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Hot Wire Cutters
A hot enough taught, thin wire will go through syrofoam "like a hot knife through butter", and it's easy to get nichrome wire this hot by simply running elecric current trough it.  A hot wire cutter like this one takes advantage of this fact, and allows the contolled cutting of styrofoam shapes.
 Table, steel 19" x 15"
Cutting Wire Support Frame,  tiltable 45 degrees, aluminum  with flexible backbone to maintain taughtness as wire expands during heating.
cutting wire .020" nichrome
Magnetic base turntable attachment for arc cutting
piece of styrofoam, about to be attached to holder disk for arc cutting
cutting fence (not in use)
TABLE-TYPE HOT WIRE CUTTER
Design and Fabrication: Carl Pisaturo, 2009
These two objects give an idea of the large range of forms that the simple table-type cutter above can create... 

The "toboggan" at left was free-handed through the vertical wire.  It was subsequently sanded and painted with gesso.

The ""theater" at right used the turntable - with the outside wall cut with the wire vertical, and the sloped area cut with the wire tilted.  The fence and vertical wire was then used to cut out a semicircular portion. It was then sanded and spray painted.
HOT WIRE ARC CUTTER
Design and Fabrication: Carl Pisaturo, 2009

This device can "scoop out" an arc with a  radius adjustable from about 2" to 12", and a height up to 8".
wire anchorage, electrically isolated  from frame
cutting wire .020" nichrome
Cutting Wire Support Frame 
Outer Support Frame 
Radius adjustment thumbscrew
Radius adjustment thumbscrew
wire anchorage, electrically connected to frame
These two objects give an idea of the  forms that the arc cutter can create... 

The "arch" at left is two different radius arc cuts.  It was subsequently sanded and painted with gesso.

The "groin vault module" at right is 4 arcs cut at 90 degree increments.
HOT WIRE PROFILE CUTTER
Design and Fabrication: Carl Pisaturo, 2009
Depth adjustment knobs
Height adjustment knobs
cutting wire, .020 nichrome,  bent to desired profile
Unlike the 2 cutting machines above which make straight cuts (because the cutting wire is straight), this machine makes profiled cuts.  It is a cousin of the router.

A piece of cutting wire is bent into the desired profile with pliers, then gripped by the holder, as shown above.  This is bolted to the machine, as seen at left, and it's ready to profile.

The setup at left can make things like crown moldings or complex grooves in blocks like the one shown below.

When coupled with the turntable, fancy turnings can be made. 

 
SOME THOUGHTS ON STYROFOAM SHAPES AND SHAPING
  • Why Styrofoam ?  When you need a one-off 3 dimensional form which doesn't need high strength or high accuracy, Styrofoam and hot wire cutters are the easiest and cheapest way to go.  Once into the 3" cube size range and above, machining out of Delrin or any other plastic becomes very expensive for materials as well as long tools.
  • LIMITATIONS. Styrofoam is great for architectural models, but you wouldn't want to build a house or machine, or even a toy out of it. It is meant to stand and be looked at. Surface is vulnerable to chemicals, impacts and abrasions.  It burns readily.  Thin sections break easily.
  • Making good cuts takes some practice. The thickness of the cut (or "kerf") is related to cutting speed.  If you slow down or pause during a cut, there will be a low spot in the finished product.  Feed gently, and let the wire tell you the right feed rate.  As the resistance goes up, slow down a bit.
  • Surface Finishing.  Surface quality of different types of Styrofoam vary, but they all tend to look pretty bad after a cut.  Fortunately, a little sanding helps a lot.  Painting with gesso and sanding, perhaps multiple cycles, can result in a fairly smooth stone-like surface.  There are probably lots of other finishing methods.
  • Use as a Core. Styrofoam forms are often used as the starting point for finished forms, with clay or other workable material applied to the surface.  When durable materials like graphite-epoxy are applied to the surface, the results can be very strong.
  • What exactly IS Styrofoam?  It is EPS (expanded polystyrene), a rigid, closed cell foam.  "Styrofoam" is a trademarked name for it, and often used interchangeable with any EPS.  There are many kinds around, including easily available home insulation slabs.  It all cuts easily with hot wire, but some finishes better than others.
  • SHOPPING. Hot Wire Foam Factory has cutting devices (if you don't want to build one), EPS, and lots of related supplies and information.
  • The Right Size Wire.  There is no single "right" size nichrome wire.  But the size used has major implications for the type of power supply required.  In some cases the wire is chosen to suit the power supply.  .020" nichrome is a good first guess.  But, for some types of cuts, the rigidity of a thicker wire is needed.

works of  Carl C Pisaturo