I have a project where I need to make really nice clean holes about 90mm diameter by about 40mm deep in upholstery foam and was wondering if anyone has done this before.
I’m thinking maybe a holesaw which has had the teeth removed and then sharpened up might do the job…
Typically, a foam converter will use a waterjet to cut such a hole.
If this is a DIY, get a steel-rule die made. It will cut the foam no problem. The die should cost more than $100.
If you are trying not to cut all the all through, that will be difficult. You could use the steel-rule die, cut through, trim to the proper length, glue back in place. Or center the steel-rule die so it can be placed in a drillpress chuck. Nick the edge of the die, use it as a drill. Compress the foam around the hole, remove the 40mm with a long-bladed box cutter.
I’d recommend a combination of the above - soak it, freeze it flat and then use a metal cutting 3 1/2" hole saw (has tiny teeth rather than the demo hole saws with big knarly teeth) with a center guide bit on low speed to the depth you need.
Just to throw out another possibility, although the best approach really depends on the qualities of the specific foam you are working with, you might consider sharpening the mouth of a piece of steel piping and using that. With a softer metal like brass you can actually sharpen it by hand with an exacto, but steel you’ll likely need to use a lathe to cut a sharp edge.
I had to cut holes some ~10mm felt recently, and any conventional drill bit would not have worked because it would catch on the material. So I chucked up a steel pipe with a sharpened edge in the drill press and it worked quite well. This is also a good tool for cutting holes in something like foam core. You can also make cuts with it free hand, twisting it back and forth.
Of course if you aren’t going all the way through you’ll need to find a way to cleanly trim out the waste.
Note though you might need some kind of core in the pipe ‘bit’ you fashion, so that it isn’t deformed when you chuck it in the drill press.
The soak and freeze method sounds pretty cool, never heard of that!