Hotwire foam cutter

[stefalo]

I don't know if this the right forum for this but I have a hotwire bow question.
I am thinking I may be able to use my Radioshack switching power supply that has an output of 13.5 volts and 3 amps. If I attach a lightswitch dimmer to the positive and run that to my bow should I run negative to the switch and then the bow or right from the power source to the bow? Hope this makes sense. I would like to use the items I have around the garage to get this thing made.

Thank you in advance!
Stefan

[slipstick]

What's a "Radioshack power switcher" ?

If "power switcher" means DC power supply then the dimmer may not work. Almost all dimmers work on AC only. And you can't put it in the AC line to the power supply because the power supply will keep the ouput voltage constant while the input changes which is the exact opposite of what you want to happen.

OTOH if a power switcher is something else......

Steve

[MatC]

Don't put a light dimmer before a transformer. This won't work and may damage the transformer (switching spikes) or the dimmer (high inrush current). The power switcher thing will also probably try to maintain 13.5 v and hence the power variation almost certainly won't work anyway. And as Steve says, the light dimmer won't work at all on the DC side.

Easier way that will work: use nichrome wire for the bow and use an extra length of nichrome wire in series (attached to something fireproof) and a large crocodile clip to add extra resistance in series and reduce the operating temperature. More wire = higher resistance = lower current = lower temperature. Check the specification on the power supply to make sure it can handle the current you need or that it is overcurrent protected (ideally with something resettable, the initial high current before the wire warms up may pop a fuse).

Mat

[stefalo]

MatC,

I think I have seen what you are describing, it was a plywood board with a zig-zag of wire on it and an alligator clip. As you attached the wire closer to the power supply the heat went up farther away it went down. Is that about right?

thanks

[jeffs555]

Stefan,
It is the same whether the wire is zig-zag or straight, the zig-zag just lets you get more wire in a smaller space. They are both basically rheostats.

Another option would be to use a PWM control on the output of the DC supply. Here is a simple circuit. http://www.electronics-lab.com/projects/motor_light/038/
Here is a kit. http://www.electronickits.com/kit/complete/motor/ck1400.htm

[MatC]

stefalo: sounds about right yes.

[stefalo]

Thanks for all the input. I think I like the idea of extra wire as a makeshift rheostat. One final question, is the 3 amps that the switching power supply provides enough. And is there a chance of destroying the sps. I am not overly familiar with wiring other than changing light switches and what not.

thanks

[Dan Baldwin]

3 amps may or may not be enough to power your wire. It depends on how big your wire is, and how hot you want it. 13.5 volts may not be enough voltage either. I'm running a 3 ft .013 dia steel wire, and it needs about 18 volts and 2 amps to smoke. Yes, it's possible that you could damage your power supply.

Dan

[ZAGNUT]

Don't put a light dimmer before a transformer. This won't work and may damage the transformer (switching spikes) or the dimmer (high inrush current).

Mat

dimmers for inductive loads work just fine with transformers. these are common for low voltage lighting.


i use a big 22 volt/11 amp toroid with a dimmer between it and mains, works beautifully. 12 volt toroids are easy to find in lighting shops but aren't always enough....maybe two could be used with their outputs in series?? also good to err on the safe side and use something that can handle at least 4 amps.


dave

[dusty IV]

I've used a current regulator P/S for over 10 years and never lost a wire.

www.uoguelph.ca/~antoon/gadgets/hotwire/hotwire.html

This circuit is similar to mine except I used a LM338 or NTE 935. That's a 5 amp instead of a 3 amp adjustable voltage regulator. Very old and very simple.

There are a lot better regulators now that will hold a closer current swing. If mine ever breaks maybe I will update it.

Pic of my swing arm cutter. P/S on the bottom shelf.

[DT56]

text

[Joe Bennett]

Here are a couple of photos of the bow and power supply I have been using for awhile. The bow is a piece of 1/2" EMT conduit bent into the "C" shape many use. It also has short pieces of dowel in the ends that the stainless steel wire is wrapped to under tension so that when it heats it will stay tight. The bow is ~46" long (bend to bend), and each end is ~6" stubbed up, with a 6" piece of dowel inserted 2" into the EMT for an overall rise of 10". The wire I have been using is just some SS tie wire I had obtained from Lowes. I had tried Nichrome and others too, but for me this seems to work the best. Have not had any overheating breaks in the wire yet. Having an adjustable power supply does help too I am sure. The feed cable hooks up to the power supply and then goes to the bow and just wraps around the cutting wire. With that setup, have never had any problem with the dowel heating at all, and they insulate the wire from the bow too. The power supply is one that I obtained from Ebay awhile back for about $15. It has both 12vdc input, or 120vac too. When under load, and the wire is hot enough to cut the foam, it draws about 6 amps according to the meter, and that is what I usually have been striving for when using it. The power supply does have a knob that I can adjust for that optimum current setting, and a 15 minute timer that will shut it all down at whatever time setting you want. The power supply is an Astro Flight Inc.with no model number on it unfortunately. Someone might be able to identify it from the photo though. All has worked very well for the cutting I have done in the past....

http://i18.photobucket.com/albums/b144/LordAvenger/Foam%20Cutter/th_BowSupply.jpg (http://i18.photobucket.com/albums/b144/LordAvenger/Foam%20Cutter/BowSupply.jpg) http://i18.photobucket.com/albums/b144/LordAvenger/Foam%20Cutter/th_Supply.jpg (http://i18.photobucket.com/albums/b144/LordAvenger/Foam%20Cutter/Supply.jpg)


The above is a quote from a thread I participated in dealing with foam cutters (http://www.rcgroups.com/forums/showthread.php?t=719355). Hope it helps with some ideas. Click the link here to visit that thread too. Later....

Joe

[dusty IV]

Nice looking setup Dusty IV! :) Any details on the unit?

================================================== ======
Sorry, not very many. Attached some pics but that's all the info I have. I generally work from sketches and no records are kept.

I don't even know if the swing arm concept is a good one. I've never seen another one like this. It's the only cutter I ever used and have no idea how well the others work. I'm happy with it and since I cut wings infrequently no learning curve set up is needed. Just set the watts link to the taper wanted , plop down the foam and turn it on.

The Arm has a watts link that adjustable for various tapers of the wings. I think I can cut up to a 55* sweep. The bow uses a spring to keep tension constant. The bow also is mounted on a universal joint so it can tilt in many directions needed. Weights pull the wire through the foam and track the templates just like most cutters.

I also made a vertical cutter for making round objects like fuselages. It will cut tapers for the nose and tail.

[Ralph Weaver]

http://www.aircraftspruce.com/menus/cm/hotwire.html

Among other things... has a schematic using a light dimmer and a 120V-12V transformer.

[stegla]

I use a 240/24V 100VA transformer and a ceiling-fan speed controller.......the controller was bought at a local DIY store and it is stated to be OK for inductive loads and can handle 5A at 240V.

Steve

[Joe Bennett]

You folks are really brave!! I have been an electrician for 38 years now, and have seen my fair share of electrocutions. Anywhere from 110vac to 120,000vac with the majority of them being in the 110vac to 240vac range. All it takes is a moment of carelessness, or slight misjudgement, or wiring error, and you are DEAD. Most people who die from electrocution do so at those voltages as they think it is not dangerous at all. Wrong!! It only takes 3-5 milliamps through the heart (hand to hand, hand to foot, etc.) to KILL you. It is ALWAYS better to use lower voltages, and DC for foam cutting rather than have the potential for death flowing through that little cutting wire. Please be cautious, and please be careful, I really don't want to lose any of you nice folks any time soon....

Joe

[Dan Baldwin]

You folks are really brave!! I have been an electrician for 38 years now, and have seen my fair share of electrocutions. Anywhere from 110vac to 120,000vac with the majority of them being in the 110vac to 240vac range. All it takes is a moment of carelessness, or slight misjudgement, or wiring error, and you are DEAD. Most people who die from electrocution do so at those voltages as they think it is not dangerous at all. Wrong!! It only takes 3-5 milliamps through the heart (hand to hand, hand to foot, etc.) to KILL you. It is ALWAYS better to use lower voltages, and DC for foam cutting rather than have the potential for death flowing through that little cutting wire. Please be cautious, and please be careful, I really don't want to lose any of you nice folks any time soon....

Joe

I suspect that you missunderstood the previous posts. None of the hot wire setups discussed have the wire connected directly to a high voltage source. All either use a low voltage power supply, or a transformer. None discussed is an electrocution hazard unless people are careless with the primary/high voltage side.

Dan

[Joe Bennett]

I suspect that you missunderstood the previous posts. None of the hot wire setups discussed have the wire connected directly to a high voltage source. All either use a low voltage power supply, or a transformer. None discussed is an electrocution hazard unless people are careless with the primary/high voltage side.

Dan

That is true, and my main concern was that they be careful with the high voltage side so that there is not any potential for it to cause any problems for them. Thanks for your observation though. I just want them to be careful around that wall socket mainly, it WILL kill if they're not careful with it....

Joe

[MatC]

I never considered RC planes as an extreme sport before ;)

[XJet]

Don't put a light dimmer before a transformer. This won't work and may damage the transformer (switching spikes) or the dimmer (high inrush current).

Don't tell that to the light-dimmer/transformer setup that I've been using for years now. It's cut many cores -- including those for a 4-metre UAV.

Works just fine and dandy, never had a problem.

It's redundant now though because I've almost finished building a 3-axis CNC foam cutter which uses a FET-based PWM controller for the hotwire which is controlled by the computer. This baby will cut just about anything you want from a block of foam of up to 2mx2mx1m

Woohoo!

[Kurpal]

Don't tell that to the light-dimmer/transformer setup that I've been using for years now. It's cut many cores -- including those for a 4-metre UAV.

Works just fine and dandy, never had a problem.

It's redundant now though because I've almost finished building a 3-axis CNC foam cutter which uses a FET-based PWM controller for the hotwire which is controlled by the computer. This baby will cut just about anything you want from a block of foam of up to 2mx2mx1m

Woohoo!

Ok, there are so many conflicting opinions as to whether or not this will work.

I have a car charger that is 12 volt and either 2 amp or 10 amp settings. If I hook up a dimmer on incoming power(120 volt side) that will control the volts/amps or what have you on the 12 volt charging side? It wont damage the charger?

I am eager to get my set up rolling, er cutting :) .

[Joe Bennett]

Why not just get one of the available Astroflight chargers like I show in post #12 in this thread? All self contained, adjustable amperage out, meter to show current draw, either 120vac OR 12dc input, and works excellent with stainless steel cutting wire (which is inexpensive to use)....

Joe

[Kurpal]

I couldn't find those chargers, and more importantly, the charger I have is free. I like free :)

[Joe Bennett]

I couldn't find those chargers, and more importantly, the charger I have is free. I like free :)

Gotcha, understand completely, good luck with your project....

Joe

[golem]

Ok, there are so many conflicting opinions as to whether or not this will work.

I have a car charger that is 12 volt and either 2 amp or 10 amp settings. If I hook up a dimmer on incoming power(120 volt side) that will control the volts/amps or what have you on the 12 volt charging side? It wont damage the charger?.
Hi Kurpal,
I recently decided to get cuttin' with wire and, like you, have taken the easy route. Using my 12v-2a/10a car battery charger with a Dremel speed controller attached between it and the 110v house power outlet it works a charm. With the charger set to 12v-2a the controller allows me to adjust the amperage in minute increments. So far I have found with .010 stainless guitar wire cutting goes smoothly at about 7v-1.4a.

This set up could not be any easier or SAFER to use. The speed controller I use is similar to those you could get from retailers like Harbor Freight and sold as router speed controllers. I hear they often are on sale for $10-$15. (normally around $20).

Cheers,
golem