It's a learning process, bit of trial and error. When I got my first pin I ended up with a little bag of about 30 or 40 parts collected from inside the cabinet. You know, "stuff" like nuts, bolts, a tilt ball, pop bumper lamp holder, screws, washers, springs and a few spare parts like a pop bumper ring. As a first test I threw the whole lot in the tumbler. I was using walnut, the type with added red rouge polish. Some stuff came up brilliantly, like the pop ring and tilt ball, almost like new. But some stuff, most notably anything that looked like alloy, e.g., some screws and the pop bumper lamp holder, just got caked in red rouge that was really difficult to clean off. Anything with small holes got packed with walnut chips. Philips screw heads got packed with red rouge.
Here are some parts that I think work well in the tumbler
before they went in:
And here's what they looked like
after:
You'll notice that sometimes even the same parts can clean up with different results. And I still polish everything with the Dremel afterwards using a 1-inch felt buffing wheel and final finish white compound. The beauty is the tumbler has done 90% of the grunt so the final 10% is just getting that final shine and sorting out any problem areas: