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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endolith (author) 8 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)  Weissensteinburg8 years ago
Yeah. I just need to find the replacement part to fix it. It looks like this:

Where does it go?
endolith (author)  Weissensteinburg8 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)  gimmelotsarobots7 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?
endolith (author)  Weissensteinburg8 years ago
Yeah, if I can find the right part.
You can't get it from the link you posted above?
endolith (author)  Weissensteinburg8 years ago
I just have to make sure that part is identical first.
Gotcha...maybe you could call Dremel and get it cheaper, though.
endolith (author)  Weissensteinburg7 years ago
Awesome, good luck with the repair.
did you try tin snips?
Flumpkins7 years ago
How bout' a chainsaw?
Rishnai8 years ago
Sawzall, portable jigsaw, or scroll saw is what you need. You could also try to file like forgesmith suggests, or use a sanding drum. The sanding drum on your dremel would work, if a little slowly. A three-inch drum the you can check into your drill is probably better, and would be handy to have around anyway.
Math is an issue. 0.5 cm diameter increase => .25 cm off the side => just under a tenth of an inch to come off, about 3/32". How thick and wide are the blades for your sawzall (reciprocating saw)? That'd be a tricky cut with anything but the scroll saw.

Does the entire circle need to be increased to get the speaker in, or just some high spots taken down?
I've gotten some pretty precise cuts with some clamps, a vice, and a jigsaw. It's less like removing a chunk than making a chainsaw carving, where you're using the saw and blade in the non-reccommended manner. I've done a little plunge-routing with a battery drill and creative clamping, too, but I wouldn't want to have tried either for the first time on anything that wasn't free...
endolith (author)  Rishnai8 years ago
I don't have a jigsaw. :) I don't have any large tools. Another option might be a little thin blade in a small hand saw.
Excellent idea. That would give good control and good cutting.
endolith (author)  forgesmith8 years ago
Yes, the entire circle. I want to mount the speaker from the bottom, so the hole has to be big enough to fit the speaker surround.
Since the dremel has already been suggested...sand paper? If the math the forge did is correct, a tenth of an inch isn't much to sand off.
endolith (author)  Weissensteinburg8 years ago
I would need to sand off about 1/4 inch. Really too much work. The dremel was working fine until it broke, and I don't know if I can get a replacement part easily. I'm going to buy a coping saw for now.
forgesmith8 years ago
Sure hope you got them for free, only 1 of 7 reviewers didn't report modifications, with one of those simply using them for a template.

You don't have to cut it, just shave off some from the hole. A half-round file with the part in a small bench vise would work, file close to the jaws to keep it from vibrating much. Likewise a Dremel with a straight cylindrical burr would work nice. Don't know about using any kind of grinding or sanding tool with the Dremel as the plastic would likely clog them up pretty quick.
endolith (author)  forgesmith8 years ago
Yeah they're free. Filing it would take forever. Dremel burr tool looks good. I don't know where i left my Dremel tool though.
endolith (author)  endolith8 years ago
I found my "Craftsman rotary tool" and it worked great until it broke. :( I got one almost done and then something inside the tool broke, so I'll have to figure out how to fix it.
endolith (author)  endolith8 years ago
Grr. There's a piece of plastic that couples the motor to the chuck, probably meant to break under stress, but it broke under normal use. Now I need to figure out what it's called so I can replace it.
Senseless8 years ago
Use a handheld router like for use on drywall, and a drywall blade that's got the threads opposite of a cutting blade for wood and go slow. It will melt you a nice line. A dremel might be able to do it to.
endolith (author)  Senseless8 years ago
I don't have a router. Dremel might work but I'd need some kind of blade attachment. I think I just have sanders... wherever it is.