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MotorradMike
04-11-2010, 12:16 PM
Hi guys:

This worked so well I just have to share it.

I'm installing a small electret mic. into a helmet. I decided to try a "circular knife" for making a hole without knowing if it would work.
I turned a piece of steel to the same OD as the mic. Then I center drilled it with the largest bit I had that would fit. My first surprise was that the steel held even with a finished wall thickness of .009".
A first attempt into scrap styro went well so here's how it went with the helmet foam.

http://farm3.static.flickr.com/2697/4511210556_41c0116f11_b.jpg

MTNGUN
04-11-2010, 12:18 PM
Good job ! Thanks for the pictures.

Evan
04-11-2010, 12:28 PM
This is the one I use. It works well on soft foam too.

http://metalshopborealis.ca/pics/holecut1.jpg

Dragons_fire
04-11-2010, 01:08 PM
Looks good! i always use a piece of brass tubing and heat it up with a torch and then just plunge it in. although depending on the foam, it usually goes kind of crusty around the edges and doesnt give as nice of a finish as you have there.

jacampb2
04-11-2010, 03:24 PM
That type of tool also works great for punching holes in gasket material. I always keep cork and paper gasket stock around for those automotive or farm jobs where it is difficult or impossible to get a single gasket w/o a kit. I will normally just find a piece of steel brake line tubing in the approximate correct size, sharpen the OD free hand on the bench grinder, and turn and push into the material. If it's thin material, I use a cardboard backer to keep the edge nice.

Cool Tool!

Later,
Jason

darryl
04-11-2010, 05:24 PM
Nice! That's how I do it also. I've used everything from brass tubing to stainless steel tubing- mostly dictated by the size of hole I need and what material I can find to make the core drill out of. I imagine cro-mo tubing would be pretty good for that- brass gets dull pretty quickly, stainless works pretty well.

Hole saws just make a mess of styrofoam and foam rubber.

nheng
04-11-2010, 05:40 PM
I've made similar punches out of mild steel and used them on an arbor press with a backing block of UHMW. Also works great on lots of materials including felt when you're cutting smaller holes for wipers,etc.

I kiss the outer edge with a steep chamfer to sharpen it so the ID is of precise diameter.

Den

TDmaker01
04-11-2010, 11:09 PM
I remember way back in high school to cut styrofoam we made a big U shape out of some 1x3 spruce and srung mechanics wire across the points and hooked it up directly to a car battery or a car battery charger and when the wire heated up it could cut through 12" of foam like a hot knife in butter.

Duffy
04-12-2010, 03:10 PM
Very nice Mike, but did you make any profit selling the swarf? Duffy

MotorradMike
04-12-2010, 06:06 PM
Very nice Mike, but did you make any profit selling the swarf? Duffy

Um, no. I don't know what you're talking about.