help cutting hardboard

[Dave Brandt]

My nephew (in Atlanta) asked me how to cut a 10" diameter hole in 3/4" hardboard. (I didn't even know they made 3/4" hardboard! Anyway, I didn't know, so I'm going to the experts. He has a jigsaw and a router available. I wasn't sure what kind (TPI) blade to use in the jigsaw. I thought several light passes with a router on a trammel would work. Any ideas?

[Dean Baumgartner]

Dave,
I'd rough cut with the jig saw to just a little shy of the finished line then finish off with the router and a pattern bit to get the best surface. You'll also have to make a 10" pattern probably out of thinner hardboard.


Dean

[John Miliunas]

I would do the whole thing with a router. Grab some 1/8" or so hardboard and make yourself a quick circle jig, using the baseplate of the router as a template. I just did this a couple weeks ago myself for a mini-project for LOML. As I was making it, I decided to extend out the jig a bit and punch some holes for several other size circles. With hardboard, I'd probably use about a 3/8" straight bit in the router and keep plungning about 1/8th to 3/16th's with every full rotation. I would also very HIGHLY recommend a dust mask!

If you need a bit better idea of what I'm talking about in making one, drop me a PM (as a reminder) and I'll go out to the shop in the morning and snap a couple pics of mine.

[Dave Brandt]

Thanks guys! I like the idea of using the routers baseplate as a guide. I didn't think of using a template because he said none of the saw blades he's tried would even cut the board! I was thinking he was just using the wrong type of blade, but more likely they were just a cheap, off-brand. The hole doesn't have to be perfect, because it's getting covered by some type of framing (for a speaker). I was just wondering how many tpi is appropriate for hardboard.

[Daniel Rabinovitz]

May I add some additional thoughts to John Milianus' idea.
On the back side of the hardboard - hot glue (12 dots) (or double stick tape) a 1x2 or 1x3 across the diameter of the hole and extend several inches beyond the hole diameter. Glue to hole and outer edge. This will keep the hole from rattling around as you cut through.
Set the router bit to a depth slightly into the cross piece but DON'T CUT all the way through it.

My thinking is that once the router cuts it loose, you flinch and the router will dig into the outer edge of the circle. Your pin is only into the center circle that you are cutting away from the main piece and the router is then able to float and gouge because it isn't pinned to anything but the center circle.

Rout from the top side and let the "pin" for the circle template extend down into the "cross" 1x2, so it holds the cut out, while the cutout is "floating" as the router slows down.
I think that's it.
Daniel
ps have I hit everything, John?

[John Miliunas]

Very well done, Daniel! You picked up the ball and ran with it right from the point where I fumbled it! (It just never fails to amaze me, the amount of times a guy does something, but then you loose it in the translation! Sheeesh.)

[Dave Brandt]

Great idea Daniel! Thanks again to SMC, something will get done without killing (or maiming) someone! Gawd I love this site!