Square tubing finish options

Hello all,

I am using 2 inch square Al tubing (0.125 thickness) as part of a frame for an experimental fixture. Part of me wants to flycut it to remove some dings, and part of me wants to leave well enough alone. Knowing that I can always start over if the tubes (11 in long each) were to warp like mad, I might just try it.

What is your best guess? Would knocking the skin off of the tubing cause it to warp or twist a lot? Is there something else you would do to make a presentable finish? It is not terribly important. I can probably get to a sandblaster, though it is not as accessible as in the good old days. Something I can do in the comfort of my garage gets extra points.

Thanks,

Bill

Reply to
Bill Schwab
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The reddish-brown Scotchbrite pads make a nice brushed finish quickly. The green household ones aren't too bad either. Inside corners are difficult, so if possible clean up the frame members before final assembly.

Jim Wilkins

Reply to
Jim Wilkins

*Maroon* scotchbrite. :) Works well, I think first degreasing/cleaning the alum helps as well. Steel wool ought to work just fine, too. 99c sell the green stuff in 6" stacks for, uh, 99c. Also, if you are doing this in any quantity, you can get scotchbrite made for belt sanders. For my 4x24, they were $10-12 *each*--goodgawd.....

Haven't tried it, but a sponge and Comet or its equivalent might work nice--mebbe great. Scotchbrite tends to get gummy from alum.

Face milling would not be a problem, as the pieces are short, and I don't think you'd have to mill off more than .010, altho that might not get the deeper dings/gouges.

Reply to
Pre-Meltdown

Try it. Or not. It's your choice.

How in the hell would you expect anyone to be able to give you a good answer, without knowing the depth of cut necessary and the finish that you want?

John Martin

Reply to
John Martin

This is one of my medium-sized "production runs" - aka two of them ;)

It probably would not get all of them, but then neither would scrotchbright. How would the scotchbright finish handle fingerprints? The parts in question form the outer casing of the offending gizmo and are likely to handled far more than the largely flycut and visible innards.

Bill

Reply to
Bill Schwab

Reply to
Mechanical Magic

Strictly speaking, scotchbrite is not a finish, but a grade of sanding, and leaves raw unoxidized aluminum, which is a magnet for fingerprints, just like polished stainless. The oxide that forms over time seems to be much less of a magnet for prints, but looks like, well, oxidized aluminum. Not necessarily bad, tho.

Before that oxide forms (if you want the brite finish), you can send it out to get "clear-coated", or even anodized, or just spray-lacquer/polyurethane/etc it yourself, which should look really nice. That's the only way I know to keep off prints, and to keep the gray oxide from forming.

Reply to
Pre-Meltdown

Which got me thinking:

If the gray oxide finish on fresh/polished aluminum is not so bad (actually, "natural anodizing", and is itself highly protective and hard), I wonder if accelerating its formation in an oven would be practical? Esp. as your parts would fit. Just don't get fingerprints on it before you put it in the oven....

Strictly speaking, say from a conductivity pov, these oxides start forming almost immediately, within minutes, enough to effect contact conductivity.

Sending stuff out to be anodized is expensive, but actually a pretty easy diy. There were/are some outstanding web sites on how to do this.

I'll have to try the oven ditty myself.

Reply to
Pre-Meltdown

Another cheap and easy finish that works on *most* grades of aluminum is the bright satin effect you get from dipping it in a lye solution. I use this for the chassis on electronics projects.

Dissolve a few tablespoons of household lye in a quart of warm water; wash the oil off the aluminum with strong detergent; submerge the aluminum in the lye solution. Leave it there until it looks good -- usually minutes. If there are any oil spots on the aluminum that show up as the lye bath continues, pull the aluminum out, wash with water, brush the spot with Scotchbrite, and put it back into the solution.

When you like the look, rinse it thoroughly, dry while it's still warm, and spray with clear lacquer or something tougher, if you prefer.

-- Ed Huntress

Reply to
Ed Huntress

In my experience brushed aluminum doesn't show them objectionably; stainless is worse. My fingerprints don't rust steel and darken brass only slightly after a few weeks but I've built prototypes, demos and trade show toys that were handled by lots of people and I don't remember any issues with the Scotchbrite'd aluminum finish. One fingerprint on polished metal shows up whereas 1000 blend into a continuous even surface.

The well-handled field return EMT equipment on the rack behind me doesn't show fingerprints on the matte beige plastic or the black anodized aluminum handles.

Jim Wilkins

Reply to
Jim Wilkins

Very interesting.

There's a guy not far from me with the biggest-assed circular vibratory tumbler I've seen, the size of a small bedroom/large closet, who puts in some chemical with the media that gives aluminum an almost galvanized look, and claims this finish is protective. I don't know if I like it, it's certainly not like a brushed/satin look, a little industrial/cobby, but not bad.

Reply to
Pre-Meltdown

Aluminum is so reactive that there have been many chemical finish treatments cooked up for it over the years. Most of them don't really protect; only a few produce a tight enough oxide or other compound to really protect the aluminum. Anodizing, particularly "hard" anodizing (which is no harder, only thicker) is among the most effective, but there are several types of anodizing for different purposes. PAA (phosphoric-acid anodizing), for example, is preferred when you want to make the surface receptive to adhesives and strong enough for adhesive bonding.

There is a mild acid treatment that gives similar results to the lye treatment, but I've never used it and can't remember which acid it is. It's also suitable for home- or small-shop use. Neither the acid nor the lye treatment provides any protection. They just leave a nice-looking surface which, if you don't let it sit around for days before coating, is also chemically active and will make a good bond with paint or clear coats. I try to coat within a half-hour or so.

Some alloys don't behave very well in this treatment -- they leave a smutty-looking surface -- but I can't remember which alloys those are. I do remember using it on 2024 and 1100, and it works well on both of those. I think it works on 6061, or so I've been told. You may have to experiment with concentration to get some alloys to work but they make not work anyway.

The lye treatment is well-known among crafty people who make things with aluminum and I've seen it recommended several times in books about making electronic projects. I said I use it for chassis; I should have said I use it for panels and covers.

-- Ed Huntress

Reply to
Ed Huntress

Well, I have developed considerable respect for the frequent contributors here. You might care to read some of their replies, because they are insightful, based in experience and professionalism.

Ironically, one of them even suggested a depth of cut, of the same order of magnitude that I had in mind.

Bill

Reply to
Bill Schwab

And to finish that point, as I deal with 1/8 wall alum tube often, I'd fall over if a .010 cut did anything to the tube ito twist or anything else, besides the very slight structural weakening due to thinning the wall.

And, if you were doing this in a CNC, you could take this cut in rapid, literally about a couple of seconds per face (or less!), with a *near-mirror finish*! Much faster than Scotchbriting, and with fewer consummables, in this case. And, no need to clean, if you cut it dry.

For tubing and long-ish material, I toyed with idea of making a home-made "time-saver" sander type-deal out of two to four beltsanders mounted in some kind of tension-loading fixture, and just run the tube through, and Walla, four finished sides--well, finished as far as the current grit you are using, which might have to be sequential.

And as other posters have said, the more satiny the finish, the less fingerprints will show, altho I seem to recall my prints showing up pretty well on the scotchbrited stuff I did (satiny), altho these were also unwashed "shop hands".

Professionalism, eh? I guess I should go back to my previous handle (Proctologically Violated®©), and remove all doubts. :)

Reply to
Pre-Meltdown

Just thought I'd ask. I know that at some point, one can release a lot of residual stresses.

Regardless of what I decide to do (or not do), thanks!

That might have been pushing it, but it seemed appropriate to come to everyone's aide.

As an aside, who/what's meltdown are you anticipating?

Bill

Reply to
Bill Schwab

OK, Bill, I'll try it again.

I, too, have a good deal of respect for most of the contributors here. You got some good information on finishes for aluminum, but no one fully answered your questions, did they?

One person "even suggested a depth of cut, of the same order of magnitude that I had in mind." What an amazing coincidence. I'd have suggested a depth of cut of .0000", because to me those dings don't look any deeper than that. From here in Maine, though, it's a bit hard to see your workpieces. You go on to say that the cut of .010" will not get off all the dings and gouges. Well, what will it take - .

030", .050", .075", more? If you'd simply stated the depth of cut necessary to clean up your workpieces, someone could have readily advised you whether you were likely to get any significant warping.

Same criticism of your "presentable" finish. To some, it might mean a mirror polish. To others, a smooth but dull finish. Or a rougher brushed, machined or shotblasted surface. I'm not a mindreader, and I don't know of any in the group. I could be wrong, though.

John Martin

Reply to
John Martin

Don't be so sure...

... several posters provided useful answers and advice. You seem to be bent on finding fault. Enjoy.

Bill

Reply to
Bill Schwab

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