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hot knife or hot wire cutter for 1/8 abs?

 
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harley guy
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PostPosted: Wed Dec 27, 2006 10:17 pm    Post subject: hot knife or hot wire cutter for 1/8 abs? Reply with quote

iam woundering if i can use a hot knife or a hot wire cutter to cut 1/8 abs plastic?i use a band saw now.but it would be real easy if a can make a cut-out jig and just go around that to cut the pattern out.because it hard for me to cut on a band saw now.i have made jigs and stand offs for my saw,but still a pain to cut the pattern.so i think it would be easier to use a hot wire and a cut-out jig.has anyone used it on abs plastic before?is it real slow?thanks for any info
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ANH trooper
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PostPosted: Thu Dec 28, 2006 5:02 am    Post subject: Reply with quote

I was talking to guy recently who said he used some kind of hot wire/blade for cutting out the eyes on his stormtrooper helmet and was much easier than using a dremel.I haven't used anything like it myself but can see how it would work well.
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drcrash
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PostPosted: Thu Dec 28, 2006 5:13 am    Post subject: Reply with quote

I haven't tried it, but it sounds like a good idea to me. Especially with a jig. If you make a cam-follower kind of thing, and a cam in the shape of each piece, it could make cutting parts out pretty quick work. (You could even have 3D cams---at least some tilting, by slanting the edge of the cam where the cam-follower goes around it, to adjust the angle of the cut.) Cool.

I think my soldering-iron-with-an-Exacto-attachment hot knife may be too wimpy for 1/8" ABS---it's pretty slow cutting 1/16 acrylic---but a hot wire cutter sounds good. (Then again, the hot knife might work fine; the melting point of the plastic may be more important than a factor of 2 in thickness.)

You may want a variable-power hot wire cutter, with a dimmer switch to adjust the voltage. (Or a reasonably big model train transformer.)

Paul

(P.S. If you send me a sample, I can try cutting it with my hot knife and my little 3-inch fixed-power hot wire cutter. The little cutter's probably in the right ballpark, being designed for cutting through an inch or two of styrofoam, which is probably roughly comparable to 1/8" of solid ABS.)
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jegner
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PostPosted: Fri Dec 29, 2006 9:43 am    Post subject: Reply with quote

Interesting!
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drcrash
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PostPosted: Sun Dec 31, 2006 12:31 am    Post subject: Reply with quote

I did some experiments with acrylic that's a bit over 1/16" thick. (I didn't have any ABS around.)

My hot wire cutter is pretty slow for that---it takes 2 or 3 seconds to go an inch, and to go that fast requires a sawing motion. (The wire cools where it's cutting the plastic, and doesn't cut as fast. If you saw it up and down, you can use different parts of the wire briefly while the parts you just used are getting back up to red hot.)

I think that might be worth it if you want a reasonably clean cut and don't need the kind of speed you can get with a saw. If you want to whip a part out in a few seconds, though, it's not going to be fast enough. (I don't know what kinds of times you're looking for.)

I'm not sure what my cutter was strung with to start with. (I hadn't used it for years.) Whatever it was, it broke when I tried to go fast. I replaced it with a piece of guitar top E string. (Something light---probably .09 or .010 nickel steel, but I don't remember that either...) That seemed to work about as well. I was applying 12 volts from my car battery charger to about 6 inches of guitar string, and it was pulling a couple of amps. I shortened the live part of the string to about 4 inches so that it got bright orange hot, and it broke pretty soon.

BTW, when I was cutting, the cut was not amazingly clean like cutting styrofoam. With styrofoam, if you get the heat right and move at the right rate, the foam is vaporized just ahead of the wire and the wire never actually touches it. With the solid acrylic, it was melting around the wire, and congealing behind it so that it was very lightly reconnected. A very gentle twist and the two pieces would come apart, but it wasn't a perfectly clean cut. Better than I get with a saw, but I probably don't have the right saw.

This may work better with a thicker gauge wire. That might make enough of a kerf that the melted plastic doesnt congeal together beind it. I tried it with some 19 gauge stainless steel, but that was too big for my power supply. (I couldn't get it hot enough to cut well, because I had to use about 3 feet to keep from pulling more than the 6 amp max of my car charger. Not enough watts per inch.)

Paul
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drcrash
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PostPosted: Fri Jan 05, 2007 11:50 am    Post subject: Reply with quote

I realized yesterday that a smart car battery charger is not the right kind of big honking transformer to use for a hot wire cutter. It adapts the voltage, and it does it in exactly the opposite way you'd ideally want for a hot wire cutter.

(The more plastic the wire blade encounters, the more that cools the wire where it's making the cut. That lowers the resistance of the wire, which should allow more current to flow through. But a smart trickle charger varies the voltage, trying to keep the current constant. So when the wire gets cooled and the resistance goes down, it will turn the voltage DOWN, just when you want it turned up to help re-heat the wire. Bummer.)

I got a dimmer switch and plan to get a big honking plain-old-transformer to do it the other way.

(By the way, a cheaper way to control the current with a fixed-voltage transformer is just to use more or less wire. Once you're in the right ballpark, you can just connect to the wire in different places so that more or less of it is live. A lot of people do that with a fixed connection on one end, and an an alligator clip on the other. You just clip the alligator clip on a little closer to the end of the wire, or a little further, to change the "heat setting.")
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Gary sculptari
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PostPosted: Fri Jan 05, 2007 6:15 pm    Post subject: Reply with quote

For hot wire machine we use a 'variac' transformer - this is variable voltage - they are available from most electronic supply houses for under $100 (for the chinese/taiwan versions). The higher the amps, the more the price but 5 amps should be good.

I have a complete hot knife set which I bought from demandproducts.com - they are used in the stucco industry to cut foam, etc. I have seen a tiny one in a craft store one time - but 1/8 abs is going to need some amps. I have also used the hot kife wire that demand sells hooked up to a cheap 110 volt 'arc welder' transformer - this is for serious hot cutting and low power is hard to control.

Another tool to consider is a 'rotozip' - is like a drill with cutting teeth on the side. It drills though the plastic, then you carefully move sidewise. Some people have trouble controlling, but they are no problem on thick materials. Better than wearing a respirator all the time!
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drcrash
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PostPosted: Sat Jan 06, 2007 12:36 pm    Post subject: Reply with quote

Gary sculptari wrote:
For hot wire machine we use a 'variac' transformer - this is variable voltage - they are available from most electronic supply houses for under $100 (for the chinese/taiwan versions). The higher the amps, the more the price but 5 amps should be good.


I was just looking at a 500-watt variac at Fry's the other day, but the price (something like $80) put me off. I'm going to try with a $2.50 600-watt dimmer (Home Depot has a two-pack on sale for $5!) and a $3 thrift store transformer. If the transformer won't cut it, I'll pay $11-$20 for a better one from an electronics place. (Wish me luck.)

Quote:
I have a complete hot knife set which I bought from demandproducts.com - they are used in the stucco industry to cut foam, etc.


I followed their link to their "foam cutting systems" page:

http://www.demandfoamcutting.com/

Cool stuff. (There are people doing smaller-scale, cheaper DIY versions of some of that kind of stuff over on CNCzone.com.)

I also came across this yesterday:

http://www.vatsaas.org/rtv/arsenal/bradrocs/napkin/napkinrocket.aspx

It has links to plans for a styrofoam cutter similar to what I'm planning to make next:

http://www.vatsaas.org/rtv/construction/hotwirecutter.aspx

(But I'll use a dimmer and a dumb transformer, rather than a big variable transformer.)

And a styrofoam shaping fixture:

http://www.vatsaas.org/rtv/construction/styrofoamfixture.aspx

I've been thinking about making something like that, but with a jig to let you use the hot wire lengthwise, rather than crosswise, for a more lathe-like effect.

Quote:
I have seen a tiny one in a craft store one time -


I was at Hobby Lobby the other day, and they still sell a little 3v, roughly 3-inch cutter like I've got for $7.00. (Unfortunately, it runs off 2 D cells, which is why I fitted mine with a biggish 3v transformer--but when I tried using the other day, it didn't work. I think the transformer wasn't biggish enough.) IIRC, those cutters come with 3 pieces of straight nichrome; some people buy them just for the nichrome to use in their own designs.
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Gary sculptari
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PostPosted: Sat Jan 06, 2007 5:55 pm    Post subject: Reply with quote

I really appreciate your research.

I realise that the hobbyist has to improvise, some of the premade stuff is pricey - $10 a foot for custom knife wire!

The big foam cutting machines, at the E.P.S. factories use DC as their energy source - is more stable over long wires and solves the problem of high amps at low voltage. Thats the fault with AC power - but I have never had a problem with the AC variac - maybe thats the difference with a variac - it keeps the amps/watts up at low voltage?

For example, an AC motor loses a lot of horse power when the voltage cuts back - a DC motor does not. This is why they are all DC stepper motors for precision work, etc.

The power from DC car batteries are amazing - I have a portable MIG welder caled 'rediwelder' which works off 2 - 12 volt car batteries wired to make 24 volts. That welder can put out a well over 100 amp weld - AC welders typically need 220 volt for that. 100 amps can also drop you dead - so be careful out there
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drcrash
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PostPosted: Fri Jan 12, 2007 7:25 pm    Post subject: Reply with quote

Gary sculptari wrote:
For hot wire machine we use a 'variac' transformer - this is variable voltage - they are available from most electronic supply houses for under $100 (for the chinese/taiwan versions). The higher the amps, the more the price but 5 amps should be good.


I've been doing some research on this, and it seems that there are two schools of thought. Some people think you should have about 4-6 amps, and dissipate 2 or 3 watts per inch of wire to cut foam. For that, you want about 21 or 22 gauge (straight) nichrome (i.e., .025-.029 inch dia) or somewhat smaller steel wire, such as a .011" dia. guitar string.

Other people make wire cutters with way less power, using much thinner wire---like 26 gauge nichrome or a .008" dia. guitar string---and pulling 2 amps or less. They seem to do just fine, including cutting through two or three feet of foam. (E.g., to make RC plane wings and fuselages.)

That may not be enough amps and watts to cut dense foam, though. I don't really know what the issues are there.

Being cheap myself, I'm starting with a 24 v 2 amp transformer ($10.50 at Radio Shack) and my $2.50 dimmer, and an Ernie Ball 8 (.008) guitar string... I'll let you know how it turns out.

If that's not enough oomph, I'll wire another identical transformer in parallel with the first one to get 4 amps. And if that's not enough, I can even add a third and get 6 amps, but then the setup will cost around $40, twice as much as I've spent so far. (But still a bit less than a variac.)

BTW, I read somewhere that if you use a variac, you should use a transformer, too, for isolation; otherwise there's a shock hazard. I'm not sure if that's true, but that's what somebody said.

I was planning on using stainless steel wire. A lot of people prefer it to nichrome because it's stronger and you can crank it tauter to get very straight cuts. In particular, I was counting on being able to get various diameters of fine stainless wire in the form of stainless steel guitar strings, but apparently stainless steel guitar strings have gone out of fashion, so that may not be the easy option it used to be.

Now I'm hunting around for cheap and easy local sources of fine stainless wire, and not having a lot of luck. (Preferably cheap in very small quantities, for experimentation. $1 for a three-foot guitar string would have been perfect, but I guess I'll have to mail-order those.)
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jegner
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PostPosted: Sat Jan 13, 2007 8:36 am    Post subject: Reply with quote

Great info! That Home Depot foam insualtion with a fiberglass shell seems an ok way to make vac forming buck patterns quick, cheap and easy.

Thanks Doc, for the research and links. I really need to make one of these now!

Jim
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moe



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PostPosted: Thu Sep 09, 2010 5:14 pm    Post subject: Reply with quote

You can use knife to cut, but you need a sharp knife.
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PARATECH1
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PostPosted: Sun Sep 12, 2010 9:15 am    Post subject: Reply with quote

Harbor Freight has hot knives on sale now for around thirteen bucks. They have a good long blade and work great. It is really fun when cutting up styrofoam blocks. This knife makes super fast and really clean cuts.
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