Intricate Finishing

 Q. I have a small foundry and do short-run custom parts, most cast from silicon bronze and ranging in size from 0.5–5 lb. Currently, we air-blast with glass beads and then put parts into a vibratory tub finisher. That works well for 80% of the parts, but I’m running more parts with intricate designs and small lettering that the client would like to see as smooth and polished as possible.

Small glass beads work well in a blast cabinet but don’t smooth the surface enough, which is why we follow with the vibratory process. The problem is that the media doesn’t get down into the crevices and details of the part. The smallest media I can find is angle-cut cylinders 3/16 × 5/16 inches in size. I tried running dry with walnut shells; results were poor. I set up the air-blast with baking soda, which gives a dull surface and does little cleaning or polishing. Blasting with walnut shells gave little or no results. Bright dip with 40% nitric acid, 30% phosphoric acid, and 30% sulfuric acid cleaned the surface but didn’t smooth out the blast profile, and smut forms on surfaces if parts are left for more than 30 sec in the solution.

What process(es) would you recommend to get intricate detail like lettering and emblems to a smooth, clean, bright finish?

 

 

A. This is one of those “let’s try this first, and if it doesn’t work we’ll try that” processes. In fact, if readers have experience with a similar process and are willing to share information, let me know.

The parts in this case have deep indents, such as you see on cast bronze name or number plates. To my thinking, that rules out hand buffing although that can be a fallback position if other processes are not satisfactory.

The shop has two small vibratory finishing machines. One is designed for dry tumbling and has no drain. It is about 1/2 ft³ capacity. The other is a 1.5 ft³ tub powered with a 3/4 HP motor. They also have a hand blast cabinet. Before addressing the vibratory process, let me suggest replacing the glass beads with crushed glass media. That will create a shinier, more light reflective surface than provided by bead blasting.

My recommendation for the vibratory finishing is to use metallic media, which can have sharp points that will get into areas too small for ceramic media. Steel media is available in many shapes and sizes. Ideal for this application would be tapered pins, which will have enough mass to impart a burnish finish while reaching all areas of the surface. They are available in carbon or stainless steel. Most steel media manufacturers offer sample processing in either tumblers or vibrators. Take advantage of such a demonstration.

Typically, the metallic media process is performed wet. The compound must be selected to give a good finish on the bronze as well as prevent corrosion of the media. There are several compounds on the market that can do this job even for carbon steel media. Test the compound before using it. Partially submerge a scrap bronze part in the proposed solution and let it sit for at least two days. Include some nails if the media is carbon steel. If the solution turns any shade of blue-green or if the nails rust, do not use that compound.
Metallic media can also be used dry. This process can generate metallic smut and darken the parts. To reduce that effect, mix two parts corncob to one part media in the small dry processing machine. After processing, the parts can be washed in a compound that passed the aforementioned test.