Would I be able to use the Dremel multipurpose cutting bit (561) to cut patterns out of 1/2 inch polycarbonate (Lexan)? Dremel "experts" said no but I want to hear other people's opinions. According to them, the bit will snap.
1/2 inch is pretty thick material to work with. A saw is the right tool for this job. You can use a table saw, circular hand saw, jig saw, or band saw to cut Lexan. This Processing Guide (PDF) has details on the sorts of blades you should use.
It does say that you can mill Lexan with at bit more like the Dremel Router bits they recommend 25,000 - 30,000 RPM. But, I would be concerned that a Dremel doesn't have the torque needed to handle cutting plastic this thick.
Here is what I would do if I did not have a suitable power saw: Rough cut the lexan with a hand coping saw. Then with a router bit (the largest that will work) go back and trim it down to the pattern. I like a router bit better than the spiral bit because it is not in constant contact with the material so I'm guessing it would be less likely to melt.
Of course I would start by testing with a bit of scrap first.
Polycarbonate likes to melt when you hit it with a high speed blade. The teeth on your 1/8" spiral cutting bit will quickly goo up, and you'll find yourself melting rather than sawing. With some skill/practice, you'll probably be able to make your plastic into about the shape you want. Biggest trouble will probably come from you making the cut too quickly, which'll cause the bit to flex, and melt an irregular path top-to bottom through your Lexan.
-I'd use a jig saw. It's a lot of work, but far less prone to disaster than 30,000 rpm cutting bits.
If the dremel is all you have, it will cut, but be careful about trying to hurry it along. The Rotozip Saber is the same blade, sometimes cheaper per bit. Chuck it as low as you can (less exposed) and use the router attachment (has less tendency to tip and cause side torque (which is what causes breakage)).
If you have access to a jig saw (saber saw), that would be my tool of choice.
A band saw would be the ultimate.