[Rokeby]

Neil,

The conditions you're encountering must be making you crazy; so close,
yet so far!

What type of cutting bit are you using now?
Are its cutting edges parallel to the bit's axis, or do they spiral around the
axis?

I didn't anticipate the overheating and "cotton candy" problem that you are
encountering. Still, I'm willing to take a stab at a work around.

Of course, the low melting temp of the foam material is a major factor. You
don't typically have this problem with wood -- the feed rate is way too fast
and/or the bit is dull -- or thin foam sheets.

I'll guess that one problem is that as the bit advances into the foam
material, it is cutting around the half-perimeter on the leading side of the
bit, and this is causing the bit to heat up.

The bit may be dragging a little on the sides of the cut as well. This may
be made even worse by the foam swelling a little when it is locally heated. I
suspect that when the foam heats up the bit then has to cut around more
than its perimeter.

I'm thinking that the solution may be to cut a channel in the foam that is
wider than the bit. This would require making side-by-side passes, with
the second pass along the actual cut line. I'm thinking an offset of 1/2 to 3/4
of the bit diameter. Cutting out would be making a series of two-pass slots
separated by small tabs.

Rather than try to suction foam chips out of the cut, it may be more effective
to blow the chips out with a stream of compressed air, which may help cool
the bit as well. But you'd also need a larger, but highly effective vacuum
hood to contain the flying chips.

He's my reasoning. On the initial cutting pass, the bit has to cut around its
leading half perimeter just to make forward progress. Here's where the heat
buildup originates. And if the foam is swelling -- due to expansion of the air
in the closed cells? -- bit heating is accelerated. FIddling with bit translational
speed and rotational speed may reduce the rate of heat buildup.

At any rate, when making the second cutting pass along the intended cut line,
the bit would be cutting around much less than half its perimeter. Also there
would be an air space on the non-cutting side of the bit. I would think this
would lead to less heating of the bit, more opportunity for cooling, faster
clearing of chips, and the probability of a much cleaner cut line.

I hope this makes sense!