Hotwire Foam Cutting Knife.

ship

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Just made a power supply for a 6' hotwire - foam cutting wand. The fab’ shop is making the wand part including figuring out what wire for cutting will be used, (trial and error). They are a little behind schedule as normal so at this point only the power supply is done. Goal is to cut blocks of 4x8x6" thick expanded polystyrene sheets made into 8' dia. convex discs for a project. Saw the design and it’s very ambitious.... glad I am only making the PSU. But the concept will in me making a variable voltage high amperage power supply be very useful for future projects also.

Dia., type of wire and it's length is given below in a linked chart (below in the links, but limited) but still very much a subject for trial and error. Dependant on wand length, the above and type of material to be cut. Hot wires are not a easy, cheap or scientific thing to do in any way to say that if you are using X foam at Y length of cut, use Z voltage, and you at least need AA amperage etc. You are re-inventing the filament of a lamp that's sufficient to cut thru foam, hot enough it cuts cleanly but not too hot or cool. Hot enough it don't burn thru the filament etc. Cool concept.

Very bad memories from when I was a scene shop carpenter in working with the above material. Dots of foam everywhere, and looking at such foam makes me hack due to the smell. Granted I was chain saw, spary paint, grinder chain saw wheel, belt sander etc shaping and never tried a hot wire before. Hotwire is the best solution - but still wear your masks and operate in a ventilated area.

Masks were optional when I was doing such things and masks got hot. Probably wore one at the scene shop but probably not at the store front theater where I also used the technique I learned in less ventilated areas. Young and stupid in spoiling the fun of wanting in some ways to take part now. I’ll probably look in on it though.

http://www.rcgroups.com/forums/showthread.php?t=628704
http://epp540.com/index.php?option=com_content&task=view&id=89&Itemid=43
http://www.instructables.com/id/Hot-wire-foam-cutter/

Above links were very informative on the concept - though read all like 300++ posts and related links as there is a lot of important information very thoroughly examined on the design, safety and various forms of such a cutter. Found it interesting that CB hasn’t had a conversation on this topic.


The above links are also balancing wall wart transformer and door bell ringer transformer "advice" in very unsafe conditions with more semi-pro and dangerous grades of the cutter in design told about. This as with other in-line transformer concepts and electronically controlled options that I wouldn't do but I could not avise on. Amazing, all posts were based on the "cheapest way you could do it." People taking pride in how cheap they did it, yet what were they producing and or was it for many safe? Really, using a CD disc storage container one found at a 5 and dime store to house your electrical components and it melted down... A few posts recommended more Cadillac like things, but everyone was still very economy in building such a thing. If a tool and building one better than one can get commercially... I at least had no budget, but wanted to keep it under like $500.00 overall in parts, and it was under that in a comfortable estimate. I’m salary so my labor won’t be counted even if very extensive - six hours alone in studying the concept.

Most powerful hotwire ever... But I have concerns also such as instead of breakered output, fusing the project to what is needed for amperage. 28v/20A variac controlled, breakered in every step with analog output dials for voltage and amperage on the output. Analog dials for low voltage was all that was available which was economical even if pricy. Will have loved to digitally dial in the settings for what material was to be cut.

EDIT: I have returned this post and only removed the portion asking for permission for it to be visible. As always, please proceed with caution on projects such as this if you do not have the relevant experience.
 
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It is always the fumes released that are dangerous and it depends on the foam you use. I stick with cutting because I do not want to breath those fumes if I don't have to.
All that being said a friend in grad school made a powerful hotwire shape cutter using a stick welder and heavy coat hanger wire.
I made one in college using nichrome wire and a light dimmer. They can be very dangerous but I do like the way it seals the cut edge.
 
Just read through the instructable, I can't fathom using CD cases as a project enclosure. Darwin award candidate for sure!
 
Just read through the instructable, I can't fathom using CD cases as a project enclosure. Darwin award candidate for sure!

I didn't like that either or in general the main goal on the websites was how "cheap" they could make one with like a wall wart transformer etc. over doing something one can depend on in safety. Store bought ones are for now the most safe and proper it would seem. My goal was to make one more high powered and better than others short of CNC machine in doing so. Links did have a wealth of information including charts and materials that was useful however on the subject. Fumes with a hot wire are less or more or equal subject for debate in other ways to carve, but I think but also to use in general with a well ventilated area and in with the protection . Hear a 3D printer just got sent up to the space lab, wonder how they deal with the fumes from it?

Attached is a photo of my version, one can see it's a little more of a professional touch to the power supply concept.

First problem they had was wire stretching too much. Two types of wire they tried and I was busy with other projects by than and didn't see or find out results or even see what sort of wand or wire solution including a thicker one they ended up using.

Second problem was main breaker/switch for on/off was tripping. I was using a http://www.mcmaster.com/#6994k34/=tvvok0 variac on it and 5A breakered before and pop up breaker after the dimmer in going to the transformer (parts from stock), than a third time in breakering on the 20A output which should have been the first to blow. While I wasn't being cheap, in needing to get their project done, and this was a used breaker, I removed and bypassed it given the load side would still protect the expensive variac and everything worked out for the power supply in use for the project. Bad breaker so far w/o testing further. Have the replacement switch/breaker but have not gotten back to the done project in re-testing.

Really been busy with other projects and the project I'm told is done. Believe they followed my no more than 90% dimmer level but overall I hear it worked out well for some 48" x 6" thick or more sheets they were cutting on a disc rotating platform once they dialed it in and found the correct possibly thicker wire I would think.

Interesting to me was the analog amperage meter (only type I could get for low voltage short of component and huge expense), didn't regester any real amperage when cutting test foam (only time I saw it in use). I was watching it in trying to figure out the above problem with bad breaker tripping. Amperage meter would have been useful to dial in and note amps/volts for any future projects, even if I had to go analog - part of the concept in spending the money was for it to pay off on future projects. It will work and appairently did, but short of all info in volts/amps/length/type of wire used/type of material in use, it would be hard to reproduce what they did and get a base set of data.

Measured 54 Volts between hot lines and ground, but only 28 hot to hot as it were. My concern is if I should add a fuse or something like that on the output side, given voltage to ground readings = should someone touch a broken hotwire could this be dangerous at the dialied in votage given the 20A output breaker? Possible for a short from a broken hotwire conductor not turned off with a 20A/28v or 54v output in someone touching a broken wire? Fuse down to the output use to max out the amperage for the application?

Thinking and not kowing in building a very powerful power supply.
 

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I see a number of designs mentioned that do NOT provide isolation between the AC line and the nichrome wire. This can be a lethal combination. Please use a proper isolation transformer (note that a Variac is an autotransformer; it does not provide any isolation) for safety.

/mike
 

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