So we've heard of hot wire cutting, but what about tiny hot wire cutting? - RC Groups
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Jun 25, 2012, 02:13 PM
Euphoric flights
GreenAce92_v2's Avatar

So we've heard of hot wire cutting, but what about tiny hot wire cutting?

I wanted to know if any of you out there knew an effective and efficient if not cost effective way to create small shapes of wire which could be used to cut say curved aileron bending surfaces and other weird facets / shapes.

How would you control the temp and also hold the cutter in an ingenious way that would offer "machine-like" cuts.

I have seen people use some sort of "dial" which acts as a voltage regulator, it seemed a bit expensive.

Currently I use a bow to cut cores that is 30" long and uses a DC power supply at 6A 12.6V, I think my temp is a little low as my wings come out slightly stringy which I sand but it seems counter-productive.

I was wondering if I could just use an attachment to control the temp of what I already have as a power source and not mess around with a bridge rectifier or a transformer etc... I have worked with these but trying to avoid it.

Suggestions are welcome.
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Jun 25, 2012, 03:00 PM
An itch?. Scratch build.
eflightray's Avatar
Not quite sure what you mean by, "curved aileron bending surfaces.....".

For cut-outs, slots etc for shapes like servos, then I use a solder-gun with a shaped copper wire element. (See picture).

The hot wire bow obviously needs tensioning so can only cut as a straight edge, (unless you move the bow too fast, then you can cut a nice curve, in the wrong place ).

Some people have made small vertical hot wire cutters, set up like a 'hot wire' fretsaw, or bandsaw, mainly for cutting out shapes in sheets or blocks.

When cost isn't a problem, then some form of CNC cutter is probably what you want.
Jun 26, 2012, 02:19 PM
B for Bruce
BMatthews's Avatar
Originally Posted by GreenAce92_v2
....I was wondering if I could just use an attachment to control the temp of what I already have as a power source and not mess around with a bridge rectifier or a transformer etc... I have worked with these but trying to avoid it.

Suggestions are welcome.
The simplest option for what you want is a high current rheostat. Given that you're doing OK with a 6 amp 12 volt supply it suggests that the wire is roughly 12volts/6amps = 2 ohms in resistance. Given this you will find that you only need roughly a 1 or 2 ohm rheostat to control the temperature.

The fairly high current suggests that you want a 35 watt capable version. These are going to be around 2 or 3 inches in diameter and made from ceramic for the core of the wire loop. The resistive element will likely be a flat band of nichrome metal and the wiper will have a carbon brush on the end. You need the higher power because wattage is given by I^2 x R. So if you're still drawing 4 amps at full resistance then you're looking at 16 watts of heat. And that's enough to produce a burn if you're running a lighter duty 12 watt model at this sort of power.

This page has pictures and lists a bunch of them but you're not going to like the prices.

Hmmmm.... the 2.5 ohm 25 watt model for $15 would likely do what you want. It may turn out to be a little coarse but it would give you more control than you have now.

If you want "cheap" another option to try is take apart a hair dryer and use the heat resistant form to wind a coil using the same wire as you use for cutting. Then make up a set of standoffs for the coil to sit up where it can't touch anything that will burn and use a stout alligator clip to simply clip on at some point on the coil. Call it a poor man's rheostat. You should be able to make one up for less than $10 worth for an old second hand hair dryer from a thrift store along with a good quality alligator clip.
Jun 27, 2012, 02:41 PM
God Created me to Create
The_Builder's Avatar
I am a certified red-neck engineer and I have some suggestions. Try a household dimmer switch. I am on my second one in 6 years of production cutting. For small shapes and such use a heavier wire, I use stainless steel fishing leader, and bend it into the shape you want, mount it to a little fixture and cut away.

I have one that is basically a very large staple that I use for cutting servo bays. The legs pass through a block of wood where they get pinched to hold the depth. I clip my wires to the legs and cut away. Since it will not cut the skin or masking tape I simply outline the pocket in tape and I get very exact cuts. Some where here on RCG there is a pic of my little tool.

If you were for example trying to radius the leading edge of an aileron the wire would be a crescent shape. Your block of wood would ride on the top or bottom of the aileron and you would get a very nice cut. Of course you would want to clamp or tape a straight edge to the aileron for the block to ride along.

Maybe this helps?....Paul
Jun 27, 2012, 06:50 PM
B for Bruce
BMatthews's Avatar
Paul, I hope you're suggesting to use the dimmer along with the power supply.

Using a dimmer by itself to directly power the wire creates one helluva shock hazard.

Assuming you intended this to be used along with the power supply then I agree that it is an option. But since dimmers don't like inductive loads, such as power transformers, you need to get one of the high power 500 or 600 watt models so they don't overheat.

I've done this with a 500 watt model to work as a speed control for a router and disc grinder used as a sander polisher. Again a use which the dimmer complains about terribly. But by using the high wattage dimmer it works just fine.
Jun 28, 2012, 01:23 AM
God Created me to Create
The_Builder's Avatar
Bruce, Now you got me thinking !!! I never thought of using a router speed control to set the temp on my hotwire. Yah I use the dimmer to lower the voltage going into a 12dc transformer.

Jul 01, 2012, 11:57 PM
Euphoric flights
GreenAce92_v2's Avatar
Wow thanks for the responses allow me to catch up

To clarify here are some images

btw the soldering gun attachment idea is so ingenious! It is so simple yet perfect, I have a soldering gun like that too! Thanks for this. The shapes are variable right?

Ahh... this is really nice. Curious if you can cut deep channels within the wings (depicted below).
Jul 02, 2012, 12:09 AM
Euphoric flights
GreenAce92_v2's Avatar
BMatthews >>>

My wings come out stringy, what does that mean? I am using polystyrene foam (the pink model).

In my physics labs in college we had these long tubular variable resistors with a slider on top, the resistor had three terminals, one on each end of the tube and one on top. How much do you think these would run? Cheaper or more expensive than the rheostats?

Thank you for the concise detail.

The Builder >>>

What you have described is exactly what I am after. If I use the stainless steel fishing leader at 6A 12.6V, you think that I would be alright?

Yeah currently I only need to cut ailerons, the channels for square spars as well as servo bays so the tape suggestion is excellent. The more professional the better eh?

Thank you guys again very much.
Jul 03, 2012, 12:31 AM
low tech high tech
vtdiy's Avatar
I wrote the a spreadsheet wire cutting calculator exactly for this kind of thing. It's here in the builder section.

You can power a wire profile cutter with a variety of power sources, even including D cell batteries, but you need to know what thickness and type of wire to make that work. That's the point of the calculator. To match wire type, diameter and length with a particular power supply, or vice versa.
Jul 03, 2012, 03:55 PM
Euphoric flights
GreenAce92_v2's Avatar
Ahhh Gratsi Gratsi, that sounds excellent and quite useful.

Thank you for your share of knowledge.
Jul 05, 2012, 05:22 PM
Euphoric flights
GreenAce92_v2's Avatar
So I tried the soldering gun method. Perhaps the metal I chose was not a very good conductor or maybe too thick?

I used a live wire piece from a coaxial tv wire (use these for building my CP antennas)

It was hot enough to cut but was not hot enough to cut fast and without dragging marks.

So... I guess rheostat would be the best option, otherwise I'm going to try a thinner wire.
Jul 05, 2012, 06:12 PM
B for Bruce
BMatthews's Avatar
GreenAce, from the short time I played with foam I was never able to cut the blue or pink without some "hairs" being left behind. But I found that these were easily removed with a few light swipes from some medium sandpaper. Another option would be a coarse Scotchbrite pad

The tubular variable resistors that you describe will work fine. The only thing is that you need a screwdriver to adjust them and you may need to wait for them to cool a little before making an adjustment. If you can arrange a spring arm with a goon connection to the wiping surface such a setup would simulate a regular rheostate nicely. Cost varies so much on this stuff that there's simply no point in saying one is cheaper than the other.

A trick for you to try is to take some 14 guage copper wire and using a vise and hand drill twist a bunch of turns into the wire. This will straighten it and work harden it at the same time. The work hardening raises the resistance of the wire. With this you can bend a shape and clamp it into a soldering gun and it will get warm enough to cut well through foam. Note that you MUST raise the resistance or the wire won't get hot enough. Instead all the heat becomes lost in the transformer in the soldering gun itself. This is likely why your coax core wire isn't working that well. It's simply too good a conductor at this point.
Jul 05, 2012, 06:20 PM
Euphoric flights
GreenAce92_v2's Avatar
Oh wow... I thought all I needed was more metal to "keep" the heat.

Thank you for this! More resistance...

I didn't think that it was using the "shorting" method to heat the wire, I thought it was already shorted at the stubs, while the wire simply kept the heat.
Jul 05, 2012, 06:28 PM
B for Bruce
BMatthews's Avatar
The whole loop of wire is the "short". You don't join the stubs off the gun with anything BUT the loop. If you used a short bit between the stubs then THAT was the part that got hot.

At least that seems to be what your last post suggests you did.
Jul 05, 2012, 07:05 PM
Euphoric flights
GreenAce92_v2's Avatar
Yes I did use a loop, however what you reasoned with having too low of a resistance makes sense, after all antennas shouldn't have high resistance right? For the accelerating charge to initialize oscillating electromagnetic waves? That's my understanding anyhow.

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