Polishing Brass best ways to do it?
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  1. #1
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    Default Polishing Brass best ways to do it?

    I started making the following pressure gauges for model steam engines. One of the more challenging parts to this is getting a good polish on the face. The gauge itself is a commercially available pressure gauge which I rip the face off of and place a custom etched face on top of.

    Polishing these tiny little things with a traditional buffing wheel and buffing compound can be a real pain to get it perfect. The hardest task is holding them while polishing and they tend to get really hot as you go about it and they never come out just perfect. Perhaps a better wheel special for brass?

    Anyone have any good ideas here? What is the best metal finish process to have done to these? I know there is electropolishing, bright dip and other processes that may work here. Any one have any ideas?

    I reached out to my etcher and asked if we could start with polished sheet and the reply back was that the photo resist doesn't really stick well to the ultra smooth surface so it seems like a secondary process is still required.

    pressure-gauge-photo.jpg

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    Have you looked for electropolishing

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    Polishing brass? Well, there's always good old Brasso.

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    2000 grit paper as used for colour coat work, wet, with a hard rubber or cork pad while rotating in the lathe.

    The issue of holding a disc is partly to do with the buffing heat and partly due to the frailty of such a part. Fix it to a larger piece of brass with melted shellac, gentle heat and a palette knife will allow removal.

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    HI Adam,
    I started out as a jeweler. I've taught it at the college level even. So I've polished more than my share of brass.
    If I were doing these, I'd prep the surface with a succession of sandpaper passes, starting at about 220 grit, going up to 600 or better.
    Put the paper down on an old surface plate, or a piece of flat glass, and then rub in a figure-8 pattern.
    Change orientation of the part 45 degrees between levels of sandpaper. That way you can see when all the scratches from the previous pass are done.
    *Then* etch it.
    Once you get it back from the etcher, continue with polishing. You could try micron graded abrasives, up to whatever level of insanity suits you, using the same technique. Somewhere around 600-800 grit, I'd probably switch to either a traditional buff, with a smallish (3") loose stitched wheel, running tripoli to start, followed by a pass with Zam. (And a trip to the sink to use dish-soap to get the residue off in-between.) The problem with a buffer is that the buffs are likely to drag on the etch edges, and may blur or erase the etch, depending on your skill. Thus the small diameter buff. But you still need to be careful. Rotate orientations quickly. (seconds) and always buff *off* of an edge. (So the wheel is rolling off a trailing edge. NEVER rolling *onto* a leading edge. That'll cause the buff to grab and throw the piece into the deck.) Also: always buff from the equator down to the south pole of the wheel. That way if it catches, it goes down into the deck, not up into your face. And use glasses. Really.

    Failing that, I might try taking a strip of leather, and either gluing it to a paint stick, or if I had a big enough piece, just setting it on the surface plate, rub tripoli into that, and go back to figure-8'ing it. That has the advantage that it won't drag on your design edges. The drawback is it'll be slow as molasses. But controlled and even. Much like a surface grinder: slow, but deadly precise.

    There are powered flat laps that exist for this sort of work, but they're not a tool that is forgiving of beginners, or easy to just wing. They use rock-hard felt laps, which tend to scratch a bit. Professionally, you use those to burn the surface down to flat, skipping the whole sandpaper rubbing phase, and then use a normal buff to clean it up from there. Which works, if you know what you're doing. If not, they'll dig an un-recover-able hole in your piece before you can blink.

    FWIW,
    Brian

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    If you use a powered buffing wheel you will soften the edges of your etched details to some degree. Using a flat surface like a piece of glass or chunk of granite counter top with increasingly finer grits of polishing paper would work and the figure eight motion. You could use dop wax (lapidary stuff) to adhere the sheet metal to something larger to hold them better. To remove, just place in the freezer and they will pop off. Another thought would be to have your etcher prove to you that the resist will not adhere to properly cleaned high polished brass. I can't imagine that the resist would care about the polish of the surface but would if it were not clean.

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    Are you polishing the case or dial or you made or both? depends on how smooth they are when you start, they look like they have a good finish already. Brasso does a good job if you're not trying to remove scratches etc.

    Next up.....how do you keep them bright? they'll quickly tarnish. On model engine parts I often spray lacquer, but you have to get the good stuff (I go to Perrins watch/clock supplies) not the garbage the big box stores sell (it goes on cloudy). No idea if lacquer will stand up to heat

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    Hold the gauge face with watchmaker’s shellac or double sided tape. Use abrasive paper up through 2000 grit, or as fine as you can get, applied to a flat surface. Rub by hand being careful to remove scratches from previous grit. Finish off by hand with rouge on a flat surface covered with felt, a soft muslin cloth, or something similar.

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    3M makes plastic backed abrasives (sandpaper, but high tech!) in to very high grit sizes, or small, if measuring in microns.
    Another good option, if you can find one, is a Micro-Mesh kit, commonly used in the Aviation industry to polish out scratches in aircraft windows and such, it contains various grit abrasives up to 15 or 20 thousand grit, IIRC.

    Stick a sheet down on a solid, fairly smooth backing, I liked using chunks of acrylic sheet stock, with a spritz of water.

    Make a holder for the gauge faces and rub them on the paper in the same direction each time you change up the abrasive to a finer grit.

    Use soapy water as a lube, to help carry away the residue produced.

    I found that it was not too hard to get a mirror grade finish with a flat surface this way. Random swirly marks resulted if you tried figure 8's or switching directions between passes, while the sameness of motion only resulted in the possibility of one set of linear lines that could be produced, if you skipped through the steps too quickly. They would not show as well in the reflection as the swirly ones would.

    I did a LOT of retirement gift plaques this way, and it does not take very long to run up through the grits and get a great looking polish that is both shiny and very flat.

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    Thanks to all for the replies. I may need to experiment with some of the techniques suggested here, TrevJ what grit would you think is when a mirror like finish starts emerging?

    I am trying to do these in batches of at least 25-50 pcs each so hoping for something that doesn't take forever to do. I certainly don't mind and have budget to send them out to the right vendor for processing. I know there are some shops in the area doing electropolishing of stainless that I may call up and see if they can run brass.Tumbling wouldn't do the trick without removing all the etch would it? Is there any other process to look into too?

    Thanks,

    Adam

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    Quote Originally Posted by adammil1 View Post
    Thanks to all for the replies. I may need to experiment with some of the techniques suggested here, TrevJ what grit would you think is when a mirror like finish starts emerging?

    I am trying to do these in batches of at least 25-50 pcs each so hoping for something that doesn't take forever to do. I certainly don't mind and have budget to send them out to the right vendor for processing. I know there are some shops in the area doing electropolishing of stainless that I may call up and see if they can run brass.Tumbling wouldn't do the trick without removing all the etch would it? Is there any other process to look into too?

    Thanks,

    Adam
    I would go up to 600 grit or so and buff from there if the stuff had to be out the door quickly, when doing the Micro-Mesh I almost never used any finer than around the 8000 grit as it gave a fine enough finish that the metal polish actually scratched it up. Or the cloth that the polish was on did...

    In all honesty, with a flat surface and a selection of assorted grits already in place, I can't see eating up more than a couple minutes per unit to do these by hand. I would start around 220-280 grit and see what the surface looked like, if it was good, carry on up the grits, if there were dings that the initial passes didn't clear, then have the option of skipping to a coarser grit to work out the dings, then carry on up. Rinse off with a squirt bottle, then move to the next higher grit.

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    I would consider double stick tape for machine buffing, on a shop made wooden or aluminum jig.
    Or, go the other way- fasten the faces down to a plate, and use a dremel or flexible shaft tool. It will be much less likely to throw the part across the room.

    Also, you can buy pre polished brass sheet, do the etching on the sheet, and cut the circles out last, after everything else is done.

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    You could try mounting at least 6 pieces on a flat piece of just about anything and lap them all at once, as suggested above. Use a thick double face tape, maybe thin foam backed, so each of the discs can settle in flat on the surface. I think it would be harder to get even pressure lapping them individually, they are so small.

    I do a lot of lapping with wet/dry polishing papers and Micromesh. I use an cast iron surface plate, and a couple of aluminum strips with magnets in them to hold down the abrasive sheets. Way easier than using adhesives, easy on and off and no damage on removal.


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