DIY aluminum keyboard tray - some q's and advice needed

I got myself a 'keyboard arm' (http://tinyurl.com/chtfdey ) for my desk and instead of paying $100 for their 'molded ABS tray'
(http://tinyurl.com/9hecxvz ) I thought I'd make an aluminum one.
I want it to be 12" deep by 32" wide. The mounting surface on the arm is 5 3/4" x 5 1/2" and it would mount in the center back of the tray.
I'd like it to be reasonably rigid, but I'm not going to be leaning on it with my upper body weight. I'd like to put a 1/4" - 3/8" bend in the back to stop the keyboard/mouse from sliding off so that should help give it some structure.
I'm not crazy about putting any other bracing on the bottom, as I want the top surface to be as close to my legs as possible.
- What grade+thickness of al? I'd like to keep it at max 1/8" if possible. - I was thinking of getting it anodized for a finish, but I'm not set on that. - I'd like to tap holes (they can go the whole way through) and mount from underneath to have a flat surface, but countersunk bolts would work.
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KEYBOTRAY


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Or if anyone knows a better forum for this, please let me know.
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A few basic concepts to start with. One, all aluminum is about the same stiffness (force required to flex it a little but not so much it doesn't return to the original shape when you let go), the differences are in weldability, yield strength, and bend radius. 6061 is the most common, welds nicely, has a good yield strength (how much force is needed to permanently bend it), but the minimum bend radius is at least 5 or 6 times the sheet thickness to avoid cracking. The common alloy to bend is 5052, which has the same stiffness, about half (depends on exact spec) the yield strength, also welds nicely, and has a minimum bend radius of about 1.5-2 times the sheet thickness so you can make nice tight bends.
Next, given the material the final stiffness is determined by the shape, and here height perpendicular to the direction of bending is everything. The stiffness goes as the height * height * height, or height cubed, so twice as tall is 8 times as stiff. If you have a keyboard sized sheet of aluminum you can either keep making the material thicker and thicker until it gets stiff enough for you (it generally works out that stiffness requirements result in designs that have way more than enough yield strength, which is why I'm focusing on stiffness), or you can take a sheet of thinner material and turn the edges up to make a three sided box (rear and sides turned up, front open so there's no edge sticking up to hit your hands on). Using thinner material and turning up the edges results in a much stiffer and lighter tray. You can turn the edges up or down but since you wanted to minimize the overall height I'd use some 5052 that is roughly 1/16" thick and turn the back and sides up so that they are at least 1" tall. Remember that height cubed? Turning the edge up 1" versus only 1/4" is a difference in stiffness of (1/.25)^3d, and the tray still is only as tall as the keyboard. Only thing left is to mount it, and that's left as an exercise for the student :-) :-) (actually I can't remember your description of how you wanted it mounted, sorry).
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On Sat, 29 Sep 2012 03:12:44 +0000, KEYBOTRAY

6061T6. With a bent lip at each edge 1/16" would be more than adequate.

Countersunk bolts would be my recommendation. With Stover nuts on the bottom, or acorns and locktite.
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1/16" 6061 should be fine, but making a 32" long bend in it without warping it will be very difficult without expensive tools or experience. I would bend 1" or larger aluminum angle into a U frame and screw or rivet a flat sheet to it. Then the metal's strength and stiffness will continue around the corners. If you bend up flanges the discontinuous corners will flex.
Practice notching and bending the angle with short pieces to see how big a gap and radius you need to keep it from cracking.
The pan will be substantially stiffer if you put shallow diagonal creases across it. Look at large panels in a commercial air ducting system.
http://www.cadalyst.com/files/cadalyst/nodes/2007/10949/2-SE-SM-CrossBrake.jpg
jsw
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On 9/29/2012 7:19 PM, Jim Wilkins wrote:

I'd go the other way. Maybe a little thicker - .080? Make form blocks and deep form the lip by hand.
I've done instrument panels that way up to 3/4" deep flange.
It takes repeated annealing to work 6061 that deep. but it works well and looks really good.
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You and I know how from experience, but what did your first attempt look like? jsw
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On 9/29/2012 8:00 PM, Jim Wilkins wrote:

This was my first attempt - the gas tank.
http://www.home.earthlink.net/~cave-2/
I think it came out pretty nice.
Truth is, a bit of patience works wonders.
Richard
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It DID come out nice, and the photo sequence is much more helpful than a brief text description.
How did you seal the joints and rivet holes?
I bought an open-topped stainless tank of similar riveted construction with soldered joints that had failed all around, then were puttied with some goop that also leaked. I finally cut it up to make stovepipe: https://picasaweb.google.com/KB1DAL/Parts#5272671642404646402 The narrow hammered flange under the clamps determines its roundness and the fit into unyielding double-wall Class A chimney pipe. I don't think I could properly explain how I fine-tuned the snug fit here in text. It comes down to learning the difference between a blow that bends the metal and one that also stretches it.
jsw
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On Sun, 30 Sep 2012 06:38:18 -0400, "Jim Wilkins"

I think you defined it just fine. That knowledge is the "art" portion of "skill."
RE: your Ranger dust shield, Revere Ware Rides Again!
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On 9/30/2012 5:38 AM, Jim Wilkins wrote:

Thank you, Jim.
A friend welded it for me. I can weld steel, But aluminum is a whole 'nother thing...

That's it in a nutshell. :)
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    Hmm ... for fairly thin metal, you want a bend down about 1/4" in the front to add to the rigidity, and to keep it from cutting into your legs, you want another inch or two bent back parallel to the top. Also, bend down about a quarter inch on each side to keep it from bending just from the weight of your hands on the keyboard. (You reality should have a wrist support designed into it to minimize carpal tunnel syndrome. And you also want it rigid enough to handle the weight of the hand on the mouse off to the side of the keyboard.

    I would suggest 1/16" instead, with the additional bends I have suggested. This will still be lighter than the 1/8" aluminum.
    Or -- 1/4" aluminum with triangular pockets milled from the bottom, leaving lots of stiffening ribs.

    Do so if it is not too expensive. It will protect it from corrosion from wrist perspiration among other things.

    If you do the milled pockets, you can leave full thickness "strong points" for the screws to thread into. Leave about a 1" diameter circle for each screw to thread into.
    I just put the keyboard in my lap as I lean back in a Lay-Z-Boy, and have the monitor to my right on an arm. There is a pull-out shelf (about 1/4" diameter chromed steel wire) to hold the keyboard when I am not actually typing on it.
    As far as the mouse goes, there is a Logitech Trackman trackball which is Velcro mounted to the right arm of the chair.
    Good Luck,         DoN.
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Thanks everyone for the info!
I like the idea of 5052 for its tighter radius. My only concern with the reduced yield strength @ 1/16" would be accidental deformation in storage or use. I'm not a metal worker though (could you tell?) so maybe I'm worrying needlessly.
IIRC, a woodworker friend of mine said the shop next to his who he's on friendly terms with has a press brake, so I'll look into that.
Failing that, the idea of form bending it seems good to me but I can see the difficulty keeping it from warping. Richard - what were your forms made from? The resulting tank looks very nice but when the ends were done, they looked warped. In your case, they had the cylinder of the tank to flatten them, but I won't have any additional structure to rely on and I'd like the surface to be as flat as possible.
If I can't expect to get a flat surface from a form bend, I like Jim Wilkins' suggestion of getting angle stock, forming a U, and attaching it to the sheet.
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On 10/3/2012 4:35 PM, KEYBOTRAY wrote:

Good eye.
The ends are bulged out just a bit to keep them from "oil canning" with temperature changes. I rolled that in with a baseball. :)
The form blocks for the tank ends were simple 3/4" plywood. One time tool. But they suffered a lot less damage than I had expected and could have been used again - probably several times.
I believe that was because I was hammering on the aluminum block paddle instead of the actual metal. That tends to spread the impact force out over a much larger area that the hammer head.

If you want this tray to remain flat, I'd suggest something heavier than 1/16". Maybe 1/8? Regardless of how you make the edges...
1/16" thick that size can easily be twisted out of flat by hand. Even with flanges!
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'Richard[_9_ Wrote: > ;2938093']Good eye.

>

Okay, this is giving me confidence in forming it if the press doesn't pan out. If I went with a similar form of wood with a 2 * thickness radius (5052), how flat could I expect the resulting piece to be?
'Richard[_9_ Wrote: > ;2938093']

>

Yeah, that's what I figured from handling various metal objects in my life. Part of me just wants to get 1/2" plate and find a CNC machine and carve in ribs. Then the more wallet-y part of me punches that other part in the groin.
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KEYBOTRAY wrote:

How about modifying a heavy aluminum food service pan? This pan is 15" * 21" and is under $7.
<http://www.samsclub.com/sams/artisan-metal-works-2-3-size-aluminum-sheet-pan/prod6070159.ip
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On Thu, 4 Oct 2012 09:06:47 +0000, KEYBOTRAY

Been there...done that...still have the curled over stagger
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I can't twist a 15" x 24" sheet of 0.093" Al significantly by hand. It bends only a little when I imitate using it as a support to get up out of the chair. The slides on my keyboard drawer aren't any stiffer. jsw
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wrote:

Not 6061T6
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On 10/4/2012 4:16 PM, snipped-for-privacy@snyder.on.ca wrote:

Yeah, 6061 T6 even. A piece that size can easily be twisted out of flat.
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