Foam Hole Saw
I needed to cut round holes in dense foam for my Pentaximus camera case. Rectangular holes are easy, whip out the electric carving knife, bread knife or use a band saw. For outside cuts my band saw works great and if you want to glue a cut closed you can do interior cuts quite nicely.
I can't claim the idea of using a food can as the cutter, but I think I have improved upon the concept.
First step was to use a European can opener to remove the lid. The can was emptied and washed out. I then used a belt sander to remove the rim of the can. A sheet metal notching plier was used to cut the "teeth"
I used Fusion 360 to design a drive flange and 3D printed it with PETG filament. Initially I had pressed a piece of 1/4" hex rod into eh flange but it only took three hole cuts before the part rounded out. Regrouping, I drilled out the flange and pressed a T nut into the bottom and a piece of threaded rod and a 1/4" coupling nut completed the tool.
The initial design shown on the left. While clever use of the design software to make a hexagon hole, the torque was too much. Right image shows the final design.
The hole saw does a fairly decent job of cutting clean holes in the dense foam of the camera trunk. I decided to make the drive flange for the cutter as I had to cut 7+ holes through the 2" thick foam sheets in two layers, so a minimum of 14 holes. Hand turning the cutter does work, but more than one or two holes will be devastating to one's wrist.
The saw worked even better in my drill press on its lowest speed your mileage may vary...
I found both my Metal (slow speed) and wood band saws cut this foam just fine. The foam was sourced from eBay: Foam Link According to America's Test Kitchen the best electric carving knife is the Black & Decker model which can be had for about $20 on Amazon.