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I am making a custom camera rig for the kite aerial photography and I need a ┏┓-shaped frame like this:

frame

(depth × width × height = 40 × 200 × 100 mm, thickness ~5 mm).

I cannot use bent aluminum as it is pretty dense (2.7 g/cm³) and I do not have any bending machines.

I cannot use 3D-printing as I don't have an easy access to a 3D printer. The plastic seems to be dense too: 1 – 1.4 g/cm³.

So, my choice is old-school wood. Plywood. As a bonus, it is very lightweight: about 0.6 g/cm³!

I could probably cut tree ▌-shaped plates and glue them together at 90°. But I am afraid that the whole frame would not be rigid enough if forces applied to like this:

forces

Just get me right: I am pretty sure that it would not break, but I am just uncomfortable with that feeling.

Moreover, as the plywood is pretty thin, I would probably need extra plinths to glue the surfaces:

plinths

So, I am thinking about CNC-cutting ~ten ┏┓-shaped parts and gluing them together:

composed

It seems for me, that the resulting frame would be much stronger to forces applied in all the directions, compared to the three plates versions above.

Question is: am I right? Or should I just make the frame of three plywood plates? Any nuances in assembling together such a stack of parts?

  • How much squeezing force will you have? None of the designs will be very resistant to being squeezed at the bottom, the wood will bend. – Mattman944 Oct 23 '19 at 18:01
  • I don't expect any squeezing in normal conditions. But if something goes wrong with the kite or wind the frame (loaded with 200g of electronics) can fall from 3-10 meters. I hope it won't happen. – madhead Oct 23 '19 at 18:11
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I agree with fred_dot_u, and I'd add a few points:

  • The grade and type of plywood is important. CDX will have 3-4 plies, gaps, knots, etc. that could dramatically weaken the critical corner areas of your piece. Better plywood has no voids, many thin plies, and higher-quality veneer.
  • A bias might be the best approach. If you cut square with the sheet, half of the plies will be aligned in the weak direction. If you were to go with a 45 degree bias, every ply would lend strength (though none would provide full-length grain).
  • Cutting a series of fairly large circles out of the center area of the panels could dramatically lighten the structure without undue weakening.

All that said, a few pine boards with a small block inside each corner (cut from the same boards, in lieu of the plinths) and high-quality adhesive would save you a whole lot of cutting.

____________________________________________
|  *     *                                   <-- top board
|
|___________________________________________
|     |     |
| *   |     | <-- block
|     |_____|                  * screw locations
|     |
|     | <-- side board
|     |
  • Thanks. I thought about the bias and I'll try to cut the parts with different angles. And what do you mean by "pine boards"? Solid wood? – madhead Oct 23 '19 at 18:13
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    Yes, solid wood with blocks running parallel to the corner on the inside. A few drywall screws and some urethane glue and you're done. – isherwood Oct 23 '19 at 18:25
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Of all your designs, the last one certainly has much to its credit. If you use sheets of plywood to create the individual inverted-U shapes, you'll gain that much more strength. Consider a combination of the design previous (with the gussets) and cut your shapes to include an arc to the inside of each leg. This provides for contiguous transfer of forces as well as looking good.

Alignment is unlikely to be critical in your application, but you'd want to have as clean alignment as possible. Drilling holes in each plate in the same location, then pinning the layers with a dowel rod will make for ease of assembly. Once glued together, you can glue the dowels in place or remove them as you like.

If your plywood source is recycled skateboard planks, your work will resemble the last image that much more!

  • Thank you for quick answer! There will be another frame inside this frame. Two frames provide rotations in vertical and horizontal axes. Gussets will force me to make frames bigger, so I'd like not to have them. A small arc, though, seems to be a good idea! – madhead Oct 23 '19 at 18:18
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If you CNC cut 10 U-shaped pieces from 1/2" plywood then glue it up, as you show in your last image, you essentially have one U-shape cut from a piece of 5" thick plywood, except that you don't have the pressure and heat used in manufacturing plywood.

By cutting flat shapes, you would have some increased strength on the corners, but I'd still want to have corner braces on them.

If you're going to the trouble of cutting & gluing all those pieces and adding the corner braces, you may as well go with 3 flat pieces glued & screwed with corner braces added. It would be much simpler construction, and likely just as strong. I'd go with solid wood instead of plywood in this case, since solid wood will take and hold screws much better than the edge of a piece of plywood will.

  • Thanks. I meant ~⅒ inch plywood (3-3.5 mm, three layered plywood). I think it's difficult to use screws with that thickness without the risk of tearing the material apart. And gussets will "steal" the space: there is another similar but smaller frame with a camera inside this one and it needs to rotate freely. – madhead Oct 23 '19 at 18:43
  • @madhead I picked 1/2" (~12.5 mm) as an example. However, the analogy still stands - you're building a new sheet of plywood 10x thick where x = 3mm instead of x=12mm. You may consider simply buying one sheet of 25-30 mm plywood and cutting one single U-shape out of that instead of laminating multiple individual sheets to come up with that thickness. As mentioned by fred_dot_u, a rounded corner instead of sharply square should give you added strength without loosing too much space for your inner frame. – FreeMan Oct 23 '19 at 18:54

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