Hot knife or hot wire supply & controller

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terrymorris
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Hot knife or hot wire supply & controller

Post by terrymorris » Sat Oct 01, 2005 10:02 pm

I would like to build my own hot knife or hot wire cutter. The commercial ones have a power supply box with a transformer and either a dial or multi position switch to vary the output voltage. They work by running a voltage across Nichrome wire some are stainless but might have Nicrome wire inside.<p>I don't know the required voltage or amperage and was hoping someone here might have the info or even a link to a schematics or plans for building one.<p>Thanks!

rshayes
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Re: Hot knife or hot wire supply & controller

Post by rshayes » Sat Oct 01, 2005 10:37 pm

Popular Electronics described a hot wire cutter for plastics (Lucite or Plexiglass) somewhere between 1957 and 1959. You might find the article in a microfilm file in a public library or a college library.<p>The article was reprinted in a book, "The Electronic Experimneter's Manual", by David A. Findlay, Ziff-Davis Publishing Company, New York, 1959, pages 90 to 92. This might be available as a used book or in a library. Try www.abebooks.com or Amazon for used books.<p>The cutting wire was 5 inches of #28 nichrome. The active length was probably closer to 4 inches. The transformer was specified as 6.3 volts, 6 amps center tapped. The wire was connected across half the secondary, so the actual voltage was 3.15 volts at less than 6 amps.<p>No provision was made for varying the temperature, other than changing the length of the wire. A light dimmer could probably be used to reduce the power if needed. This might allow the use of heavier or shorter pieces of nichrome wire.

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Chris Smith
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Re: Hot knife or hot wire supply & controller

Post by Chris Smith » Sun Oct 02, 2005 1:54 am

Every time I need a hot knife, I use my 5 amp variac with my stainless steel wire for the Mig.<p> I have several miles of the wire on a spool, so the wire is almost for free or pennies. <p>Both of these in combination work quite well and don’t cost me any thing to assemble OR disassemble. <p>Less than five amps is the current, and the variac never exceeds around 10 volts while the wire glows real nice. If it goes cold in the cut, you just bump it up a tad. <p>I helped a friend make his own buy taking a standard 120v to 15 volt transformer out of a old VCR, and he broke off the metal shell around the core and wire wrapped a few windings of 12 gage wire around the whole core, and hooked that up to produce his main current. <p>Then he used a simple light dimmer to control the main core for his Foam cutter. <p>Either way works real good. <p>I prefer the variac method, but if I needed a dedicated cutter I would go the transformer way with a light dimmer. <p>Less than 20 bucks.<p>[ October 02, 2005: Message edited by: Chris Smith ]</p>

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Re: Hot knife or hot wire supply & controller

Post by rshayes » Sun Oct 02, 2005 3:26 am

Try a google search for "hot wire cutter". The usual hundreds of thousands of hits, but a fair number actually show how to construct different versions of hot wire cutters.

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jollyrgr
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Re: Hot knife or hot wire supply & controller

Post by jollyrgr » Sun Oct 02, 2005 9:29 am

You can build a controller quite easily using parts from your local home improvement store (i.e. something like Home Depot or similar). While not absolutely needed, a GFI would be a good idea. Wire a GFI outlet or get an extension cord with a GFI outlet at the male end. It might even be possible to liberate one from a "newer" hair dryer or curling iron. (I see these at the local "Salvation Army" and "Goodwill" stores for a buck or two.) The GFI is just a suggestion and not a mandatory item.<p>Feed the output of the GFI device to a variable speed control for a ceiling fan. This would be the controller you would put in the wall switch box and use it to control the speed of the fan. While it works like a dimmer, you DO NOT want a dimmer control! Some dimmers have DC outputs. You want something with an AC output. By getting a fan controller you are getting something that will work with inductive loads. (Probably you COULD get away with a dimmer. Maybe I'm too cautious.)<p>Feed the output of the speed controller to something like a 24 Volt, 5 Amp transformer. Alternately you could use something like a car battery charger; something you already have. Many of these can put out 10 Amps or more. If you don't mind sacrificing the battery charger you could mount the fan speed control right in the charger case itself and control the AC line input to the transformer.<p>Drive two nails into a board far enough apart for you to work with whatever foam you are working with. Stretch the "cutting" wire between the two nails. Connect the transformer, battery charger, etc. to the two nails and adjust the speed controller until you get the desired heat.<p>[ October 02, 2005: Message edited by: Jolly Roger ]</p>
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Edd
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Re: Hot knife or hot wire supply & controller

Post by Edd » Mon Oct 03, 2005 3:46 pm

I had occasion to fabricate a replacement internal foam pad for a neighbor
hood kiddo’s motorscooter seat and used a sheet of plywood “C’ clamped
to my table saw and used nichrome salvaged from a ww power resistor.
A 24VAC 5 A with its primary power supplied thru a Variac. Years later
Saw the same procedure written up on a web site:
http://www.techlib.com/hobby/hotwire_foam_cutter.htm <p>73's de Edd
[email protected] .........(Interstellar~~~~Warp~~~Speed)
[email protected]........(Firewalled*Spam*Cookies*Crumbs)
;) ;) <p>Do Roman paramedics refer to IV's as "4's"?

terrymorris
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Re: Hot knife or hot wire supply & controller

Post by terrymorris » Mon Oct 03, 2005 4:51 pm

Thanks you everybody for the replies. I have found a wide variety of information from here and searching deeper on the web. It looks like the amount of power varies highly depending on the gauge of wire and the length. I have seen some that run on 2 D size batteries and some that use a 24v 10 amp supply.<p>Mine is only going to be a few inches long and I am starting with 20 gauge wire. From what I read this will require about 9 volts at 1 to 2 amps for good results. I also found a great do it yourself kit info here
http://nsrca.org/technical/tip_tricks/f ... supply.htm<p>if anybody is interested.<p>Thanks again for the suggestions!!

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Chris Smith
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Re: Hot knife or hot wire supply & controller

Post by Chris Smith » Mon Oct 03, 2005 5:45 pm

Terry save your self a lot of money and buy some Stainless steel MIG wire. Several thousand feet can be purchased for 20 bucks or less through most hardware stores, welder shops, or Harbor Freight. Its also useful in other areas where strong small wire can be used. I use it as passive heater wire, on my pipes and many other uses. <p>Nichrome wire is expensive.<p>[ October 03, 2005: Message edited by: Chris Smith ]</p>

terrymorris
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Re: Hot knife or hot wire supply & controller

Post by terrymorris » Mon Oct 03, 2005 6:48 pm

Chris,<p>Thanks. I already ordered some Nicrome wire, got 40 feet for $5. Should last me a lifetime or two.<p>Have you found you burn up the stainless wire? <p>The reason I ask is that when I was young we had a neighbor that owned a foam business where they made and installed styrofoam covered in stucco around beams and posts in shopping centers, made them look like greek or roman pillars.<p>Anyway he made them in his garage and he used a home made wire cutter, he used stainless and complained it would break all the time, when it did it would sometime ruin the blank piece which were about $60 ea.<p>I told him about Nichrome and gave him a piece. Almost a year later he was still using the single piece I gave him, it never broke. His business went under but the wire cutter was still working.

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Chris Smith
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Re: Hot knife or hot wire supply & controller

Post by Chris Smith » Mon Oct 03, 2005 7:14 pm

Terry<p>NO<p>I have 25 thou wire off my Mig welder, and it lasts forever if you can control the current. <p>I have yet to burn through a single piece of wire, and this incorporates over 15 years of trying. <p>Yes, I could do it but that is over amp-ed. <p>We have come along a long way since the 60s, 70s, and 80s, [chemistry] when everything including chemistry has changed. I have failed to burn out a single 2 cent piece of SS wire so far in the last 15 years.<p>And even if I could, its not even a penny worth of wire.

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