I need to cut kind of thick plastic

I have these "universal" plastic speaker adapters:


But the hole in my car is too small to mount the speaker on top, and the hole in the plastic is just slightly too small to fit the surround of the speaker if I mount it from the bottom. I figure it will be a lot easier to widen the hole in the plastic rather than in the car's sheet metal, but it's still not easy. Any tips? I've tried a box cutter knife, pocket knife, soldering iron, torch with soldering iron attachment, chisel, etc. Probably gave myself cancer from the fumes from trying to melt it. It's not terribly tough plastic, but it's kind of thick, so it's not easy. Plus it's difficult to cut a circle from the inside. I need to increase it from 13 cm diameter to 13.5 cm.

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it's this part


endolith (author) 9 years ago
The coping saw worked well. I now have new car speakers! (And a broken Dremel tool.)
Too bad about the dremel, but at least the speakers are working!
endolith (author)  Weissensteinburg9 years ago
Yeah. I just need to find the replacement part to fix it. It looks like this:

Where does it go?
endolith (author)  Weissensteinburg9 years ago
It's the part that connects the motor to the chuck.

Ooooh. You shouldn't have torn it apart. That would have been covered under warranty. Mine Broke and I sent it in. A few weeks later I got a new one in the mail! If you had it less than 5 years it's under warranty. But you opened it and the warranty is voided. Although, I like to fix things. Good luck with the repairs! not sarcasm
endolith (author)  gimmelotsarobots9 years ago
The tool is super old, probably from the 80s my dad said, and the part only cost me $10 and was easy to replace. :)
Cool! I'll always take the easy way out if I can because I usually have many projects going at once so any spare time I can get is much appreciated.
Will it be easy to replace?
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