foam cuting with a hotwire

hi
ive been trying to cut leading edges for f2d models
and the foam is turning out all lumpy and stuff
i think its the density of the foam but i am not sure
has anyone had any experience with doing this
thankyou
tom

It sounds like the wire could be too hot or is moving too slowly. Another possibility is substandard foam. Some beaded foam has recycled bits in it which the wire tends to pass around or pass through slowly, overheating other areas. Good beaded foam will feel uniformly soft. The substandard stuff has hard spots you can feel when you run your fingers over it.

What are you using for templates, wire, and power?


Howard Rush

>hi
>ive been trying to cut leading edges for f2d models
>and the foam is turning out all lumpy and stuff
>i think its the density of the foam but i am not sure
>has anyone had any experience with doing this
>thankyou
>tom
>

Tom,

Alan Mason up at Caves Beach on the southern end of Lake Macquarie is clued up on foam cutting. He's not too far from you, do you want his contact details.

regards Ray

Foam from a lumberyard(Lowes, Home Depot, etc) is cheap but almost always has lumps of rework in it. You'll never get a decent cut with it, but it's probably good enough for F2D leading edges.

Try to buy virgin, 1 lb. EIFS(External Insulation Foam System) from a company that either produces the foam, or a lumberyard that can buy it from them. You may have to buy a lot of it.

>Try to buy virgin, 1 lb. EIFS(External Insulation Foam System)
>from a company that either produces the foam, or a lumberyard
>that can buy it from them. You may have to buy a lot of it.

A virgin in a corehouse ??

Hi Tom,
I have plenty of experience cutting foam wings, you better give me a call at home after about 6pm, or send me an email.
[email protected]
02 45 722 157

Andrew Heath

I knocked out a few foam leading edges with some polystrene from Home Depot, no problem.

Used scrap .018 flying line as the wire. Simple hand tools.

The power source was a Variac I pulled off the bay for about $20-$30. I might use it for a controller on my new electric stunter, if I don't fry myself first.

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Try a drywall contractor for the EIFS foam. Nice stuff.

If you're using a variac, make sure it's an isolated variac. Or use an isolation transformer with it. Better yet, feed a 120V in 20V out transformer with the variac - that lets you use the entire range of the variac rather than trying to nudge the dial a little bit.

Be really careful using a Variac without an isolation/stepdown transformer. The usual Variac circuit can put 110v directly on one end of the wire which can be real shocker. Use a voltmeter to check against a real ground. Also take some care to select a wire diameter/length combination that keeps the voltage across the wire below 42-25 v. or so. Higher voltages can stop your heart, just like 110v.

These are all safety precautions against the time you accidentally touch the wrong place and send 110v. through your heart. It's quite possible to use an unsafe setup for years if you take other precautions, like rubber-soled shoes and keeping all the connections covered up so they won't get accidentally touched.

I got this baby wired up and checked it with a light bulb.

Zero is zero,
20% is 30 VAC,
30% is 40 VAC,
40% is 45 VAC, and
100% is 120 VAC, just as labeled.

It cuts foam at about 20% to 30% of power, depending on the length of the wire. No need to go higher, unless I'm questioning a detainee for CTU.

----------------------------

At Phil & Isky's suggestion, I did some really scientific research. Now don't get me wrong. I'm not modernizing. All of this technology is OTS-legal (pre-1952).
--------------------
In the interest of safety, the "hot section" is in a box.
Do not try this at home, kids, order from The Corehouse, exclusively.



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>In the interest of safty, the "hot section" is in a
>box.


Hey Paul:

This is a potentially lethal setup..

I can't see all that is in the box but if it is simply a variac feeding the outlet, then it is very dangerous.

Variac's are normally what is called an autotransformer--the difference from a regular transformer--and what makes them dangerous--is that that there is NO ISOLATION from the feed line.

you have a very nice three prong grounding outlet and then you plug in a two prong plug from the wire--that wire could potentially (no pun) kill you if you come into contact with another piece of equipment that is plugged into the mains. It could even have 220V on it if the other unit is on the opposite "half" of the incoming line.

To make this set-up safe (er) , (and you might have this, I can't see) you need a 1:1 isolation transformer in the system somewhere, either before or after the variac. These are cheaper than the variac.

If cost is an issue, and variacs aren't cheap these days, get a 1:1 transormer and use a 400 watt household light dimmer--perfect set-up.

FWIW

Curt

PS I stopped reading before seeing Isky and Phils response which were essentially the same warning--

>At Phil & Isky's suggestion, I did some really scientific
>research.



Hey Paul!

Did the research involve any exploding watermellons?

Curt

Nothing exploxed. I just took some readings on a Sears multitester.

Please explain to me how running 110 VAC 60 cycle through a transformer and getting 110 VAC 60 cycle out makes it safe. You're probably correct, I'd just like to understand why.

All 110 VAC appliances are somewhat unsafe. What make my foam cutter any worse than the ChiCom devices people buy at WalMart and hold in their hands?

Would it be any better if I plugged it into a ground fault recepticle?

I run into this on the tractor forums all the time most are just looking out for your safety but sometimes the warnings and do-it-exactly to code suggestions are a bit nerve-racking. Especially when you know what you are doing isn't all that risky.

The problem with an un-isolated variac is that one side of the hot wire could very well be tied to one leg of the single phase 220 coming into your house. All that is needed to complete the circuit is a ground. There is a much greater chance to complete a circuit to ground (through your body) with this setup than with an isolation transformer. If you have an isolation transformer in the circuit you will have to actually get to the other wire on the transformer to complete the circuit. Much harder to do accidentally than finding a ground.

You can make it a tad bit safer by making sure the terminal on the variac that isn't adjusted (the one that goes straight through to the wall plug) is tied to the neutral wire in the wall plug. Take your meter and measure between the two sides of your hot wire and ground. You want the wall plug wired so you read nothing or very little on one end and 50 volts or less(whatever the variac is set at) on the other. Then do not hook up the ground on your 3 wire plug, you don't need the ground going through your variac box and without an isolation transformer the last thing you want close is a ground wire.

Good luck and don't blame me if you kill yourself I just tried to tell you the best way to minimize the risk.

THanks Bob:

I was struggling with trying to explain without diagrams.

The origin of the problem is the way house wiring is made--and begins with the 220 pole transformer outside the home with its centertap grounded. Thus we get two 110 circuits in the house with a common neutral tied to ground. You can verify this is the case by checking the voltage from a left row fuse or breaker and a right row fuse in your normal two column panel. You will see the 220V.

As Bob says, by mixing up polarities on appliances and if any of them has a grounded common inside, you can "see" the 220 from the opposite circuit since there isnt any isolation. Also, if you were to touch the cutting wire in your rig and be grounded at the same time, depending on the way you had the wiring done, you could see the full 120 line voltage regardless of what the variac was set for.

Some will believe that these wire cutters are benign since the voltage is typically pretty low--I run mine at about 30V to get the right temp--but even that can kill you under the right circumstances--even with no ground or isolation faults. If that wire were to break while you were cutting with it and lash back around your wrist or penetrate your skin--you could easily get a lethal shock.

I use a DC Bench power supply with variable current and voltage limits. Roughly the same cost as a variac on the Bay.

BE VERY CAREFUL.

FWIW

Curt

Again, what about these "ground fault" recepticles that are supposed to protect you if your wife throws the hair dryer into the bath tub?

As explained to me, if the current goes anyplace except round-and-round the racetrack like good little electrons, it faults out in a microsecond.


I understand that a house has a white neutral in the middle and two switched, fused blacks on the outsides. White-to-black gives 110 and black-to-black gives 220, as in the stove. I can't see a way an accidental black-to-black circuit could happen from one outlet.

As suggested,

If the black wire is on the "rheostat side", and
The cutting wire snaps and sticks me, and
I happen to be somehow connected to the other black phase,
I'll only get 150 VAC in lieu of the 220 VAC I would have gotten with the other polarity.

But if I add an isolation transformer, I could still get a 110 VAC jolt?

You,re correct.

THe GFI can protect from issues with ground path problems.

The 220V scenario is rare but has happened. It must involve a second device outlet. If the device is faulty, you can get hit with 220.

Lets say you are working on a bench with your wire cutter and your metal table saw is right next to the bench and plugged into a non-polarized or improperly wired outlet from the next room. If the motor has a fault, which could exist with no operational evidence for a long time, and your touch your wire and the saw frame while grabbing your cup of joe, then boom you get it!

Part of my point with the broken wire thing is that we all need to be careful with this stuff--it is NOT like working with a 3Volt battery powered device.

The small tool industry has re-designed the entire environment of small tools like drills.

The days of all metal-housed tools is more or less gone. Now they are all double insulated with plastic cases to eliminate a part of this ground fault/ reverse polarity problem. Same with the kitchen appliance stuff.

But it seems in our shops, we all have lots of OLD stuff that can have ground faults, non-polarized plugs, etc. because the came from Grandpa's shop before ours.

Anyway,

FWIW

Curt

>Nothing exploxed. I just took some readings on a Sears
>multitester.
>
>Please explain to me how running 110 VAC 60 cycle through a
>transformer and getting 110 VAC 60 cycle out makes it safe.
>You're probably correct, I'd just like to understand why.
>
>All 110 VAC appliances are somewhat unsafe. What make my foam
>cutter any worse than the ChiCom devices people buy at WalMart
>and hold in their hands?
>
>Would it be any better if I plugged it into a ground fault
>recepticle?

Fault currents come 2 ways Line to Line and Line to ground. Isolation transformers remove the line to ground risk.

The isolation transofrmer make it safe becasue neither side is grounded an you can litterly tie one of the legs to ground with no effect. So you can touch either terminal of the transformer without a shock.

If you go touch both terminals of the transformer (line to line), you are not safe, becasue the fault is line to line.

The risk from the a line to line to line shock is still there, in a hot wire foam cutter we use only 12-20v line to line, so that is pretty safe, line to line. Less than 40 volts is considered safe in most situations.


With 110 house current, the ground is defined back at the transformer, and may not be where you expect. The only thing you know is that the line to line voltage is 110. That 12-14 volts coming out of the variac may be still 100 volts above ground.


You can get a 24v control transformer surplus cheap cheap especally in Detroit. Or even get a single phase 460-240 (2:1) transformer. It won't be super efficent but it will work fine. I think about 30-40VA? Put this transfomer ahead of your variac, and you have the best of both worlds.

GFI circuit will prtector will protect you in a fault condition, but a transfomer will eliminate the risk entirelly.

Dave
Just a sport flier

Don't know how available they are in the States, but my current hot wire rig uses two very inexpensive ($3 or so apiece) 12V 50W transformers meant for halogen lamps. Hooked up with primaries in parallel and secondaries in series, that will supply 24V at over 4A. Built a circuit based on a triac I found on an RC web site to control the voltage. I suppose a variac in series with the primaries would have worked as well.

A standard triac-type light dimmer ahead of those would work dandy! and about 6 bucks.

FWIW

Curt

Two things I've wondered about on this subject.

1. Could I use a standard 24v transformer from a heat and air unit to power a foam cutter or is it just not enough power?
2. Could I use my car battery with a dimmer switch to control the output?

I remember when Tom's dad (Andrew) and I were teenagers, we just used a 12V Car battery and Nichrome resistance wire and nothing else. In hindsite we had no idea what we were doing but it worked. We used to cut full wings out that came out pretty good.

I guess the car battery had pleanty of Amps and we must have had just the right length of wire. ie we got lucky.

It's not really an exact science is it?

Warren Leadbeatter
Port Stephens - Australia
AUS-14782

Car batteries work fine, but with my wire of choice (steel control line cable, .015, or single strand stainless fishing line, .015 or .018, or a steel guitar string) it'll only drive a 25" length or so. I want more like 30 inches, to cut those wings with lots of taper or for extra long cores.

Similarly, nichrome works OK, but is far too weak for the type of tension I like to use. The more tension, the less sagging you get. I like it so tight you get a fairly high note when you pluck it.

In the olden days I used my 24V American Flyer Train transformer. But now the train set is back together. Plus, I found out how expensive it would be to replace that if I screwed it up.

Mainly, though, I've started to work with longer wings (longer than Combat wings), and found that 24 volts just doesn't cut it. I need 30-40 volts for the length of cut I'm doing. It takes energy to vaporize foam, and a slow cut is a bad cut. But I will use finesse and keep the rheostat below 40%, I promise. I won't inhale the fumes, either.

--------------------------------

I had the prescience to sell both of the old power saws in my basement. They were cast iron and of pre-OTS vintage. As suggested by Curt and Isky (2 guys who don't want to electrocute me, yet) these old relics had questionable circuity and might have actually competed the fatal 220 volt circuit.

24V sounds high. There may be some resistance there. What voltage do you measure at the ends of the cutter wire? The less the series resistance, the better regulated the wire temperature will be.

Howard Rush

You certainly don't want to "vaporize foam", you'd end up with a 1/4" kerf and a horrible looking surface. You want to just melt it. Ideally you should have little polystyrene whiskers on the surface of the core when you're done. And slow cuts aren't bad - you get smoother cuts that way. If you're running low wire tension, yeah, your wire will sag if you cut too fast and your wire is too cool. But if you realy stretch the wire, you can get reasonable speeds with a fairly low wire temperature, really helps for highly tapered wings.

I can't imagine needing 30-40 volts for CL model wings - you'd need about five to six feet of wire to soak up that voltage. Those are mighty long wings... I suppose if you were cutting some of Ken Burdick's GFAPs... Even then...

>Two things I've wondered about on this subject.
>
> 1. Could I use a standard 24v transformer from a heat and
>air unit to power a foam cutter or is it just not enough
>power?

Like Howard mentioned, it kind of depends upon the wire in use. I use .020" Monel safety wire and the Bow is about 38"--I use very heavy wire from the power supply to the wire and it runs at about 25V and a about 5 amps I think. With shorter bow the 24V control tansformer might work but my guess is that something like a furnace or AC unit wont be rated for enough current--it will work but might not last very long.


> 2. Could I use my car battery with a dimmer switch to
>control the output?

Household light dimmers wont work right with DC. 12V would only run a very short wire or a very large diameter wire (low Resistance)


The best and least expensive setup I know of is a 200-400 watt light dimmer driving TWO 24 Volt control transformers rated for around 5 Amps. Use heavy wire to run to the cutting wire so the current heats the cutter instead of the whole thing. Like #12-14 or so and use wheel collars or similar clamps to connect the wiring to the cutting wire.

FWIW

Curt

I remember speculating about using lighting dimmer switches in series with the primary of a transformer once and was warned I'd end up with smoke coming out of it. Something about dimmer switches not mixing with inductive loads. So I didn't try it. Maybe I should, if I build another hot wire power source.

In the old days, I used nichrome and a 12V motorcycle battery. To regulate the temperature I drove some nails in a board and strung nichrome between them in a zigzag pattern and used an alligator clip to choose how long my step-down resistor would be. In other words, it was like adjusting the length of the hot wire. Then I switched to steel, and that didn't work any more.

Yes..that was the case with most of them early on.. They are pretty well protected these days tho..almost all I have seen have supreesor diodes in them to protect from inductive spikes.

They are basically variable cycle width half-wave devices so are switching the load on and of once or twice a cycle. The switching can generate spikes if not surpressed.

Curt

Come to think of it.. Maybe I should start building that "hot wire scrollsaw" I've been wanting to build just about forever. Something so as I can cut really square block ends and holes. Since the wire will be short (8" or so) I think I'll get by with one 12V halogen light transformer. They make these in a low-profile form factor identical to a small (20W sized) fluorescent light ballast, so I can mount it in the open, on top of the saw table. And I can use a dimmer switch with it. And I can use one of those guitar tuners I bought to tension the wire.

Hi Isky:

If you want to be certain of the output protection, use one of the zener spike supressors. (back to back Zeners in a single package. Get the spec so it's rating switchs at about 200V or so.

Curt

Interesting... any comments about my using a 12 volt auto battery charger? I must have cut hundreds of wings for combat planes here locally using my Sears 10 amp battery charger and some .012 flying line.
Bob Furr

"See ya in the circle!"

They do supply 14-15V, so they'll probably drive a steel wire OK. Never tried using .012 wire - the way I figured it, my templates are far from perfect, and the larger the wire, the better it would ride over the odd roughness.... You do lose some in foam thickness, so this might be a problem if you use the "bed of nails" method, like I do.

If you don't vaporize at least some foam, it'll just fuse back together behind the wire. I had that trouble when I was power-limited. I use old .018" 7-strand now. I might have used .015 when I was on the old train transformer.

Somebody sold me spool of nichome wire, specially good for faom cutting. I still have about 98% of it. Didn't hardly work at all.

I can't swear to the exact voltage. I just finesse the rheostat until it feels good. But I certainly don't go over 40 VAC, that would be unsafe. This thread was the first time I put a meter on it.



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A lot of tooling for one spar slot, but it got cut.

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No..

One of the things this thread has provided is that there are alot of ways to do this.

I like having a way to change the drive to the wire, although I rarely change it after getting the bow set up. I don't even change it when going from the blanking bow to the foil bow which is about 10" longer.

I'm sure the battery charger works fine.

One thing I did notice from your video is that you like to cut pretty slowly.

Just for grins. you might try a larger wire--like 018-020 with more tension, and drive it so it will go faster. Just a different way to "fly the pattern". like going 4.9 sec laps v. 5.4 sec.

You are right I do cut slowly. I like the feel of the ultra light bow that I use. That requires cutting slower to avoid bowing the cutting wire. I do have a heavier bow with .018 solid wire (old rat race flying lines)that will cut faster but I find I make more mistakes cutting faster. So the heavy bow gets used for straight line cuts like taking a 4 by 8 sheet of 2 inch down to size.
Bob
PS. Nice to see all the discussion about cutting foam... I was beginning to think that the art was going to be lost with all the ARFs coming out of China.