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What is the req.thickness of sheet metal to prevent buckling

  1. Sep 8, 2015 #1
    My project involves making a 5 ft*3 ft*4 ft (l*b*h) aluminium/stainless steel sheet metal box at the back of a slow moving pedal vehicle. There is no load on the sheet metal, except from small vibrations from road travel.

    My question is: 1. What gauge of sheet metal thickness would be enough to prevent buckling from its own weight? How do I calculate this without experience?
    2. Would you recommend using a stiffener just for a higher factor of safety? If so, which type?

    As an aside: Can one join the aluminium box without welds/braze i.e just using hems, lock seams etc.?

    Thanks in advance!
     
  2. jcsd
  3. Sep 8, 2015 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    What is the platform/box for?
     
  4. Sep 8, 2015 #3

    Baluncore

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    It is the topology of the surface rather than the thickness of the material that confers resistance to flapping. Machines for working sheet metal are optimised for cutting and forming straight seams in flat sheet. That leads to inherently noisy structures.

    Notice that large flat sheets sometimes have a slight X pattern bent onto their face. That is there to prevent flapping and can be formed with available straight bending equipment. The pattern generates four curved surfaces. Parallel corrugations also increase rigidity.

    Membranes that are doubly curved tend not to flap like flat sheets. To get a double curved surface in a membrane you need a minimum of four corner points, that are not co-planar, then tension the membrane.

    Aluminium boxes are usually built with folded corners and TIG or MIG welded seams. But for your one-off production, I would consider pop riveted joints with a silicon or mastic sealant in the seam that will also quieten the box.

    Design your box to have slightly curved or corrugated faces, i.e. parts of an 'S' shaped, cylindrical or conical profile. That should prevent flapping of flat faces. You may need to cut and bend the seams by hand.
     
  5. Sep 17, 2015 #4
    Apologies for not replying earlier. Thank you for your kind reply. The platform and box are placed behind a mobile 3-wheeled vehicle i.e in between the rear 2 wheels. The purpose of the box is to store refrigerated food, which would then be heated using a gas stove placed at the back, and sold as fast food. A draft of the box can be seen from the image I've uploaded along with this post.The red part you see is the side door for ingress of the boxes. The orange door you see is a pivoted door that can be swung-up and pushed inside.

    The box is divided into 2 compartments: one to store the refrigerated food in boxes and another that houses a cylindrical gas cylinder and stove.
     

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  6. Sep 17, 2015 #5
    Apologies for not replying earlier. Thank you for your kind reply.

    FYI: I have decided to use a 1.22 mm Mild Steel sheet-metal, because of the non-availability of 20 gauge aluminium in our village and a lack of pop-rivet guns.

    When you mention a slight X pattern on face, does one use the term 'x-shape surface seam' in the market catalog? From my internet research, it seems making seams on sheet-metal requires the use of a die and a press, which are not available in the workshop. Any alternatives to make surface seams? For now, I am just welding-in rectangular profile stiffeners for a gap of every 18 inches.
     

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  7. Sep 17, 2015 #6

    Baluncore

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