Polishing

Question:

I have a tub style vibratory finishing machine. I always have customers asking for someone to polish wheels, motorcycle frames, etc. Would this machine work for that application? If so, what would the best media and fluid be to accomplish this? A.B.

Answer:

Some pretty impressive polishing work can be done in vibratory finishers and tumbling barrels. There are some limitations, mostly having to do with the time available to achieve the finish and the numbers of pieces you have to process at one time. Two important questions must be answered before you will have a complete answer: 1. What is the starting surface finish, and 2. What is the desired final surface finish? Not knowing the answers to those questions, I can give you some general guidelines.

It stands to reason that the smoother and shinier the finish is at the beginning, the less time and the fewer processes it will take to polish the part. Very small surface imperfections such as handling nicks, grinding lines, tool marks, and the condition based on manufacturing method, such as extrusion or sand casting, will dictate the parameters of the process. Do you want to eliminate every imperfection? Do you want a mirror finish? Will you be plating or painting after polishing?

In many polishing applications, customers want a highly-buffed appearance and a mirror-like luster. Often, those parts will then be plated. The substrate is definitely important prior to plating, but in some cases shiny is no more important that low RMS. Vibratory finishing can get close on both counts. It is important to understand that the more critical you are of the final finish, the more it is going to cost.

One company imposed an unrealistic, jewelry-type polishing standard on an industrial tool that was to be plated in the duplex nickel process. They set up finishing inspectors with four-power magnifying glasses, and they rejected any nicks that could be detected. The volume was very high, and hundreds of parts were being processed together in large vibratory bowls. The standard imposed limitations on production that led to fewer parts per load, slower machine settings, longer time cycles, hand unloading, and eventually, a two-step vibratory process. The final outcome was very sad: A previously profitable product line was sold at a loss.

You must determine what surface defects you can tolerate. More perfect finishes require more preparation. If you will paint the parts you can be less particular, depending on the thickness of the paint layer. On the other hand, plating will actually highlight the smallest surface imperfection.

The first step is to cut below surface imperfections that cannot be tolerated. Abrasive media is generally called for and may leave a dull, matte finish. Some people prefer plastic media for this step because it won’t damage most metals with “media dinging.” If a light cutting ceramic media does not damage the surface, it may both cut and polish, perhaps adding a burnishing compound for the polishing phase. The polishing step may require a special media, such as small steel burnishing balls, or a heavy ceramic polishing media. It seems that media weight plays a role in obtaining a shiny surface. This requires some experimenting. Surface damage caused by the media can be reduced, or eliminated, by running fewer parts, or even a single part in the machine, or you may consider fixturing the parts.

Sometimes machine settings are changed for the polishing cycle. Or, a second machine is set up specifically to polish the parts. In many cases we can use the same media for cutting and polishing (light ceramics are best for this). A burnishing compound will cushion the part/media action, giving a better luster.

Not to be overlooked is dry buffing. Corncob is generally used for this, although walnut shells, wood pegs, and other agricultural products can do the job. The media can be coated with polishing rouge, such as a metallic oxide, or used by itself. This step may take several hours, but the results often meet realistic polishing standards.

An alternative to all this is offered by a chemically-accelerated process. In this process the same media is used both for metal removal and polishing. Chemicals do the work of removing high spots and surface imperfections. This process is worthy of investigation and may reduce the overall time cycle. A discussion of that method goes beyond an easy answer to your question.

Recently, a company that promotes metallic media published some wear rate information. Their data shows the wear rate on hardened steel media as 0.01% of the weight per hour. This is 20% in a 2,000 hour year, a little higher than I would expect. Also, they published a wear rate of 0.014% per hour for cast aluminum media. Somewhere between my figures and those you should find the actual experience. When you apply these numbers to the cost of a volume of media, the cast aluminum will be about half as expensive to use as the hardened steel, and both are less costly than abrasive ceramic media.

Regarding deburring we always recommend sample processing before purchasing. Only you can judge the results.