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Hot Wire Foam Cutting

fliteadmin

Administrator
Staff member
Admin
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#1

Welcome to Hot Wire Cutting with David and Josh!



David made his hot wire cutter out of PVC piping and extra RC components, including a Turnigy 30A BRUSHED ESC and a servo driver.



You can use building pins to hold down your template.



Hot glue also works on very thin parts of the template. The hot wire will cut through the hot glue easliy.



When cutting you want to allow the heat of the wire to cut the foam. Try not to pull or bow your wire.



This specific project was cut in sections using templates found here.

Check out the review of this plane here, and see David's build log of the plane here!
 

earthsciteach

Moderator
Moderator
#2
Great tips! My Physical Science classes have been hot wiring airfoils to test in my "wind tunnel rig of science." Once through the drudgery of exporting coordinate data and plotting in Excel, they've had a great time with it. I'm going to show this episode in class tomorrow!
 
#7
Two questions if I may:

1. What dimensions / specification is the nichrome wire you used?
2. Would selling the formers in a kit be something you would consider?

You do an awsome job guys!
 
#8
Can we get a little more information on this hot wire cutter? I appreciate the link to the brushed ESC, but would like to know what size PVC pipe was used, and anything else that may be helpful.
 

MrGravey

Senior Member
#9
I'm wondering lately... I have, as most of us do I'm sure, a DC power supply that I use to charge batteries indoors. Its a modified server power supply and it puts out a mortal ton of power when it needs to. Why can't I take a pair of clips and wire a positive and negative to a dimmer switch, then wire that to a cutter and control the heat to the wire with the dimmer? I might actually have to calm down the voltage a bit before the dimmer as I think the power supply will put out over 1000v and I'm not too smart in the ways of electric current.
 

Craftydan

Hostage Taker of Quads
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Mentor
#10
HG,

Should work, but:

- put the dimmer in series - positive to dimmer, wire to dimmer, wire to negative (with jumpers where appropriate)

- may need a power resistor inline to make sure you can't turn it up to the point the NiCrome wire fuses.

If you Could post the specs printed on the power supply and/or measure some things w/ a multimeter, we can probably help you refine the circuit to something simple, but fool resistant (fool proof doesn't exist, but we can slow some down enough to see reason ;) )
 

earthsciteach

Moderator
Moderator
#12
My set up is very simple. I use a 6 - 12 v car battery charger and steel E string guitar for the heating element. I think its 0.024 diameter or something like that. for cuts of short width I use 6 v and for long width cuts (like wings) I use 12 volts. I just attached the alligator clips to the guitar string. I can't post pics of my frames at the moment because I'm in the hospital, but basically all you need to do is be able to hold the steel string tight. I drilled a hole through a brass wood screw which I use as a "tuning peg" to tighten the wire as it stretches under heat.

It would be advantageous for me to be able to control the output with a dimmer switch. I just haven't gotten there, yet.
 

pgerts

Old age member
Mentor
#13
Can you use a dimmer for DC? I Think that the smartest way is to use a brushed speed controller and a servo tester like in the video David did.
 

Craftydan

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#15
Old style dimmer (not CFL compatable) where nothing more than potentiometers sized for 120v w/ current to match a few 40-60W bulbs. As long as you go cheap or old, they should work. The new nice ones -- maybe, maybe not.

Yes I agree the Brushed ESC is a slick, clean solution (best I've seen in a while), but not everyone has one. If you have an order going in, HK has a few nice cheap ones. If you're careful about it, you *could* use a 12v supply with a 3s brushed ESC. Keep the servo tester to "just enough" to keep the current draw below the ESC's limit.

Be careful with the cheap potentiometer you pick up from a parts store -- it still needs to be sized. Range and power rating is a concern, since a short strand of NiCrome is near a dead short and can pull a good bit of current. If you don't size it right you'll either end up with poor control (it only gets hot in the last 5 degrees of turn), or it will burn out from too much power. You may still have control issues w/ a light dimmer's range since it's sized for 120v, not 12v, but it should still be close enough to use.
 
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Craftydan

Hostage Taker of Quads
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Mentor
#16
Old style dimmer (not CFL compatable) where nothing more than potentiometers sized for 120v w/ current to match a few 40-60W bulbs.
*sigh*

Ok, well Wikipedia is a good thing, but soemtimes it reminds me of how little I know (and why it's best to double check before speaking).

Old-school rheostat dimmers are potentiometers, but they haven't made those for years, it seems. The sorta modern, old dimmers use a newer technology (thyristors). Looking at the "typical" circuit, they still should work, but they'll pulse the DC to do so.

Dunno if you'll have control range problems, but it can handle the power and should pass enough current to work.
 
#17
Hot Wire Electronics

David & Josh
Did you ever post the drawing for the electronics. I would like to know if there is a substitute for the servo driver you are using. Is the out put to the wire 5 volts?
 
#18
In a recent build I had to plane a 1" thick piece of foam down to 3/4" thick so I needed a hand held hotwire rig. I do have a larger rig made up of my 12V car battery charger and a Harbor Freight router speed control, but this needed some finesse and it had to be done fast. It took me about 10 minutes to scrounge the shop and put it together so there you go. :)

Enter an old ProTech charger I had from a 1990 truck racing season

The timer went bad years ago so I replaced it with a switch. it has 120VAC input, fused 7.2 and 8.4VDC output with adjustable current so it's perfect for a small hot wire rig. You can find these on ebay for low cost every now and again.
The bow is made from a 12" scrap of 2x4 that had been ripped in half, a 1/2" diameter pine dowel (two 12" sections) and some very thin OOK! brand picture hanging wire. I lost the package so I have no idea on the size. Get as thin a wire as you can get. Drill 1/2" diameter holes in the base of the bow slightly angled out. this will help preload the bow legs. Wrap one end of the wire on a dowel, bend the two dowels "in" and wrap the wire around the other dowel. This will tension the wire. the wire stretches as it is heated so pull it snug. Like others have done, I used simple gator clips to get power to the wire.

I notched the dowels with my band saw to fix the wire in place.


Finally a shot of me planing the foam. I used two 3/4" thick oak boards and just slid the aileron through.


So if you have an old charger in the closet, don't throw it away, you just might need it in 23 years. :lol: