I got one of those small tumbling machines from HF and it seems to work fairly well. I also bought a box of crushed walnut shell media for it. So far I have used it to clean up some old nuts and screws I have.
I also tried to finish some small aluminum parts with fairly good results. After with my success with the nuts and screws, I was initially disappointed with the aluminum parts. After several days of tumbling in the walnut shells they were only mildly improved so I added a bunch of nuts to the drum and they seemed to turn the trick. Apparently you need something in addition to the walnut shells to actually finish the aluminum parts. I thought of some abrasive like sand but did not want to use anything too coarse as it may have too drastic of an effect.
On another shopping trip I noticed that they do sell abrasive in small packets apparently to be added to the walnut shells. This sounds logical and there were four grades of abrasive in the box: I got one. But no instructions. So what I am wondering is exactly how much abrasive should I add to the shells for different kinds of parts? Steel, aluminum, brass, plastic? Can anybody provide guidance here or do I just have to experiment?