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Vibratory finishing: Continued total failure plus a new clue

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  • Vibratory finishing: Continued total failure plus a new clue

    Hello folks,

    I'm continuing to try to use vibratory prep for plating old woodwind parts. Just a hobby today, but you never know... I'm trying to get my process down.

    I've posted a few other questions, but to sum things up: I'm trying to finish tiny key mechanisms made of yellow brass and german silver (a white brass containing nickel). The pieces started out with worn nickel plate, which I stripped easily using a deplating tank (got tired of trying to vibrate it off). I've since that point tried about a month total of trial and error in the vibratory tumbler with various wet and dry media.

    So far, not so good.

    The short of the problems is that the tumbler has failed, so far, to produce any cut down, finish or burnish whatsoever to the base metal of these pieces. Even using coarse triangles for an entire week with the tumbler on 'high' failed to produce any noticable change in the pieces (I took pics to compare, and tried with and without a few different tumbling soap additives. I've been tumbling wet. The flow pattern has been vigorous). According to every doc I've seen, this should have made the parts disappear entirely; brass isn't all that hard. Instead... you can't even tell they've been touched since stripping.

    ...except...

    The one noticable result from tumbling is that the pieces have been coming out almost black (a dark charcoal gray). The base metal is either a light yellow or white-gray (each piece is silver-soldered out of smaller components of differing composition). Today, after the latest full-week run with 'coarse' triangles, I noticed that the only other result was that one particularly exposed corner had been thoroughly rounded off, and the rounding was indeed quite bright and shiny. The rest of that piece was untouched (and dark gray). after about half an hour outide the tumbler, the rounded off corner had also dulled.

    'huh', I thought. Followed by, "oh, black. I'll bet the brass is oxidizing. Brass does that in water. I'll bet that oxide layer is really hard (after all, my tumbling media is itself a metal oxide abrasive). I'll bet it's oxidizing faster than the tumbler can take the oxide off."

    So, after a little chemistry involving ammonia, vinegar and acetone, I'd made an ammonia acetate solution which dissolved this black layer and turned bright blue. "Bingo," I thought, "copper and zinc oxides". The base metal was again a light whitish-gray. (I know that ammonium acetate will take off zinc, copper and iron oxide and turn them into nice bright-colored salts, but I'm not sure about nickel. Does anyone know?) I pickled the piece in pickle #3, just to make sure there was no lingering nickel oxide, rinsed, and tossed it back into the tumbler, this time with a healthy splash of ammonia (a reducing agent which should prevent oxidation).

    Tomorrow I'll go see if this has improved things at all. At this point, even if there's nothing left but a shiny stub, it'll be a huge victory Does anyone have a comment or sanity check for me here? I feel like I've been flying blind on the tumbling route as everyone else seems to be using rotary buffing tools. On the other hand, I know this *can* work, so I've not given up yet.

    Monty

  • #2
    results of last few experiments....

    First off, a correction in my chemistry from last post: Ammonia does not prevent oxidation. It only prevents acetic acid from attacking the base metal by turning it into an acetate. I found this out after trying a tumble-run consisting only of ammonia. Once again, the brass turned black (and amazingly quickly). I can't believe how fast it oxidizes. A splash of ammonia plus a *much* smaller splash of white vinegar once again stripped off the oxide, and I continued tumbling.

    Next: Finally some polish! Although the instructions with the medium triangles call for only a few hours, I saw good results after about a day-and-a-half. Actually, my tumbler manual seems to think all media need a few days to work. It appears to be correct. So long as I'm not doing 200 tiny bassoon castings (per bassoon) by hand with a Dremel, that's good enough for me.

    Last: It looks like about half my problem turns out to be the brass in question. It has an exceptionally coarse grain (somewhat visible under good light all the way to one piece that looks like it's galvanized!), and the grains appear to be dissimilar composition. They don't oxidize at the same rate; when I stripped off the oxide after the failed run above, I was back to square one with a very rough, almost sand-cast surface! Even if I have the acetate in the tumbler stripping off oxide as it forms, this is clearly working against the polishing process.

    Whew. I'll post more once I get a piece from 'stripped' to 'perect mirror replate' successfully.

    Monty

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    • #3
      Success, next things to try....

      Whew. I've taken a piece all the way from 'corroded, pitted, ugly, worn specimen with multiple layers of peeling plate' to 'just like new, mirror perfect'. The ammonium acetate did the job, and the pieces are now buffing up pretty well.

      I do wish I was losing less metal in the process though; the brass oxidizes rapidly when wet and although the acetate is immediately taking the oxide off the surface, it feels like I'm losing more metal than I need to be. I'm going to try a brass-specific tumbling additive next, one that claims to prevent corrosion as opposed to just dissolving it once it forms.

      Monty

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