Ed Smith

I do not know why there is emphasis on using various laminates for foam wing templates. I have cut countless foam wings. For templates I use 1/8 ply. It is readily available, inexpensive and easy to work. When I have the profile I want I apply CA glue to it and I have a hard surface for the wire.

Cut through the nonsense, keep it simple!

Ed S

bkdavy

My Feedback: (1)

The aluminum flashing is even easier to work with than plywood, and you don't have to harden it with CA. Cutting the shape from aluminum flashing took slightly more effort than cutting it out of paper.

Brad

KI8FR

I have to say that the aluminum flashing was very ezy to do.. I have not cut the wing yet.. I will try to do it next weekend.

Montague

Fwiw, I didn't come up with the flashing idea, I stole it from someone a while back, I just can't recall who or when or from where . (almost, but not quite, credit where credit is due). I've seen it mentioned in other places as well.

KI8FR

Now I'm going to try to do something here that I see ppl doing. I going to try to make a fuselage out of foam. I made a frame out of wood now I want to cut foam to put on the side of the frame to make the fuselage look round. I got a idea on how to do it. but I was hoping that there was some one that has made something like this that can give me some ideas

drcrash

It seems to me that you could run the wire through a pair of cylindrical rollers (dead center through a cylinder on each end) and run the cylinders over the template rather than dragging the wire itself over the template. Cylinders of non-negligible diameter would be much less likely to catch on tiny non-smoothnesses in the templates. If these "rollers" are smooth and slick enough, they don't have to actually roll---they can just be round skids. Their larger diameter would make them slide right over minor imperfections in the templates. The roundness would ensure that they always put the wire a constant distance from the template contour.

To make this work, you'd need to adjust your templates. For example, if your rollers/skids are 1/4" diameter, you'd need to make your templates 1/8" smaller everywhere than the actual foil you want.

The hardest part would seem to be getting the wire exactly centered in the circular roller/skid things. If they roll, any non-centeredness would make them wobble up and down and make ripples. But if they're just skids, and you hold their orientation fixed, they wouldn't. You'd get some slight divergence from the intended shape spread across the whole foil, but I'd think that'd be okay if you get them pretty close.

Anybody tried anything like that? Anybody see a problem with it?

Paul

Tired of buying cheap plastic crap? Now you can make your own!
http://www.tk560.com/phpBB2/viewtopic.php?t=466
http://www.rcgroups.com/forums/showthread.php?t=621858

Montague

I've seen pictures of a guys setup that does one better. I can't find the links right now, but I think I can describe what he did.

Basically, the wire is mounted on ball bearing sliders that can move vertically. There is a ball bearing wheel mounted so that the bottom of the wheel is 1/2" below the wire. The foam blank is set on a sliding table, so the blank moves front-to-back, and the wire only moves up and down. The template mounts in to a slot next to the blank and spacers are used so that the foam blank is 1/2" higher than the template. The biggest problem with this setup is that you can't easily cut tapered cores.

But you could adapt parts of it yourself, and rather than trying to run the wire through the center of your roller, mount the wire above the roller, and just use an offset spacer under the core.

drcrash

If you do find the links, please do post them. That sounds very interesting.

I've been thinking about which degrees of freedom should be wire motion and which should be moving the blank. The basic setup you describe (to the extent I understand it) sounds easy to generalize if you move the blank non-linearly in some simple way. (Like Jim Young's setup, or something.)

Paul

drcrash

About flashing templates...

If there's any problem at all with heat sinking right there at the template, I'd think you could just put a small spacer between the template and the end of the foam blank, so that the chilled part of the wire isn't in contact with foam. This would elongate your shape a tiny bit, and make your blank-end profiles very slightly smaller on the big end and bigger on the small end, but you could compensate for that in the template shape. (But maybe it's a complete non-problem? Do you get any artifacts at all right there at the ends?)

Montague

If I run too much heat on the wire then I do get a very small but non-0 lip at the ends of the cores where they touch the templates. A quick touch with a sanding block fixes it, and the cores are plenty accurate for what I'm doing. Running the wire cooler reduces this effect, probably by making the kerf smaller. I think all that is happening is that the kerf is a bit smaller at the end of the core. But since you take the kerf in to account anyway, sanding off the lip is a non-issue to me. YMMV of course.

And yes, I have heard that 1/32" or 1/16" balsa as a spacer fixes the problem, I've just never bothered to try it. And the size of the small lip is such that I believe the spacer would take care of the problem easily.

I couldn't find the pictures of the foam cutting setup I described. And I can't remember exactly who it was that did it.

drcrash

Thanks, Montague

What gauge wire do people use for hot wire cutters? From various things, I infer that it must be something with a resistance of about an ohm or so per foot... so for nichrome roughly 22 gauge (.029" dia.), or for a plain (steel wire) guitar string, an .011 or thereabouts. (According to my measurements of my loose guitar strings, anyway.)

That would give 2 or 3 watts per inch at 5 or 6 amps, and let you use up to a 2-foot string with a 12v transformer, or a 4-foot string with 24v transformer.

Is that about right? Do people use thinner wires for shorter bows, and thicker ones for longer bows?

I would like to build a wire cutter that can do 3 feet or so. It sounds like 12 v is not enough, because I'd need really thin wire that I can't find (nichrome) or which would break (steel). So I need a 24V power supply that can do 5 or 6 amps, I guess...?

(Anybody know where to look up the electrical resistance of fine stainless steel wires? My music store doesn't stock stainless guitar strings anymore; I guess they've gone out of fashion.)

If anybody else is interested in this sort of calculations, I can post some stuff I've looked up/figured out.

Montague

Honestly, I think you are over-thinking it. I haven't done the math, or even measured the voltage I run on my bow, nor measured the wire thickness.

I get my wire by buying "picture hanging wire" at home depot. It's 3-strand braided. I clamp one end in a vice, the other end in a power drill, and spin-until-ubraided. Cheap and easy source of good-enough steel wire. Basically, I don't think wire thickness is that critical. Just remember that if the wire is too thick, it will require too much current to heat it up.

I do use a 24volt transformer from Radio Shack, but the imput is through a hardware store ceiling fan dimmer switch, so the actual voltage somewhere less than 24. I suspect it's close to 12, but I'm not sure. I say that because I did manage to burn out a couple of hardware store doorbell transformers, which were rated for 20va, if I recall. The RS transformer is rated higher, and works great.

I do use an amp meter to measure the heat of the wire, and run about 1.6-1.7amps, which gets just the right heat for the wire I use and the foam I'm cutting.

I have two bows, one is about 6 inches for cutting wing saddles in fuse blanks, the other is 4 foot, though the longest span core I've actually cut is 32". I cut shorter span wing cores by moving the alagator clips in on the wire as well, so I don't heat the entire wire, just the part that's doing the cutting. (I find I burn myself less often this way. Less hot bits for my fingers to grab at the wrong moment.) (this may be a "bad thing" in theory. I don't know. I'm 100% home-brew, figure it out as I went here.)

Anyway, I use the same current regardless of the cut span. That means I use less voltage (by turning down the dimmer) for shorter span cuts since the alagator clips are moved closer together, but the ampmeter should always reads the same.

I bet guitar string would make pretty good cutting wire, though maybe more expensive than other options.

Btw, one last tip about using metal templates. Remember that your wire has electricity running through it. And the wire is touching the metal templates. Therefore the templates are "live" as well. If, by chance, you manage to touch both templates with something like a metal yard stick while cutting, bad things can happen. Not that I'd know by personal experience or anything. (at least not that I'd admit to).

math guesswork: Rounding up to 2 amps during a cut. Voltage, guess 12v, since I think the dimmer usually winds up at around "half way". So, 24watts, over a 32" span of wire. But the current and voltage are AC in my case, not DC. I seem to recall that changing the power calculation a bit. Anyway, it looks like I'm about .6-.7 watts per inch, much lower than your numbers, and I know I'm way under 5-6amps.

drcrash

Could be, but I'm being a bit more careful than usual about this because I'm trying to come up with easy, reliable, safe instructions for other people. I'd be pretty embarrassed if they burned out their transformers, burned down their houses, etc. If it was just me, I'd be more inclined to wing it. As it is, I don't want to run any components over their ratings. (Which half the plans I've seen seem to do, or the designers don't really know.)

That's also part of why I'm looking at guitar strings. They're easy to buy just about anywhere---you can walk into a music store and ask for an 011, and that's what you'll get---a little over 3 feet of good quality wire for about a dollar.

Interesting. I've seen several people say you need about 5 amps, and I'm wondering what that really means.

Does your wire actually touch the foam, or vaporize it before it touches it? It may be that you need 5 amps to vaporize the foam without touching it. Or something. I wish I knew.

I have two bows, one is about 6 inches for cutting wing saddles in fuse blanks, the other is 4 foot, though the longest span core I've actually cut is 32". I cut shorter span wing cores by moving the alagator clips in on the wire as well, so I don't heat the entire wire, just the part that's doing the cutting. (I find I burn myself less often this way. Less hot bits for my fingers to grab at the wrong moment.) (this may be a "bad thing" in theory. I don't know. I'm 100% home-brew, figure it out as I went here.)

Sounds good. I haven't included an ammeter because RadioShack and Fry's no longer sell ammeters. (Just multimeters that either can't handle whole amps, or cost too much.)

That sort of thing never happens to me, either.

Interesting. Thanks for the input.

Montague

Next time I'm doing some cutting, I'll try to get a voltage reading. I have a second meter around somewhere, so it should be easy to get.

The wire is hot enough to not touch the foam as you describe. I try to minimize the kerf which means keeping the cutting speed up as fast as is reasonable, so it might drag a little bit at the front edge, but it works out ok anyway. If you feed too slowly, you get a huge kerf and your core comes out thinner than the template.

I suppose if you used a thicker wire you might pull 5 amps. The wire you use is going to make a huge difference here.

I also honestly don't know if AC vs DC makes a difference.

I HIGHLY recommend you get an ampmeter in your cutting arrangement. Unless you always have the same length of wire you'll want to be able to adjust the power going to the wire. And the amps == heat. So you alter the voltage to always cut with the same amperage, and you can cut anything from 2" wide to however big your bow is, and get the same cutting properties. With out the meter, I'd either have to always heat the same length of wire, or I'd have to use trial and error to adjust the heat, making test cuts to see if the wire was hot or cold. Or figure out a thermometer rig somehow.

drcrash

I'd appreciate that.

Yes. I've seen several sets of plans for using roughly 1 ohm per foot wire (21 or 22 gauge nichrome, or 012 guitar strings), but clearly some people are using much thinner wire and much lower amps, and it works. (I just found some plans with somebody using 32 AWG stainless, which is just tiny, and I'd assumed that would just break. They say 1 watt per inch is about right. The 32 AWG might be a typo, but it's something small.)

Maybe you only need bigger wire & amps if you're cutting dense foam? (I do want to be able to cut 4-lb foam for moldmaking, but usually 1 or 2 because of weight and/or cost.)

My impression is that it doesn't make much difference heatwise. It does make a difference in how nasty it is to be shocked and how hard it is to measure the voltage. (AC makes muscles clench; non-regulated DC pulsates and confuses cheap meters so that they read the peak voltage rather than the average voltage.)

Yes, that's a huge plus. I need to look into sources of suitable cheap ammeters (maybe an auto parts store?).

By the way, if you're interested in trying guitar strings, here are some (room temperature) measurements I made:

Ernie Ball 8 (.008" ) 2.0 ohms/foot
Ernie Ball 9 (.009" ) 1.40 ohms/foot
Ernie Ball 10 (.010" ) 1.17 ohms/foot
Ernie Ball 12 (.012" ) 0.83 ohms/foot

I'll try the .008 with one 24 volt RS transformer (& dimmer) and see how it goes.

I also found these people selling very thin nichrome for hot wire cutters---a 30-ft roll for $9. (But 4 dollars handling per order, plus shipping.)

http://www.aeroconsystems.com/electr...ichrome.htm#26

It's 26 gauge, 2.67 ohms per foot.

I may try that, too.

drcrash

I found out that Fender and Ernie Ball still make stainless steel guitar strings, and you can get them (in sets) for $3.50 or so from Musician's Friend (www.musiciansfriend.com) and Sam Ash Music (www.samash.com). The light sets start with a .009, a .012, and a .016 (or something close to that) and the not-quite-so-light sets start with a .010, a .013, and a .017. (The heavier strings in a set are useless both because they're too heavy and because they're wound.)

I'll look for someplace that sells the single strings for a buck each.