Do I have it right that the marginal gain of the magnetic finisher
is in the time it takes to burnish a piece? Is there any other
advantage? Does tumbling do anything better?
The advantage of the magnetic tumbers is that they work much faster,
and get into extremely small details. Ordinary steel shot, which
isn’t all that slow either, but does take a bit longer, is better at
peening surfaces and burnishing them, sometimes helping with porosity
and tightening up surface pores. Shot can leave a better brighter
surface, but won’t get into the tiny details like the tiny pins in a
magnetic finisher. In essence, the two machines actually do fairly
different jobs. Both finish the metal, but the results are different.
Some items are better processed with shot, some better done with the
mag finisher, and some end up best processed first with the magnetic
finisher to get the fine details bright, and then run in shot to
improve the finish on broader more exposed surfaces. If your tumbling
things that will then get manually polished more, you don’t need the
steel shot most of the time. But if you’re hoping for a final polish
on things, you might now and then want both.