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Unread 15-12-2004, 12:18
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Gary Dillard Gary Dillard is offline
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Re: Has any team or anybody made a 80% - 100% lexan bot?

A few points about Lexan that we've learned over the years:

1) All polycarbonate is not created equal. We have tried cheaper forms of polycarbonate and found the the GE LEXAN brand name is definitely stronger / more rugged than others. There are different varieties of GE LEXAN as well, but in general they are all better than other manufacturers.

- Also, please don't confuse Lexan or polycarbonate with acrylic. You might as well make your robot out of glass if you go that route.

2) We started using Lexan when it was an unlimited allowable material because it made very nice assemblies. It is easy to machine and it makes nice square corner joint assemblies - we drill pilot holes through the edges using 3/8 inch thick Lexan and use self tapping drywall screws. You need to use Lithium grease or the screws will shear while torquing (use the low torque setting on your cordless drill when installing them), and don't use locktite because it heats up, expands and cracks/delaminates the lexan.

3) An important point to understand when talking material choice is that for impact loading (like running into other robots or them running into you or your components) the imparted load is NOT INDEPENDANT of the material or construction of your robot. The kinetic energy input (1/2mv^2) is absorbed by the structure based on its spring rate; a very stiff structure will take a very high effective g-loading, where a soft structure will see a much lower load. So you could say for instance that aluminum is twice as strong as Lexan, but it the aluminum structure will see 10 g's and the lexan structure will only see 2 g's when they run into something, the lexan may be the better choice if you can tolerate the deflection. This is frequently the case with some mechanisms that don't require precision, and why PVC is a very good choice for them in many cases.

One other point about aluminum if you're making the comparison for a structure: 6061-T6 loses 2/3 of it's strength when welded. The Aluminum Association Handbook takes it from 35 ksi yield to 12 ksi yield in the heat affected zone (within 1 inch of the weld) so be careful.
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