidewall, then the media de-accelerates as it moves up the wall and re-accelerates down to the center of the disc. This continued flow of media acceleration and de-acceleration produces energy 7-to-15 times that of vibratory machines. The CD media flow also produces better finishes than the hammering effect of a vibratory machine. And the CD method is one of the few high-energy processes that can be automated.
In short, the advantages of the CD process are its high-energy, better finish capabilities, ease of automation, flow-through water/compound system, and quick time cycles for cellular manufacturing. Disadvantages include wear life of the ring and rotor lining (with relining cost of approximately $1/hr), and its inability to run large quantities of large, heavier parts without part damage.
Centrifugal-disc finishing is perhaps the most versatile and production-friendly, high-energy system produced. Machine sizes range from 1 to 10 cu. ft. Examples of parts that are typically run in CD machines include medical implants, bone screws, aircraft fasteners, ballscrews, O-rings, seals, fuel injector parts, blanks for uncirculated and proof-minted coins, machined parts, and jewelry.
The correct peripheral speed is an important consideration in getting the right results from your buffing operation. A buff that is turning too fast or too slow may result in damage to the buff or to the workpiece.
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
Here’s a primer on the types of finishes required for equipment used in sanitary applications.