Modding Question about structure rigidity

[justadude]

Hey everyone, i'm new here.
i would like to make a pc case (duh). I was thinking about materials for the structure and this led me to this question:
How strong is aluminum? really.
I made a quick sketch of a simple cube carcass in sketchup, composed of

3/4"x3/4"x1/8" alu angle,
1/2"x1/2" alu square rod and
3/4"x3/4"3/4"x1/8" alu channel

How effective do you think this setup would be in holding everything together? using screws for joints
Here are the pics:




Thank you

[jhanlon303]

Actually, a lot depends on the overall size. A 6 in cube or a Mountain MODs size case. Inherently a small cube is self supporting but as it gets larger some gussets or triangulation will become necessary. It also depends on what the outer covering is to be. Plexi, alum., Stainless steel? we really need some more info here.
John

[crazybob]

Aluminum is extremely strong for its weight, but not particularly strong on the whole; steel is a much sturdier material.

Your frame will almost certainly have pretty low rigidity. If it were very small, it would be fine, but yours looks pretty large. If you wanted to make the frame stiffer, you'd want to add diagonal braces to keep everything in position. However, if you're putting a skin on and fastening it directly to the frame, you'll be totally fine - it'll form a shear web and make everything extremely stiff.

[Cheapskate]

What bob said. As far as rigidity if the frame is a stand alone object...That would depend on how well you built it. If the fit is not snug, you will get some wobble.

[justadude]

Thank you very much.there's so much for me to learn. i was planning to cover front, back and bottom with 18 gauge stainless steel sheet and use top and both sides as windows, covering around 30-50mm from each edge with the same steel sheet to make a window. Like this:



would this be enough to support pc components? I have in mind scythe ninje copper or orochi cooler and that alone weighs over a kilo.

[crazybob]

Yes, that should make everything plenty stiff.

[justadude]

thank you for good news, crazybob. As soon as exams are done, i'm doing this

[Javerh]

Even though aluminum is weaker than steel, it is tough enough for most mundane tasks. On average, a simple 20x20x1.5mm aluminum angle bar of 350mm will support a weight of over 2000kg without buckling if fixed firmly at both ends. Do note that this is on a good day! Supporting these kinds of forces will require a lot from the supports and alu angle ends. If you use pop-rivets properly, they will support insane abuse.
I'm joining some alu 20x20 square tubes together with corner brackets and 5mm pop-rivets. The corner brackets will give before the alu or the rivets. Screws or bolts should hold with equal vigor.
A small structure such as yours could easily face any kind of use and withstand it. You could probably even jump on it if you wanted to.

[karolis]

I highly doubt i will he jumping on it though On a related issue, the way i see it, joints reduce the strength, right?
So if say i was making something like this and bent part of the frame from one square tube, would it increase the strength? Or more specifically, would it be stronger than using brackets+bolts/rivets?

[Javerh]

Joints are usually made by drilling holes or welding. Holes reduce material and welds might initiate cracks. So yes, your are right. Joints usually reduce strength. Furthermore, because joints are abrupt changes in material they often suffer from the highest stresses.

The strength of a bend depends on the radius of the bend and the aluminum alloy used. The most common aluminum alloy used for extruded aluminum profiles is 6061 or 6063. 6061 has an elongation at break of roughly 25%. For some aluminum alloys the elongation is only 3%! To keep a bend of a 10x10mm tube within the 25% the outer bend radius would have to be of the order of 50mm. A bend like this reduces sharp corners in the structure, which create stresses. A bend that keeps within the elongation can be even stronger than a bracketed one.

A good point to consider is that aluminum work hardens. If you bend it, it will turn harder but more brittle. If you bend it enough, it will crack. It might be a good idea to warm the bend with a blowtorch to anneal the residual stresses within.

[karolis]

Wow thanks man. i never expected such concrete info. Great. So if i received 6061-0 alloy with 25% elongation, and heated the bend area while bending it 90 degrees with 30mm outer radius, would it be even possible without proper tools? i.e. by hand

[Javerh]

You'd want the aluminum to reach a superplastic form. For 6061 superplastic area is somewhere around 290-325 celcius. Unfortunately I can't find a clearcut answer on what the maximum elongation is at that stage but my sophisticated guess is between 100% and 1300%

For a version of 7475 alloy a stress of 5.25MPa at superplastic would give a 310% elongation. You could reach that kind of stress by mounting a 10x10x1 square tube in a vise and pressing a 300mm free length with a weight of 0.1kg. Sound definately doable! 6061 is even more elastic than 7475 so bending should be even easier.
Don't take my word for it though. You should really get a small sample and try how it behaves.

[Burnout21]

Originally Posted by Javerh

You'd want the aluminum to reach a superplastic form. For 6061 superplastic area is somewhere around 290-325 celcius.

little tip from me, if you want to bend aluminium easily coat the sheet in soap yes soap (bar form) that stuff you use for washing your hands. when heating the aluminium up you need to guage the temperture, at ~300 degree's the soap turns black and thats when i stop heating. then let the sheet cool slowly. you will now have a stress free area where the heat was applied. now for an insanely easy bend.

And yes bend by hand is piss easy! note it will be soft so heat it up again and cool it in water. this will harden the aluminium.

[karolis]

Nice. So it is possible. Thanks for the info. this will hopefully mean 8 less joints for me. oh well. back to studies now. or i'll never get into uni

[B[x]]

You'll do fine with 1/16" wall thickness on your angles and channel. Just make sure your joints are tight. No matter how strong your case's members are, weak joints will be its undoing.

[ricickle]

You could work out how much force will be on it and then use bows notation to work it out.

[quietguy]

I imagine it will be more than adequate as long as you're not balancing elephants on it. The only thing I would suggest is to shim the low spots on the exterior; that way, when you add the skin, it will make it stronger.