hot wire cutter

Looking for some information on how to build a hot wire cutter (with pictures). I found one on the internet that used a 9V cellphone/ accessorie battery charger. Also I saw one using a light dimmer with a 110 outlet that you plug you car battery charger into. Has anyone used either of these methods? What is the best/ most economical way about constructing one?



Hey Opie,

Check on the Remote control sailplane websites

Look for hot wire wing cutter.

There is at least one pre made cutter available. The Feathercut I think.

I use the light dimmer method, but you NEED to step down the current with a transformer, got mine at R-shack. I built mine a few years ago and I don’t think I have the exact info now. But if you need help I bet I can find something for you.

You must be careful using house current. It needs to be stepped down, or you could DIE!


Eureka CA

Opie - A basic 12 volt automotive battery charger works fine. Some have switches to control the the watts… i.e. slow charge at 2 watts or fast charge at 10 watts. Depending on the thickness and length of your wire, you might experiment with different settings. I leave mine on 10 watts and it works fine.

Stepping down household current to 12 volt via battery charger or other type of transformer (model train controllers have been used) is a very good idea for safety.

Gary Geist up in Portland has had success carefully using direct household current through a series of lightbulbs.

if you don’t want to mess around with electricity and DIY…

i found this place online that makes all kinds of hotwire tools complete with adapters you can plug right into the wall and start carving away.

Do yourself a favor and buy a real variac I built my own and it seemed like a great idea at the time I built it, but it turned out that I wouldn’t go that route again. I purchased a real (but very low end) variac and its made a world of difference. It only cost $44 and the difference it makes in your cuts is amazing. I’m pretty sure this is the company that I bought mine from

As far as the harp goes, search for one that Greg Loehr and Airframe built and use. Its easy and effective.

b.t.w. for the variac useres…

  1. what setting (range) do you find you use when cutting a 24" eps blank (looking for a guideline as a starting point).

  2. 3 amp vs 5 amp variac, is that in the noise? or is one more desirable than the other. Now have a source for both.


I know that my 3 amp homemade one didn’t cut it (no pun intended). It seemed as if the power was inconsistant.

I feel that a variac (also called an autotransformer) would be best.

For more precise control of the voltage put an isolation transformer of around 24-30vac @ 5-10amps after the variac, which would also make it safer too. About 20-30vac is the setting on the variac for a 24" bow depending on the wire used.…air/cutter/page9.jpg

You might also do ok with the setup that aircraft spruce sells:…pages/hotwirekit.php

The one thing about the above aircraft spruce diagram that is weird. The black wire is usually the hot wire, white is neutral, green is ground. I don’t know whay they show only a black and green going through the rheostat to the transformer and don’t say hot or neutral. It should be the hot and neutral going through the rheostat and transformer, with the ground wire going to the casing of both. But maybe they send better instruction with the unit. I did inquire about this setup and the transformer is a 24vac @ 3 amps.

Dan, that variac deal you posted above is a good deal.

Ive got a 12v @ 5 amp transformer from a halogen lamp and some wire guitar strings that I’m gonna give a go. I’f its too much current I can step it down further with a extra resistor. I like the dimmer switch idea.

I think I paid about £12 for the transformer, but that was for a different project originally, any lamp that uses a hallagen bulb will have a 12v tranformer (in UK at least) in the base, along the wire or at the plug. For a cheap option look in the buget stores o op shops for a second hand one, probably only set you back a few quid.

I use a car battery charger plugged into the light dimmer/outlet switch which plugs into the wall. I found plans from a link on this site

Works well for me although my bow is a piece of shite. I am trying to find the plans for the one airframe made that you see on Sway’s all the time. anyone got a link for that(yes I checked the archives but had no luck on plans lots of good photos though)


I didn’t use a variac (yet) but if I did, this is a great diagram I found for the wiring…

Is the extra transformer to prevent you from cranking it up all the way (thus cutting down on the electrocution risk)?

I guess so.

Mine goes wall(110)-> dimmer-> 24vXfmr-> plug-> bow

So I “dim” down the 110 before it hits the transformer which is like a $5 version of the variac. I like the variac you linked to yesterday, Dan, with the fuse & all…I might order one of those. And $44 is awesome - most are closer to $100.

My wall receptacle is GFCI protected so I haven’t worried about installing an inline fuse.

The only down side of those variacs is that they are not precisely calibrated. It doesn’t really matter since I don’t use them to run precisely calibrated electrical equipment :wink:


I just bought the 2in1 Prokit from the Hotwire Foamfactory with these two tools their variac and the adjustable sled…

Next year when I get back into the home depot blank building routine, I plan on using these to cutout a perfectly smooth outline from my MDF templates and doing all my railbanding without creating alot of EPS pellets flying around the neighborhood. Can’t wait to give them a try. Like MikeJ and his electric turkey knife adventure it should be neat to see if you can actually shape most of an EPS board like Tom Sullivan does using these hot wire tools without a planer… and just finshing it off with a 24" fairing block…

This is the stuff:


The foam cutting tool the world’s been asking for. A combo hand held scroll saw and power drill that cuts foam with fast clean surgical precision. The 4" blade is made from thin tough steel, with a unique round shape that slices quickly and cleanly in any direction like magic.


Comes with 5 ultra-stiff, yet shapable 8" wires that bolt onto the arms. Arms are short for a high level of control. Serious sculptors, modelers and architectual designers helped design this tool to fit their needs.

Control both angle and depth of cut with all hand tools. Simply clamp the tool of your choice onto the Sled at the desired angle and depth and start cutting.

here is an other variac source:


The variac/transformer is what I did. I’ve got this guy near me that cuts foam model airplane wings for a living with a cnc hotwire table. Aside from using the variac/transformer he had an amp meter inline and thermocouple attached to the wire. Maybe a bit overkill but he said he never has an unperfect cut. He said he’d tried everything and the variac/transformer works best.

One thing that I forgot to ask him when he was showing me around was what is the perfect temp of the wire for cutting eps. He was cutting other foam types as well, the pink and light blue stuff.

But I would think that there must be a best wire temp for EPS. And the perfect temp might be a bit different for different density too. I tried emailing Insulfoam but never got a reply on that question.

A few things I did with my setup is to put a outlet and switch on the variac/transformer setup. Also, I can plug the hotwire bow into the variac/transformer. But I’ve also have a hand switch on the AC supply wire about three feet from the bow so I can turn it off while I’m hotwiring. That picture I posted above shows me with the switch in my hand. Also I installed quick blow fuses in and out of the variac/transformer for saftey. And plugging all that into a GFI outlet would be best.

I’m a bit of into electrical saftey since that I work with it everyday for a living.

Here’s ACP’s hotwire foam cutting stuff:

They have some cool looking stuff but quite expensive compared to the 10 dollar one I made from home depot. It was quite easy actually I just bought the stuff and had them cut up the wood and when I got home I just bolted everything together. Like a $10.00 hotwire kit.



quite expensive compared to the 10 dollar one I made from home depot. It was quite easy actually I just bought the stuff and had them cut up the wood and when I got home I just bolted everything together. Like a $10.00 hotwire kit.

Rio, Ten dollars is the lowest price I’ve heard so far. Can you post a picture of the hotwire bow and power supply you put together for ten dollars? Are you getting a perfectly smooth cut on your blanks too?

I’ve hotwired an XPS blank by just connecting the hotwire bow straight to a 12 v car battery.

Worked just fine, maybe a little bit slow cutting with this setup.

Don’t know why since I don’t really know much about electricity.

Maybe not enough power?

Give it a go, see if it works out. It’s a very simple setup.

Also tried with just a car battery charger (no variac or transformer) connected straight to the bow.

This worked fine too, a little bit faster cutting than with the 12 V car battery.

Is it dangerous to connect straight to a car battery charger without the variac and/or transformer???

If so please explain why.

Good luck and don’t get zapped