Hi Gerald,

The stongest way to do it would be to mitre the corners of the box section (like a picture frame) to make them intersect at 45 degrees. You could then use a small amount of structural polyurethane adhesive or structual epoxy adhesive to bond them together and then reinforce the joints using gusset plates made from 1mm pre-preg carbon fibre sheet, cut to shape, and bonded (again using structual adhesive) to the top and bottom of the box section.

The 3mm bar that you want to have as a central divider divider is a little more tricky because its OD is so much less than the OD of the 10mm box section. I think I might be tempted to use some much thicker pre-preg carbon fibre sheet (like 3mm, or 3 layers of 1mm keyed and bonded on top of eachother to make 3mm) to make a block of 10mm x 20mm and then drill a 3mm hole in the middle of the block. If you made 2 of these blocks you could then bond the blocks to the inside face of the box section and then assemble the 3mm bar into the 3mm hole in the block (where the box section would act as the back-stop) and then bond the whole assembly together. I don't know what your application is but I would consider this to be a very strong (and quite simple) way to make this assembly.

Best regards, Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Wish to make square frame of `10mm box section with one central divider of 3mm bar. please advise of jointing methods.