QuickTalk 3 - HOT WIRE IDEAS
- Category: Q-Talk Articles
- Published: Friday, 30 April 1982 07:11
- Written by Robert Herd
- Hits: 1466
There is probably not a tool more essential to the composite builder than the hot wire cutter. And as sure as planes fly at Oshkosh, everyone seems to have their own idea on how to support that simple piece of wire.
Most frames you will see around are derivatives of the basic "2x4-and-tubing" cutter which you should be familiar with in your plans. The concept is very simple and requires a minimum of parts. However, there are a few negative aspects. At greater tensions, the wire has a tendency to make the tubes "unwind" which could produce slack in the line. Second, the straight tube is very awkward to twist to tighten the wire, usually requiring vice grips or the like. One builder suggests putting a 90-degree bend in the tube at the 2x4 end to facilitate hand tightening. Another way to achieve a very tight wire is to replace the tubes with threaded rod. Using a couple of double nuts and washers, the rod can be located through the board and tightened easily with a wrench. There is also the sect that prefers leaving the supports alone and pulling directly on the wire. This is usually accomplished with the use of eyebolts and turnbuckles on the ends of the wire. The user simple twists the turnbuckle until the desired tension is reached.
After trying several cuts, you may have noticed your arm getting tired from holding up the bulk of the 2x4 frame over the foam block. This usually doesn't help your concentration during a critical cut. An easy way to solve this is to support the cutter by the use of rope cord and counterweights. Simply tie one or two ropes to the frame and loop these through pulleys directly over your cutting bench and a second set conveniently away from the immediate work area. Finally, tie weights at the end of the ropes equal to the weight of your hot wire set-up. A milk jug filled with water makes a good, adjustable mass of this. This balance system should allow a feather-weight feel which can be a blessing when performing many cuts at once.
The second item most people face is how to push all those electrons. This usually divides into a power unit combined with a controller. I've heard of people using everything from batteries to small welders for an electrical supply. Just be sure that you have a source, which will provide constant voltage over the time of your cut. If you can't obtain a variable resistor box for your controller, the nails and safety wire block from the plans are an effective, although clumsy, method. The easiest (and most expensive) way is to have the supply and controller all in one unit. A QBA member found that an electric model train transformer worked very well. Any good variac (VARIable AC supply) will be sufficient to provide a relatively portable unit. Just be sure that the unit provides for fairly accurate settings, which may require very minor changes. Also, be certain of any possible electrical dangers. More than one builder has fried his wire by accidentally switching on full house current to the cutter! It is not a bad idea to have your power switch on the frame near your hand. It is convenient and requires less fumbling in case of an error.
The size and type of wire can have a significant effect on the results achieved by individual builders. A smaller diameter wire requires less current to achieve a particular temperature and makes a smaller cut as it passes through the foam. However, it is also more prone to lag in the center, especially if the user is rushing the cut a bit. Larger diameter wire requires more force to be pulled through the foam block because of the larger path it produces. The natural reaction is to slow down because of this resistance, which may produce gouges in the foam. For these reasons, experimentation of wire size and temperature is a good recommendation to the new builder.
From time to time you will find your hot wire is considerably longer than the foam you wish to cut. This means your hands (on the frame) are quite a distance from the actual piece. It can be difficult to accurately guide the wire around the template. Try taking a couple of wooden sewing spools (or any wooden dowel) and putting a metal tube sleeve inside each one. Thread these on the wire during your original installation. You now have a couple of "extenders" for hand guiding the wire around the template during narrow cuts.
As we mentioned earlier, the best way to learn about different hot wire set-ups is to observe them first hand. You will soon find that one of the most basic tools of the composite builder can be a showcase for one's personal cleverness. -RH
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