Which way of joining is stronger?

I have to build a box to extend the length of a projector bracket:

The projector bracket weighs 7kg and has an arm of about 500mm. The projector weighs another 5kg.

The box has a depth of 140mm. That is the length by which I need to extend the length of the projector arm because the wall on which the bracket is mounted is recessed by 140mm behind the image surface.

Does one of the following two ways of joining offer a stronger structure?

I don't have tools to make complex joints. I can only screw ½" plywood at right angles.

• Duplicate of engineering.stackexchange.com/questions/26407/… – fred_dot_u Mar 17 '19 at 0:05
• @fred_dot_u has you covered in his Answer on the other SE, but if you'd like to read a bit more see this other Q from here from late last year for other relevant input (be sure to read the Comments). – Graphus Mar 17 '19 at 7:25
• Why not both? Put a block inside as in A, and then cap the assembly with another block, as in B. Will require aligning the screws so they don't run into each other. – Brydon Gibson Mar 18 '19 at 20:04
• Are you constrained to making a box from thin material? A solid block, possibly laminated, would be cheaper, stronger, and easier to make. – Spencer Joplin Mar 23 '19 at 5:49

1 Answer

IF you consider the connection of the bracket to mounting box as a hinge (or rotation axis) the projector will exert a downward force that will try to rotate the bracket counter clockwise in your diagram. This force will attempt to pull the top of your box out top left connection. IN option 'A' this rotational force is resisted by the hold that the wood has on the threads of the screw. In option 'B' the rotational force is resisted more by the sheer strength of the screws. In this situation I would trust the strength of the steel screws more than the resistance of the wood to prevent the screws from pulling out making 'B' the better choice.

Since the weights are fairly light and the bracket is not setting out from the box very far, it is probable that both options would be satisfactory especially considering that there are multiple screws across the top of the box and together they should be adequate to prevent the screws from pulling out of the top of the box. I would also not hesitate to skip screws all together by creating a rabbet at the corners and gluing the wood pices together.

• That was my initial analysis. Then I started to worry about what happens if the plywood against the wall delaminates when the screws go into it? The rest of the box and the project would just rotate about the bottom corner of the box against the wall. – Martin Bonner supports Monica Mar 22 '19 at 12:49
• I think it would take quite a lot of weight on the projector support to delaminate the plywood especially if there are multiple screws. – Ashlar Mar 22 '19 at 20:36