How can we eliminate damage to aluminum parts finished in a vibratory machine with steel ball-cone media?
Q. We produce an aluminum motorcycle taillight housing that requires brightening. Our current finish process is a 1-hr cycle with a 3/8” steel ball-cone media in a vibratory machine. It brightens the parts, but leaves small scratches and dings on the surface. What would you recommend to eliminate the damage?—M.E.
A. Steel media works well for burnishing parts. It is polished, cased-hardened and produced in many shapes and sizes. The steel media also runs approximately 65 HRc, which is much harder than your aluminum parts.
The vibratory machine should be set to a gentle action with the amplitude setting between 2 and 3 mm. A burnishing soap compound with slight foaming and good lubricants will cushion the mass, reducing media damage. A flow-through compound system will help stabilize the process.
If part damage is still a problem, try a different media. A mix of 3- and 4-mm porcelain balls with the right burnishing compound produces a higher luster and eliminates media damage on aluminum. The porcelain balls will weigh 100 lbs/cu. ft., which is much less than the 300 lbs/cu. ft. of the steel media. Heavier media can damage softer metal alloys.
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 ASTM A480-12 and EN10088-2 are two; BS 1449-2 (1983) is still available, although no longer active.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
Metal fabricators that laser-cut with oxygen take steps to prepare parts better for powder coating.
Consider these five variables to determine what fits your application.