Making A Hot Wire Cutter

By Paul Williams.
Article posted 26th October 2007.

A hot wire cutter ("hwc") is a basic tool for carving blocks of foam to shape. It consists of a simple "H"-shaped hinged frame, across one end of which is strung a length of ni-chrome resistance wire. The opposite end of the "H" frame has a tensioning mechanism to tauten the wire. My hwc bows have a tensioner made of cord with a length of waste carbon tube threaded through as a winder.

The wire length changes as it heats up and cools down. This means you will need tension and loosen the wire as you switch it on and off. My routine is:

This may seem a lot of work, but if you leave the bow tensioned when you kill the power, you risk the wire snapping as it cools and contracts.

Hot wire cutter.

Depending on the size of foam blocks you have, you may need to build two hwcs, one to cut blanks from large sheets of foam, and another, much smaller cutter to do the actual carving of the profile itself. Regardless of size, the construction principals are identical.

What you will need

Make It

Start by deciding on the width of hwc required. For my large size, the sheets of foam I have are 600mm wide, but I'm going to make my hwc with a capacity of 800mm and the lengths quoted will be to suit 800mm.

Cut one length of timber 850mm. Cut two lengths 400mm long. Using your sanding block, tidy up the cut ends. On each of the 400mm lengths, mark centrelines across one face using a square. Measure off and mark a line parallel to your centreline one-half the thickness of the timber you're using - in this case 19mm. Offer up each of the hinges in turn and mark the holes for the mounting screws on both the 400 and 850mm lengths. On the 400mm lengths, mark the position of the eyebolts for the tensioner, on the longitudinal centreline 25mm from each end.

On the "business end" of each 400mm length, mark the centre point of each end face. The wire clamps will screw in to these points. To prevent the timber splitting, pre-drill each hole with a small diameter drill bit. To mount the wire to the bow, I'm going to use some electrical clamps from the local DIY shop.

Once you have the wire clamps in position, attach the three pieces of the bow together with the hinges and screw in the tensioner eyebolts. Cut a generous length of ni-chrome wire and attach to each of the wire clamps, pulling the wire taut as you tighten the second clamp. Once the wire is secured to the bow, add the tensioner cord and winder.

All that remains now is to run some wire from each cable clamp to wherever you want the bow to connect to the power supply. I used the cores from a length of heavy duty twin-and-earth mains cable to wire up my bow, attached to the frame with staples. As you can see from the pictures, I routed the wires to one side, and terminated them in 4mm gold banana connectors.

Power Supply

If you use a DC power source, for example a car or leisure battery, the ends of the wire will run at different temperatures - ie., one end will be cooler than the other. An AC power source will heat both ends of the wire, with the middle coolest. For this reason, I recommend using an AC power source. I use one of these:

Final Word

A hwc bow is a simple tool. It's essentially a length of resistance wire strung across a frame and wired to a power source. You don't have to slavishly follow the method detailed above to make one. As long as you understand the principle, feel free to make one to your own design using whatever hinge and wire mounting suits you.

Safety

When running, the bow wire will be more than hot enough to burn skin and clothing, and will be carrying a voltage - be careful! Hot wire cut foam leaves a melted residue behind on the wire, that gives off noxious fumes as it breaks down. To minimise fuming, wipe the bow wire clean each time you use it before you kill the power. This also keeps the wire ready to use for the next cut.

© Copyright Paul Williams and www.fastelectrics.net, 2010.

This article may not be reproduced wholly or in part without the written permission of the author and www.fastelectrics.net. If you would like to use this article or the accompanying pictures/diagrams please email articles@fastelectrics.net.

Last modified: 08th July 2010 @ 09:06