Turkey cutting knife does a good job - back and forth opposing knives.
Martin

On 11/10/2010 7:12 AM, Wild_Bill wrote:
For cutting thin foam, most of us would just use our benchtop water jet
machines.

An old sewing macine could possibly be adapted/modified to cut thin,
soft materials.

When I was making straps and various other accessories for boats that I
had years ago, I stopped in a local furniture upholstery shop to find
out what types of needles they used for thick, multiple layers of
fabric, and the friendly owner was happy to inform me of all sorts of
interesting info regarding thick materials (straps, parachute harnesses
etc).
They use needles that cut as they pass thru thick materials, where
common single-pointed needles will get jammed in dense, thick materials.
I was able to gently grind the shanks of the commercial machine cutting
needles so they would fit an old Necchi home-type sewing machine, and
the cutting points made multiple layers of strap material and various
fabrics a breeze for the tiny Necchi motor.

I believe the cutting points were ground similar to an endmill, with the
tip ground as two bevels in opposing directions and with a greater
relief angle, which would likely require a good magnifier and possibly a
mini abrasive disk or stone in a rotary tool.

A cutting point that reciprocates rapidly could be very useful for
cutting fine lines in synthetic materials such as foam. While feeding
the material by hand (without using the sewing machine feed pawls, or
thread), closely spaced perforations/holes will essentially become a cut
line.
The freehand feed mode is used for embroidery-type work with thread,
such as creating fabric patches with graphics on them (I watched this
being done years ago).