Mass Finishing Q&A: Mass Finishing for Maximum Part Brightness

Q. We have small, flat, stainless stamped parts with have slight die marks that need the highest shine possible. What mass finishing machine and process can help us?

A. Maximum part brightness achieved with mass finishing is the centrifugal barrel machine running dry media impregnated with a polishing compound. This process will remove slight die marks and keep flat parts from sticking together.

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The dry media most commonly used is a ground corn cob, available in fine, medium and coarse grits. The polishing/brightening compound additives vary with the manufacturers producing it.

The centrifugal barrel, centrifugal disc or spindle machines produce the high compressive impact energy required to obtain polish or deburring with dry media. The centrifugal barrel, running at least 120 rpm, at 20–60 minute process times, will do the finishing required.

A few process tips for dry finishing in centrifugal barrel machines:

• A pre-dry process of the parts may be required to remove the stamping oils so the light flat parts will not stick together.
• A post-wash or dry untreated cob process will be required to clean the oily residue left from the finishing process.
• The dry cob process generates heat, so an alternate batch of cob media will be required under constant production. While one batch is processing, the other batch of media is cooling.
• The barrels of the centrifugal barrel machines should be filled to 60–75 percent full by volume.
• New polishing cob can run for 12–48 hours and longer in production before replenishing. Replace just 25–50 percent of the cob at a time.
• New polishing cob media requires 3–4 cycles for break in. The initial few processes create an excessive oily haze on the surface of the finished part.
• Heavier parts require divided compartments to eliminate part-to-part damage. Dry media do not protect the parts like a wet media. Heavy parts will accelerate through the dry mass and ding into each other.

The centrifugal barrel with cob polishing media is an excellent choice for brightening your stainless stamped parts. A separate deburring process for small stamped parts can also be accomplished by adding cutting abrasives like silicon carbide into the cob.

Originally published in the November 2015 issue.