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Oct 24, 2010, 11:40 PM
"Those Other Guys"
11Hotel's Avatar
Thread OP

Home-made Hot Wire Bow

Hey guys and gals, I have been getting into making my own foamies recently and decided to build my own hot-wire bow for cutting larger chunks of foam than the one I have now. But, after assembly I wired it to a 12v 1.25A plug and it got hot, but not hot enough. I then wired it to my PC power supply I use for my chargers so I could use the 12v 10A and it still seemed really under-powered. It was still cutting, but not very fast.

So please take a look at my reference pics and I welcome any and all suggestions as to what I can do to help my hot-wire bow.

FYI, the bow is 26" long and uses .25 gauge wire (I think, it might be .10).

Thanks all!
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Oct 24, 2010, 11:53 PM
U down with EPP? Ya u know me!
Aerogance's Avatar
That looks much better than my first setup years ago. The internet / rcgroups is the best reference our hobby has ever had.

As for suggestions, a better power supply and longer bows. Also, use a suspended bow for best results. Once you cut a few blocks of foam, you will start to realize what your system is lacking and then can zero in on solutions to the problem.

All the best.
Oct 25, 2010, 02:12 AM
Reduce the drama...
rick.benjamin's Avatar
move the clips closer together, directly on the wire.
Wire should get hotter.
Oct 25, 2010, 05:00 AM
"Those Other Guys"
11Hotel's Avatar
Thread OP
The clips are on the wire now. I tried them on the eye bolts but thought they might be acting as big resistors.

If I shorten the distance between the clips, I won't have enough clearance for some of the bigger foam blanks I will be using.

I think the PC power supply might be only 500mA. There is a chart that shows +12V @ 10A, then another for the 5V plug, then a column that shows -12V @ .5A.

So it might only be supplying .5A to my bow.
Oct 25, 2010, 05:52 AM
No pressure
Check your voltage at the aligator clip and see what it is. Should be right at 12 v.

Also, if what you say is true about moving the clips closer together and it getting hot enough, you are not supplying enough voltage for the length of wire you are trying to use. If that is the case, you need a higher voltage power supply to run the length of wire you are trying to us.

Hope this helps
Oct 25, 2010, 08:46 AM
Registered User
You don't say what type of wire you're using. It makes a big difference.

Oct 25, 2010, 09:17 AM
Registered User
rimshotcopter's Avatar
What I have read but have not tried it but soon will, the idea cutter is low voltage and high amperage. So maybe lowing the voltage to 6 volts will do the trick?
Oct 25, 2010, 09:53 AM
Curtis Suter's Avatar
If you're cutting blue or pink 25 psi foam from the big box stores you'll need about 1.9amps.

You may be interested in this article:
Build an Automated Foam Wing Core Cutter

and this video:

Last edited by Curtis Suter; Oct 25, 2010 at 10:15 AM.
Oct 25, 2010, 01:02 PM
"Those Other Guys"
11Hotel's Avatar
Thread OP
I think the wire is either .10 or .25 gauge, not sure. It is Ni-Chrome though.

I was testing it on the big window foam from Home Depot, but have some 9mm Depron I can try also.
Oct 25, 2010, 07:51 PM
Crafter of flying plugs
Owen Hedger's Avatar
The current running through your bow obeys Ohms law
Volts = Resistance * Current

Or more helpfully in the case of a foam cutter this can be rearranged to
Current = Resistance / Volts

Your power supply is fixed at 12 volts and your resistance is determined by the type and guage and length of wire you use. Also the rating on the wall adapters and rails of the PSU are the maximum ratings for the supplies. The 0.5A 12 on the PSU for example would be the -12 volt rail. You cannot force more current through wire just by using a higher rated PSU that is the same voltage.

You can increase the current by moving the clips closer which will lower the resistance of the cutting wire but as you said you cant cut what you need to if you do. You can also increase the voltage to get the current higher but that would be difficult/not an option with the power supply you have. You could change the cutting wire to something of lower resistance but that could be too low and get too hot, but that could be fixed by making the bow longer.

Ideally you would want a variable voltage power supply but the only adjustable component you have currently is where the clips are on the wire.
I use 4 Ohm/ft nichrome and find it takes about 1.3 amps to cut EPS and 1.6-1.8 amps to cut blue foam well.
Last edited by Owen Hedger; Oct 25, 2010 at 08:05 PM.
Oct 25, 2010, 10:42 PM
"Those Other Guys"
11Hotel's Avatar
Thread OP
Owen, that helps a lot, thanks!

I went to my local Fry's Electronics and picked this up today. Seems to work okay, but I think I need more than 2amps to do what I want.

I plugged the wire I'm using into my 10amp tester and it broke within 1.5 to 2 seconds, so at least I know that's too much!

I'm headed to Home Depot tomorrow and get some of the stuff from this video:
DIY Hot-Wire Power Supply (Vid 1 of 2) (9 min 16 sec)
Oct 26, 2010, 07:02 AM
Curtis Suter's Avatar

What about something like this?
Back in the day this is what Burt Rutan recommended.

Must read:

Excellent source for many things!

Please be careful, you want to make sure the power supply is isolated from the source or you could be shocked severely.

Last edited by Curtis Suter; Oct 26, 2010 at 07:16 AM. Reason: Added links.
Oct 26, 2010, 08:54 AM
"Those Other Guys"
11Hotel's Avatar
Thread OP
Curtis, thanks for the heads-up about the power supply and all the links. I am *fairly* comfortable with the power supply I bought, but don't want to cut at 12v and 2a, so I am gonna try to get some of the stuff from those websites and build my own with the dimmer switch and see how it goes.

I'll keep you all posted, and thanks again for the help!

By the way, if I don't post in awhile, can someone please call my wife and ask her to check on me in the man-cave?

Oct 26, 2010, 09:57 AM
Curtis Suter's Avatar
Originally Posted by 11Hotel

By the way, if I don't post in awhile, can someone please call my wife and ask her to check on me in the man-cave?

Good One!

Oct 27, 2010, 02:08 AM
"Those Other Guys"
11Hotel's Avatar
Thread OP
I would like to thank everyone for their help in my hot-wire project. I've finally finished it!

Special thanks to Crash Hancock over on YouTube for his great video on making a power supply. Following his video I finally have this:

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