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Hotwire, Foam Questions


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I'm about to reveal just how unhandy I am but I have a couple of very simple questions regarding hotwire construction and foam ident.


First, hotwire. I have a variac power supply with a transformer on the front of it. It has a 3-prong outlet on it for connecting whatever it is I need to connect,. Well, I need to connect a hotwire.


I searched and reviewed many of the hotwire links on the web (there are several with very nice page layouts showing a step-by-step of the hotwire) but none of them really explain how to safely connect the hotwire handle to the power supply. I assume I connect the common to one end and the hot to the other. Is this safe or do I run the risk of shock? A guy at work said I needed to place resistors (big ones) on the wire to be safe but none of the sites I've seen mention this. I need a laymen's explanation of the connection I need to fashion (3-prong to hotwire handle).


Second, I have the composite practice kit and it mentions in the booklet that cutting urethane emits a hazardous gas and not to hotwire it. Question is, since the foams aren't labeled, how do I tell what's urethane and what isn't?


I hesitated posting these questions since they seem so basic but then decided I wouldn't know for sure unless I asked. Be gentle =]



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A number of things to look for, most of which I am not an expert on!


Isolation: Is the output voltage isolated from the input? If not, then you could be working with one end of your wire at 120 volts and the other at 100 voltage. Could be shocking, to say the least. If you have a transformer, there is a good chance that it is isolated.


Connecting the output to the hot wire rig: Here is a picture that shows some detail: http://www.gourmetdamage.com/images_01/PIC00179.jpg


I attached a length of wire to each of the output leads. You may have to make a plug up for your transformer. For what you describe, I would get a cheap two wire extension cord, 8 feet long, and cut the female end off. Strip the wires and fasten these to each end of your hot wire. These were about 8 feet long or so. You need enough so that you are not dragging the transformer around when you do your cutting. I put an aligator clip on each end that I could use to clip the wire to the tension tube (as seen in the upper rh corner). You can also clip it directly to the hotwire near the tension tubes. Now that I think about it, you could roll the stripped end under the hot wire as you tighten up each end. No misc connectors needed.


The resistor comment won't work. You are either going to make a lot of heat or burn out the resistors.


The foam we hot wire is the blue styrofoam. Thick stuff. Urethane foam is soft and crumbly, similar to the florist foam that they stick fake flowers into. You can hotwire styrofoam insulation from the lumberyard for practice. The 'bead board' is quite different, but most yards will have either blue or pink extruded styrofoam.


I insulated the torque tubes on my hot wire rig with vinyl electrical tape. I also had my helpers wear nitrile gloves (like what we wear to protect against epoxy). If I remember correctly, we were around 20 volts for cutting. If your secondary is isolated, 20 volts is pretty safe.


Hope this helps!



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That looks very similar to what I have, except that yours has a box on the side of it with the outlet.


One thing that you want to be careful with is that if you turn the knob the wrong way, you can put high voltage across the hotwire. It will turn orange and burn out (for some reason I know this)! A person could put a temporary stop on the dial to keep it from getting too hot, or just pay close attention.


Good Luck!



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One thing you might find handy is to make a small hotwire and power it with a 10 A. Battery charger. Works like a champ for the small jobs. I don't know how it would work for the biggies, however.


You do not want to use a charger that requires some battery power for excitation.

I Canardly contain myself!

Rich :D

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