Brian Adam recently noted that tumbling only work hardens the
surface of metal. I have searched the archives (as best I can) and
I see no on exactly how much of the surface is
hardened. If, for example, it’s hardened to a depth of .5 mm, then
1 mm wire should be thoroughly work hardened–right? I’m also
concerned about how much a piece is compromised by merely having
the surface hardened. If this is going to be your only method of
work hardening, are there certain tolerances you need to observe
(e.g. don’t use thicker than 24 g sheet)? What are the potential
consequences of not staying within these tolerances, particularly
when working with fine silver?
Thanks in advance for all your patient and brilliant responses!
I can’t tell you how deeply metal is hardened by tumbling, but I can
tell you that just last night, I made a money clip from 18g sterling
with soldered-on decoration. It was no longer springy by the time I
finished, so I threw it in the tumbler before I went to bed. This
morning, it was as springy as I could wish. This may be one of those
“it might work in practice, but it’ll never work in theory”
situations.
I can’t tell you how deeply metal is hardened by tumbling, but I can
tell you that just last night, I made a money clip from 18g sterling
with soldered-on decoration. It was no longer springy by the time I
finished, so I threw it in the tumbler before I went to bed. This
morning, it was as springy as I could wish. This may be one of those
“it might work in practice, but it’ll never work in theory”
situations.
It probably can vary quite a bit depending on the type of shot you’re
using, as well as the shape and nature (castings vs construction) of
the original silver material. But if this helps, I recall some
silver castings with a deeply carved design of narrow channels cut
into a flat surface. That left a lot of sharp crisp square edges on
the design. Putting those into the tumble was a bad mistake, since
those cast sharp corners all got peened over rather deeply (at least
a half millimeter of mushrooming over, pretty much closing up the
narrow grooves that made the design… The castings were not
salvageable. But the metal sure was well burnished, bright, and what
with all that gentle hammering, pretty well work hardened. Smoother
forms not subject to that damaging distortion might expect a similar
depth of work hardening, perhaps a half millimeter or so, I’d guess.
That would pretty much penetrate most of the thickness of many sheet
metal or wire items, once you figure it’s coming from both sides…
The shot we were using was a heavier/larger sized mix of balls and
cones, and the tumbler was a somewhat expensive hex barrel rotary
type specifically made for steel shot, giving a rather fast and
effective tumble with steel shot. Typical lapidary type tumblers
likely would be slower, but no less effective. Remember that much of
this depends on the weight and type of the tumbling media. smaller
media in things like vibratory tumblers, where the shot doesn’t get
moving with as much inertia, fast, may burnish nicely, but will
penetrate less deeply and thus work harden less. My magnetic tumbler
with it’s tiny pins gets into extremely tiny details nicely, but I’ve
not noticed any significant work hardening with it. Too gentle. And
ceramic burnishing media also doesn’t seem to work harden much, just
surface burnish the metal. It’s not heavy enough to get any sort of
deep penetrating impact on the metal.
The hardening depth will depend on how powerful your tumbler is and
how long you run it for. Steven Alviti of Bel Air Finishing Supply
wrote a paper for the 2002 Santa Fe Symposium on steel shot
burnishing titled “The Effects of Burnishing on the Surface of Cast
Gold and Silver Jewelry” in it he states that “The surface hardness
was increased by an average of 22% using vibratory, roll and pin
burnishing and 13% using disc burnishing.” His measurements were
only of the surface hardness, You would need to do a cross section
and a micro hardness tests to determine how deep it goes but a 22%
increase is significant.
HTH
Jim
Jim Binnion
James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160
@James_Binnion
Member of the Better Business Bureau