I needed to cut a bunch of styrofoam for an insulating project. I found that serrated blades like cheap steak knives or various power tool blades made a horrible mess on the floor of my garage. The best thing I found to cut styrofoam was a sharp fillet knife. After a while (like 50 different rectangles cut out later)I found that it made my wrist hurt. Strangely enough I wanted to avoid carpal tunnel so I came up with a reciprocating power knife. I also didn't want to mess with hot wire cutting. It stinks!
This could certainly be used to cut anything else. Have fun finding out what.
Step 1: What You Need
You'll need some kind of power saw. You could do this with a jigsaw, but you'd want it to have a long enough blade to cut through what you're working with. I used a reciprocating saw blade.
You need something to shape your knife blade with. I used my bench grinder, but you could use an angle grinder, rotary tool, or metal files.
Step 2: What to Do
You wanna sharpen that thing. I used a 20 to 25 degree angle to simulate most knives you'll find anywhere in the world. You can then sharpen this with the same sharpeners you might use in your kitchen for your chef knives. A sharpening stone or files will keep it sharp for you.
Wear ear and eye and hand protection. Ignore that I didn't bother with wearing gloves.
Step 3: Putting It to Good Use
Mount your blade and take it for a spin. I found that what's most important in using the reciprocating saw for this is supporting your foam so that the last of your cut doesn't break off because there's a lot of shaking and movement and what little foam there is pretty weak.
I think it works wonderfully and doesn't make much mess compared to scoring and snapping or serrated blades. If you don't like the smell of hot wire cutting this might be the way to go.
I think a multifunction oscillating tool would work well for this. Take a dulled blade from that and make it into a knife and go to town.
Go and do likewise!