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Well I am using M4 bolts and 10mm HDPE, seems very secure to me. As for stripping threads, I would say drill a 3.5 (or slightly smaller) and drive bolt in with a drill/good electric screwdriver to help cut a deep thread and heat up the plastic around it to hold it in better.
what about something like these
http://www.ebay.co.uk/itm/M4-x-8mm-U...item3f16213303
or these
http://www.ebay.co.uk/itm/M3-x-8mm-S...item41696e7003
The thread makes them better for fastening.
Thanks for the responses guys. All taken in. I may do a combination of all these: just tap it to m4 for things like top panels, but melt in proper nuts for the major joins, or go the thread insert route.
Also, here's yet another design, partially based on team RCC's Valkiri 2.
http://13q.imghost.us/Yc/Beetle%20new%202%20pic2.jpg
It may be possible to bend the plastic into shape by heating it up then bending it around a table edge- that way you avoid needing a fixing at all. You could also make up some little brackets from bent aluminium or little blocks of hdpe with holes drilled through them at 90 degrees to each other- then you can bolt the walls to these blocks with a nut on the other side rather than having to tap the hdpe.
I'm going to have to do the latter to attach top panels etc ^
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Was chatting to Harry earlier, and we decided having a silly-fast beetle would be great fun. I had to go and put T2 as back together as possible anyway, so after that I fiddled for a bit. Got this far, angle grinding comes next and not sure how well that'd go down at 11pm, so I'll continue some other time!
http://13q.imghost.us/CT/DSC_0041shrunk.jpg
The question now is, do we make it go stupidly fast.. or gut wrenchingly stupidly fast?
the gut wrenching fast is good :) I will make one soon.
That drive system looks awsome! Is the gear inside the wheel the main gear out of the drill gearbox? And for stupidly fast, what voltage and gearing will you have?
Yep, that's the ring gear from a drill, with an HDPE hub pressed in (drill press poor-man's lathe style), and then an old Meccano wheel and tyre, pressed around the ring. If it needs it I'll solidify it with grubs but for testing it seems stiff enough.
The ratio is, calculated roughly, 5.3:1. With the 12v motor at 3s, that's 20,000rpm on the input, so after the reduction that's 3,750 odd rpm on the wheel. The tyre in the pic is 72mm, results in 30mph, the one I'm more likely to use is 55mm dia, giving roughly 20mph.
I think I'll go for the latter but the chassis I'll build around the drive will allow for the larger tyre, should it be desired!