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>> Tumbling affects only a thin surface layer, perhaps a half
>> millimeter deep at best,
> Not even that on jewelry scale machines and objects. You cant get
> enough energy into the work. Maybe with a Centrifugal Barrel
> Finisher that has a 5HP motor and your work would look like it had
> been beat to death. It takes real movement of metal to get that
> kind of depth.
Jim, I based that statement on one instance recently, where I tumbled
a bunch of cast pendants. These have a milled design of almost
vertical cuts into the surface, with the cuts all too narrow for any
of the shot (imagine the lettering on a class ring...), so all the
tumbling was confined to the outer surface. In that situation, I
found the originally sharp corners/edges of the recesses to be
slightly mushroomed over, since the shot could hit one side of the
corners, but not the other. The depth of that effect was roughly half
a millimeter, or seems so to my memory. (enough to mess up the detail
on the castings, which I recast, and tumbled the next time in my
magnetic tumbler, which worked well). It was only these half
protected corners which got uneven burnishing which showed any
appreciable and noticable distortion. The rest of the surfaces were
merely nicely burnished. I think it's the unequal distribution of
the tumbling to only one side of each corner, and the fact that the
corner shape offers less resistance to the tumbling forces than a
flat surface would do, that resulted in that degree of effect, and
would agree with you that this would not be the usual depth of the
effect on the metal.
My tumbler is fairly small, using an octagonal faceted barrel (think
of a shape something like a large single cut stone...) set an about
a 30 degree angle. It's designed for small volume jewelry tumbling
with steel shot, not a repurposed lapidary machine, but it's still
not an exceptionally large or powerful thing. I got it from allcraft
a few years ago (If memory serves, it was about a hundred fifty bucks
or so...), and it does work better with steel shot than previous
tumblers I've used.
Peter Rowe
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