This is my second post about building a foam cutting hot wire. From the first, I found 9 sets of plans at:
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Very helpful. Of course, I had to do it my way, so I put together aspects from several of these plans.
What I now have is a 30" ni-chrome wire (2.5 Ohms per foot)stretched horizontally. For power, I first tried a model train transformer (as several plans used) but it didn't get hot enough (not sure why). I then went a transformer/household dimmer switch arrangement per several plans. First I tried a doorbell transformer, then a 12V/3Amp transformer. Neither one got hot enough.
I am trying to avoid making a power supply IAW Tom Weeden's very plans - I've never worked with copper clad circuit boards or the other electronic components called for.
So, here are my questions... Why hasn't what I've tried worked? Is there something I can change to make a it work? One plan used a car battery for power. If I tried that, what could I use to adjust the temperature?
This is my last try before I dive into Tom Weeden's power supply....
30 inches = 2.5 feet, and at 2.5 ohms per foot, you've got a 2.5*2.5 ohm resistor. That 6.25 ohms. Let's call it 6 to make it easier.
Running 12V through it will result in a current of 12V / 6 ohms = 2 amps, using Ohm's law (voltage = current * resistance, in volts, amps, and ohms respectively).
Using the other rule (power = current-squared x resistance, in watts, amps, and ohms), we get
Power in watts = 4 * 2.5 = 10watts
That's sort of lukewarm (you can hold a 10W bulb in your hand quite comfortably). Not hot enough.
If you get a 50Amp 12V supply...you'll get the same 10Watts. The supply voltage is typically regulated -- the supply puts out that many volts into just about any load...UNTIL the current gets too high, when either (a) the voltage drops off, or (b) the power supply melts, or (c) a fuse/breaker goes. So changing the amps of the supply is just not the right way to go. You can hook up a car battery, but it STILL won't get hotter.
What you need to do is either
Get a lower-resistance wire OR
Get a higher-voltage source.
Combining the two formulas gets you something useful:
Power = I^2 R = (V/R)^2 R = V^2 / R.
So if you double the voltage, you get 4 times the power. And 40W starts to sound like it'd make that wire fairly warm. So if you had TWO 12V batteries (in SERIES), you might do OK.
But I think that I might follow some other suggestions and consider going with fishing-wire leader rather than nichrome wire -- almsot certainly lower resistance.
The message from Randy Maheux contains these words:
I have used a 12v DC power supply rated at 7 amps which originally powered a CB radio, works fine but I suppose it all depends on the thickness/resistance of the wire,
NiChrome wire needs a certain amount of initial current to get it past the 'warm-up' phase - for you rlength of wire, you'd probably want somewhere between 18-20 volts run voltage (at load) w/ a 20-25V initial voltage.
Instead of a 'dimmer', which just chops the AC waveform (it's no longer a sine wave, so the transformer can't work efficiently), try a variale autotransformer (Variac is a good brand name). You can find them online or on Ebay readily. I got my 10 AMPO unit for $15, and that was a 'consumer' product - already encased in metal w/ cord, outlet, power switch / light etc. By keeping a sine wave, you'll gain efficiency.
You can use them in two modes:
1) 0-120V - this is standard.
2) 0-140V - this allows a 20 volt 'boost' to provide a little extra power when you need it.
So you'd install a variac in line with your 12V (or 18V) transformer to allow variance in the supply voltage. This effects current flow, therefore watts etc.
Remember the wire in open air will take more energy to maintain its heat. Once it starts cutting through the foam, there's no fresh, cool air to absorb the enrgy, so the wire temperature may rise dramatically if you don't keep the foam moving!
From my own experience, this is a good solution. Use your train transformer, your other small 12V unit, or the dreaded car battery, trade your nichrome in for fishing leader, and it should work perfectly.
The one I built works extremely well, and I didn't make it adjustable. It's about 42 inches of fishing leader, 4 feet of lamp cord, and a car battery. Just make sure your battery isn't going dead.
And if you use a transformer make sure you're not exceeding it's rating. See if Radio Shack has a VOM that'll measure up to 10 amps, buy it, and use it to check the current in your wire -- were it me, and I wanted a fancy setup, I'd wire the current meter in permanently.
But were it me, I'd get a 24V transformer of the correct current rating and use it on the nichrome.
Don't let boards scare you for not building your own power supply. Radio Slack has books on building power supplies as well as the web and your local library. Some of our Flying club members are also into ham radio. See if there is a club in your area and do a cold call and talk with some of its members. These guys are always wanting to help new people out. Also try a trade school in your area that has an electronics department. Again do a cold call, visit the department, introduce yourself. Soldering on boards is not difficult. Do a google search on soldering and you will find sites that take you through the whole aspect. Check hobby electronics websites you will see that that there abundant and all have good sources for your power supply and they will be safe in operation. Batteries can be nasty, heavy and need charging anyway. Doc Ferguson I would have added the websites, but search and find the one that will be the most useful and safe for what you are doing.
Also remember that a variac is NOT an isolation transformer. The hot side of the line from a variac is some significant voltage (up to 140 V depending on where you have the variac set) above earth ground. So if you manage to touch the electrical hot end of the wire you are going to get a nasty shock if you are grounded in any way. All of this is really no problem as long as you are aware that you can not touch the electrically hot end. I and everyone I knew used variacs in the lab for a great many years and I do not recall a single incident.
If you are willing to risk the variac why not buy a simple light dimmer? They are a lot less costly then a variac and have exactly the same small hazard.
You still end up using your 12 or 18V Xformer, too... That provides the necessary isolation. Buying a 'consumer' grade variac gets you the nice gounded case etc.. In fact, some have Volt and Ammeters, too.
Keeping either a true sinewave, or using a very well regulated DC voltage is your best bet. The DC voltage regulating method is a tad cheaper with reasonable components, bu the variac is useful for many other things besides a foam cutter....
Currently, I have one to work to prove out UPS's would not send us proper messages when line voltage fluctuated... what better way to fluctuate it?
I believe all the replies so far are making this more difficult and expensive than it has to be.
Look in your garage (or shop the garage sales) for a battery charger with a capacity of at least 8 amps. This provides around 14 volts isolated DC. Mine is an old NON-solid-state unit. I don't know if the newer solid state types will work.
Plug it into a socket wired to a ceiling fan speed control, or, if you don't feel comfortable messing with house current wiring, purchase a cheap router speed control from Harbor Freight.
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$12.49...on sale right now.
Obtain some .032" stainless steel aircraft safety wire from your local aircraft mechanic, or buy your own through Aircraft Spruce & Specialty
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or other supplier of aircraft tools and supplies. $7.95 for a 1 lb. spool.
The SS safety wire will not be as brittle as nichrome, will not take as much current to heat it up, and is much easier...and cheaper...to obtain...AND in long lengths
I made a light, simple bow from a length of 2-1/2" X 3/4" plywood with two 12" long pieces of 1/4" dia. music wire rods(from the hobby store) on the ends. On each rod, grind a notch around the diameter on the "wire" end, about 1/8" from the end to allow the cutting wire to be looped around it. Drill a hole in the edge of the plywood at each end, so that the rods can be pressed into the wood and will be 2 inches or so "out" from parallel. I used an approximate 15 degree angle from the perpendicular to the length of the plywood. This allows the rods to be bowed-in to keep tension on the cutting wire, and not slip out of the wood. Plan ahead so that the distance between the notched ends of the rods will be at least 1 or 2 inches longer than the longest wing core you want to cut. As you will see, this assembles and disassembles easily, so you can make many different length bows simply by making different length plywood supports, or by drilling extra holes in a long one. Once constructed to your satisfaction, you can make it even lighter by cutting lightening holes in the plywood with a hole saw.
Warning! Use caution when working with safety wire, as the ends are sharp and the wire may snap under tension. Safety goggles should be used! You have been warned!
Twist a loop in one end of your cutting wire and place it over the notch on one of the music wires. At this point you can straighten any kinks that may be in your cutting wire by chucking the loose end in an electric drill and, under tension, spinning the wire along its length. This stiffens (work-hardens) the wire somewhat, and also lessens the tendency for the wire to stretch under tension. Don't spin it so much that it gets brittle, or breaks?.A dozen or so revolutions on a 36" wire should be sufficient! Now spring (bend) the other music wire in about an inch and use this length to twist a loop in the other end of the cutting wire. Slip this on the other music wire and it will hold sufficient tension in the wire for cutting cores.
The power supply leads can be connected to either the rods or on opposite ends of the cutting wire using alligator clips.
With the power supply turned OFF, set the cutting wire on the lead-ins of your airfoil templates, against the leading edge foam. Connect the power supply leads to each end of the bow.
Using just enough force to hold the wire against the foam of the leading edge, start dialing up the variable power supply until you see the wire slowly move into the foam. Use the variable supply as a speed control for the cutting speed.
If you get in a hurry and use too much heat, the wire will cut a curf much wider than the wire diameter, and will cause ripples in the wing core surface.
If you hold the heat constant, but try to force the wire through the core, the center of the wire will lag behind the ends, and you will produce a core that is thinner in the center than at the ends. This will be most pronounced when cutting over the top of a high-camber airfoil.
I have a .pdf covering all this plus diagrams, sources, etc. I'll try to get it posted somewhere.
Pardon my ignorance, but is there a difference between the router speed control and a 120V lamp dimmer - I have one of those and Harbor Freight is 30 miles away!
On Tue, 23 Jan 2007 07:32:30 -0500, I said, "Pick a card, any card" and Randy Maheux instead replied:
A lamp dimmer has a far lower load surge capacity and no tolerance at all for inductive loading. I know the inductive load may be irrelevant to this particular discussion but if you wanted to add an isolation transformer (for safety) it would be very relevant.
Randy, I use my Sears auto battery charger. It gives 10 amps at the charge setting and that seems enough to heat the wire I am using (.012 control line flying wire). To control the temp I use small alligator clips the attach the power cord to the cutting wire and move one in our out on the wire. Move it too far in and the wire becomes a flash bulb! I have used this setup for over 10 years and cut hundreds of wings for control line combat and stunt models. Bob Furr
PS If you are interested I sell a video on cutting foam wings through ebay. The buy it now option is only $9.99 and postage.
Somewhere in the damp dark recesses of my Emporium (the shed) is a 200 foot spool of .035 stainless steel wire which may make a hot wire cutter or two, will dig it out (literally) tomorrow and see if it works,
Maybe... remember as the wire size goes up it takes more power to heat it up to where it will cut foam. I tried .018, .015 and .008 seven strand stainless before settling on the .012. I have another bow made with .024 single strand stainless. I am guessing you will need more than 10 amps at 12 volts to make the .035 wire work. Go to the Brodak website and order a spool of .012 control line flying wire. Look at their website for how to make up the ends of the wire. Bob Furr
Here's my summary of wire & power supply issues for hot wire foa
cutters:
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Comments welcome.
Some highlights:
Most people seem to go for a 2- to 4-amp transformer heating a .016" t .020" stainless wire at roughly two amps and one to a few watts pe inch. 12 volts is good up to 2 feet or so, maybe more with thin wire.
24 volts seems to be preferred, for up to 4 feet or so.
Doorbell transformers can't cut it; they're only rated for about a amp, or less.
Train transformers generally can't really do the amps either. New one with overload protection will shut down, old ones will run beyond thei ratings, but it's iffy
The easiest way to get a variable heat control is to plug a hefty $ wall-wart into a $12.50 router speed control.
Or if you don't mind connecting the wall-side AC bits yourself, you ca wire a regular power transformer to a dimmer. A regular dimmer isn' really designed for inductive loads, but the amps you're drawing ar not much on that side of the transformer. (E.g., a 4-amp 24-vol transformer full draws less than amp on the wall side.)
To get more amps, you can wire two transformers of the same make an model in parallel, but you should know how to check that you've got th polarity right. (See the link above.) This works for wall warts too---to "wire" the wall side, just plug them into the same power stri or multi-outlet extension cord, and wire the secondaries in parallel.
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has good prices on reasonably beef transformers.
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has good prices on fairly beefy wall warts.
You can get 30-foot lengths of 304 stainless steel wire for $1.80 t $2.60 in the size range we're talking about from Small Parts Inc.
Cabela's sells big spools of stainless wire for (salwater fishing line for about $9. (You get two or three hundred feet.)
A lot of people prefer stainless to nichrome because it's physicall stronger, and has about three quarters the resistance. Regular stee wire like guitar strings has much lower resistance, meaning you nee much thinner wire to avoid running too many amps for a reasonabl transformer. (Use a really THIN guitar string if you must use a guita string.)
If you want thin nichrome, maybe for a setup with a marginal (low amps power supply, you can get 30 feet of 26 gauge nichrome (2.67 ohms/ft from
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for $9.
An ammeter is handy for setting heat levels. If you can read the amp reasonably consistently, you can find a "good" current level for given type/diameter of wire, and then dial up to that fo different-length wires.
You can get a multimeter that reads up to 10 amps as an AC ammeter fo about $4 from Harbor Freight. To use it with a transformer on a dimme or router speed control, you must rectify the output to DC. You can ge a rectifier from RadioShack for $3, and the wiring is trivial
What I have knocking around is a 13.8v 7 amp /9 amp surge CB power supply that I use for my mini drills and other gear, somewhere in the shed is a 13.8v 7 amp power supply that I kept as a spare when I used to run a Cobra 148 sidebander CB and a 300 watt linear amp so needed both power supplies to power them. As i will not be building anything with foam wings bigger than about
48'' span I wont need anything bigger than a 30'' cut so should be able to put something together with one of those to power it,
Hi Lee, excellent videos but that lump of concrete slab looks a bit too technical for me....;-), joking apart it supplies all the info you need to get started in foam handling and could be the way I make the more complex wings for the Spit 22 which could be made in two sections also wings like the Corsair and Stuka as built wings tend to get a bit complicated with some of them. I tried a jury-rigged wire out using the .035 stainless wire I have (about 200 feet of it) and 13.8v at 7 amps and it went through high density white foam that you usually find as packing in Video and Hi-Fi boxes at the rate of about 1'' a second so works well, will build a much better system in a couple of weeks time when I have changed over to model aircraft altogether but have a couple of sailing ship models to complete for people first,
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