Untitled Gardening Project: Aside
Oct. 14th, 2007 08:08 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
xiphmontToday's aside topic: vibratory polishing of tooled aluminum
Surprisingly, there's lots of discussion of vibratory finishing of aluminum on the web, but little practical, documented advice. There is lots of vague, unsubstantiated advice that sounds like it was quoted from a glossy brochure short of details and big on pretty pictures--- even less useful when the pretty pictures are nowhere to be found. Now that a few people have actually written me directly to ask about it, here's some real experience that includes the pictures.
I already talked about initial cut-down using 'green triangles': these are medium-grit silicon carbide in a plastic binder, and do a good job of removing most milling marks in about a day of tumbling on the highest setting in my modified Minisonic MT-14V with a flow-through water/detergent system. The time will vary for other tumblers, of course. The result is an excellent smooth, matte surface.
Past the cut down, advice on getting a smooth, near-mirror polish diverges wildly and vendors are no help. Some recommend three further steps: First burnish with shot, followed by walnut with rouge, followed by corn cob with a brightener. Some recommend skipping burnishing with shot suggesting the end result is just as good, it merely takes longer (usually not a problem with supervision-free vibratory finishing). Several recommend skipping straight to corn cob or staying with walnut shell. Rouge? Burnisher? Each 'expert' gives a different answer, then generally admits 'I haven't tried it with aluminum'. Some of the confusion comes from the fact that most of these recommended sequences work. But which is the best and/or the fastest? There's only one way to settle it: try several and see.
The first trial is the old, reliable stand-by: walnut shell and red rouge. It's not nearly as effective as on brass or copper (possibly due to aluminum's oxide layer?) Several more days would continue to add to the shine-- but we don't need to spend several more days.
Second trial: A product called 'DriShine III' which is apparently corn cob + some proprietary treatment. It claims to perform 'equal to or better than walnut and red rouge, but leave no residue.' I found no effective difference between this and walnut shell.
Next, a 'brightener' or perhaps more accurately, an 'activator'. TurboBrite claims to 'reduce tumbling times by half' and be 'most effective on non-ferrous metals'. This claim is overly modest. TurboBrite, by far, made the largest difference of any change.
TurboBrite really isn't anything mysterious or special-- it's just one of a number of available metal 'polishes' (like Flitz or Noxxon 7) that contains no abrasive and an ion-exchange activator to remove the aluminum oxide layer. Any of them would work just as well, TurboBrite is just the cheapest. One caution: Brasso and some other polishes contain an abrasive (usually feldspar) that will scratch the living Hell out of the finish. Make absolutely certain the polish you choose has no adbrasive, just an oxide stripper, unless you're going for a random brushed/matte look.
There is a downside to TurboBrite aside from being $12 a bottle. It strips the oxide layer, but the aluminum will want to keep oxidizing resulting in more oxide to strip, so it does remove a little metal. This metal ends up in the walnut shell/corn cob/whatever, so the media will not last as long as it clogs up with dulling oxide. This is the price for speeding things up by at least a factor of four.
Polishing times over 36 hours did not dramatically improve the finish.
Many vendors recommend shot-burnishing metals as a second finishing step for the highest possible final shine. I cannot comment on the use of shot for other metals, but my own experience shows it to be mostly useless for aluminum. It neither improves nor degrades the final finish. The photos below used porcelain shot rather than steel or stainless steel (which would be too heavy for aluminum). The result is as expected: a lightly peened, grainy surface not substantially different from the matte surface we started with.
There is no significant difference between the 18 and 36 hour mark.
But does an intermediary shot-burnish step make any difference in the final polish? Apparently not; the below pictures show a piece burnished in porcelain shot for 18 hours followed by 18 hours in corn cob + TurboBrite for a total of 36 hours of polishing. The finish is not better than simply skipping the shot burnishing step. Pieces shot-burnished for 36 hours, followed by 18 or 36 hours in corn cob + brightener did not differ noticably. Thus we can conclude that the shot is not particularly useful for aluminum.
End recommendation? The simplest, easiest way works both best and fastest: your choice of treated corn cob or walnut shell, optimally with a brightener like TurboBrite to speed up the process severalfold at the expense of going through somewhat more media.
