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.