The plastic blade tension knob on my bandsaw broke and here's how a zinc knob can be cast and machined.

The foam will vaporize when the hot metal hits it. Rough shapes can be cut from foam and is a simple solution for a quick lump of metal. The time spent from cutting the foam to finish machining was about an hour.

A circle shape for the knob is cut from paper card stock and pinned to a piece of 3/4"  beadboard. The nichrome hot wire will melt the foam but not burn the paper. There is more info about hot wire cutters http://www.granthams.com/Movie/ that you might find interesting.

A sprue to feed metal to the foam pattern can be made by wrapping a piece of construction paper. This piece is 2" x 12" and will make a tube about 4" tall. The form is a piece of 1" PVC thin wall plumbing pipe. So the paper will hold it's shape, both sides are coated with rubber cement and let dry. Rubber cement is contact cement  and will bond only to itself. To make sure the spiral sprue can be easily slid from the pipe, a thin coat of petroleum jelly works well. The ends of the pipe act as a guide so the ends can be trimmed nice and square.

The gas formed and resulting burp when the foam melts will be lessened by making the sprue from paper rather than beadboard. A 4" length of soda straw for small patterns makes a good riser and helps the mold fill completely.

My preference to bond the sprue and riser to the pattern is pink sprue wax. At 140 degrees F the foam will not be melted. The wax in the photo is heated in a small 5" fry pan and filled with lump pumice.

While not necessary, this pattern was also dipped in the wax to fill the surface voids.

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