At work, I need to cut a lot of closed-cell polyethylene foam, in thicknesses from 0.25" to 1.5", to make custom Pelican case inserts… I’m currently doing it all by hand, using cardboard templates, but would like to reduce the amount of my time that it takes, and improve the neatness of the cuts. However, all of the commercial and DIY foam cutting solutions I’ve found are based on a stretched hot wire, which is fundamentally incapable of making interior cut-outs, a feature that I absolutely must have. So, I’m looking into the possibility of building something of my own, most likely based on an X-Carve (due to the extreme flexibility in ordering parts for them, and my experience with the original Shapeoko machine).
There are hand tools for making plunge cuts in foam, that look like this:
There’s even a CNC version of this tool, that has a mounting bracket instead of a handle, although I haven’t found any references to anyone actually using one. The problem with these tools is that the needle is really, really bendy - you have to move extremely slow (multiple seconds per inch of cut) to avoid damaging it.
As a proof-of-concept, I’ve tried a test cut with this hand tool jammed into the vacuum hose mounts of my old Shapeoko 1. (The machine is nowhere near big enough to do my actual cuts, which are about 22" x 17".)
Even with the feed rate slowed down to 5 IPM, the results aren’t great… here’s the top view:
And here’s a bottom view of the 1" thick foam, with even more deflection in the needle. All of those edges in the star are supposed to be straight!
Frankly, I can do a better and faster job by hand. Does anyone have any ideas on how to do this right?
I have three ideas, none of which I’m really happy with:
-
Give up on the idea of heat-cutting, and use a normal spindle with a specialty foam cutting bit. I’m hesitant to spend the $50-$80 for the bit, when I have no idea how well it will work or how long it will last, and I expect that this would produce a tremendous mess (with heat cutting, you only need ventilation, there’s no swarf).
-
Build the machine with two X gantries, one above and one below the foam, with carriages that move in unison. The top carriage has the usual Z axis mechanism, the bottom one simply has a hole (with a tapered lead-in) that receives the tip of the cutting tool when it is fully extended. That should brace it well enough to be able to cut at a reasonable speed - there would be some bowing in the middle of the needle, but I would expect it to be minimal. This is getting pretty complicated, and I have no idea how to mount the foam in the machine - the thinner sheets aren’t rigid enough to support themselves in the middle if held only at the edges.
-
Somehow adapt the machine to use this style of foam cutter:
which is what I use for hand-cutting in areas without any small, curved details. I don’t think a drag knife kind of setup would work (when hot, that blade will happily cut sideways about as well as it cuts along the proper direction), so an actual rotary axis would be needed. That rules out any sort of GRBL/G-Shield control system, and would probably require a completely custom CAM program - it seems unlikely that any off-the-shelf CAM solution (that doesn’t cost several fortunes) would know how to drive this. The necessary swivel mechanism is probably beyond my mechanical design & fabrication capabilities, anyway.
