Hot wire cutting, and the wire that cuts wider than itself
Thermal kerf, and how to compensate for it in the pattern.
A hot wire cuts wider than itself because it does not slice through foam the way a blade slices through paper; it melts a channel by radiating and conducting heat outward from its surface in every direction, and that heated margin around the wire removes foam the wire itself never touched, leaving a cut path measurably wider than the wire's own diameter.
Heat that reaches past the wire
A thin electrically heated wire, drawn slowly through a block of foam, has no need to push material apart, since the foam directly ahead of it liquefies and retreats from the heat before the wire arrives. That heat carries on a short distance past the wire's surface before it fades below the temperature needed to melt anything, so the cut channel has the wire's path down the middle and a margin of melted foam on either side. The machine described in the previous article surfaced this within its first few test cuts: shapes traced directly from a finished drawing, with no allowance for the wire, came out consistently undersized.
The name for that removed strip is kerf, borrowed from sawing, where it means the strip a blade's own width takes out. A saw's kerf is fixed by the blade's thickness and can be looked up once. A hot wire's kerf depends mostly on how far the heat spreads, and that depends on the wire's temperature and on feed speed, since a slower pass gives heat more time to spread before the wire moves on. The same wire at the same setting leaves a different kerf when fed faster or slower, so kerf has to be measured on the actual machine at the settings it will run.
A felt-tip marker drawn across an absorbent paper towel makes the same point. The ink bleeds sideways through the fibres, leaving a line far wider than the tip, while the same pen on glossy card leaves a line close to the tip's true width. The visible line records how far the ink travelled, and a hot wire's channel records how far the heat travelled.
Drawing the pattern oversize
A pattern for a part meant to land at a specific dimension is drawn oversize, offset outward by half the kerf on every edge, so the foam left after the wire passes is the size wanted. Ignore a one-millimetre kerf and a block meant to be 100 millimetres across comes out 99, since each of its two edges loses half a millimetre. Because the error is consistent, every part in a batch shares it, which at least points straight at the kerf offset instead of leaving a dozen possible causes to chase.
A single offset stops working once temperature or feed speed varies during a cut, from an unstable power supply, foam whose density changes within one block, or a hand-fed pass that speeds up and slows down. The kerf then shifts partway through, and the part is right in one region and wrong in another. Steady feed speed and wire temperature matter as much to an accurately sized part as the offset itself, and a hand-built machine has to hold both before its output can be trusted.