Foam Wire Cutter - Controlling the Temperature

[Chris King 3]

Hi,

I've built a foam wire cutter and have tried various power sources that I already had. I was controlling the temperature by using a dimmer on the mains side of the power supply.

Even with the dimmer out of the circuit it didnt quite get hot enough to cut well. Was a bit slow which caused the wire to drag.

So I thought I needed more power so bought a 100VA 240-12V Torroidial transformer.

My problem is that my dimmer wont work with inductive loads so I need a different dimmer. Ive see this Variable Voltage Regulator which looks like it would do the job well https://www.ebay.co.uk/itm/393140331113?hash=item5b88fd6269:g:FSgAAOSwQbVe12JC but wondered if it would be better (and more cost effective) to vary the power on the 12V side of the transformer with a 12V AC Speed Controller?

Just wondered what others do.

 

Edited July 10, 2021 by Chris King 3

[kc]

It's many years since I cut foam wings, back then  I never used any temperature control but just varied the speed of movement through the foam to suit the temperature produced by the old transformer I used with a length of 'nichrome wire' unwound from an old electric fire element.   Of course the length of wire actually used can be varied by clipping one connection a little further in along the wire.    That is to say the temperature would be increased if the length actually used was shortened.   No need to alter the bow - just connect a little further in.

Simplest way is often best?

[Chris King 3]

I'm using nichrome wire but it's a pretty beefy transformer and I'm worried it will melt the wire if I use it without some way of controlling the power.

[Brian Sweeting 1]

Considering that they use that type of wire to power furnaces I don't think that you would  melt it without some serious power input.

[RottenRow]

Chris,

What is your original power supply and what is its output voltage without the dimmer in use?

I would be surprised if your old power supply didn’t have a transformer inside  and you say the dimmer worked with that.

If the original power supply was more than 12V then your new transformer may not make much difference. Beware that toroidals take quite a kick of current on power up. Perhaps that’s why your dimmer doesn’t work with it.

You won’t melt the nichrome wire unless you get it glowing almost white, which will be too hot for foam cutting anyway.

In the ‘good old days’ we used to use either a car battery and regulate by adjusting the length as KC says, or an old-fashioned model railway controller.

Brian.

[Martin McIntosh]

I use the same method to control temperature as KC above but I managed to obtain a beefy 24V transformer. 12V is not really enough for a 1m or longer cutter but a 12V battery in series with a battery charger works, or just two 12V batteries in series. I wind the wire round one end of the wooden frame and attach a croc clip to the optimum point.

[Chris King 3]

The original power supply was 12V at 30A but was DC so had some other electronics involved as well/instead of a transformer.

Measured the Volts from the new transformer using the dimmer and is always below 8V even when turned fully up.

The usable length of the wire (where the connectors are) is about 700mm but theres 1M of wire so could play around with changing lengths. Have some different thicknesses too and am on the thinest currently which is 32swg.

[Piers Bowlan]

Chris, I recently wanted to hot wire cut a polystyrene foam wing so I bought one of these 'AC/DC adaptors' and used it with really good results. It is sold as a power supply for laptops and tools and has voltage controllable from 12v-24v. My cutting bow is about a metre long, although the wing panels were only 53cm each. I use Rene 41 wire, .35mm dia. and found that 19v worked well, producing a smooth cut without ridges. Its maximum output is only 5A but sufficient for what I was doing. With thicker wire more current would probably be required but Rene 41 is the 'go to' wire for cutting foam as it does not stretch or break, so a good investment. 

 

I can't help you with using a dimmer with your powerful transformer as I don't know about such things.

[Piers Bowlan]

My recent foam wing cutting efforts!

 

[Stephen Jones]

To control the heat / voltage I have used two methods.

 

1. Use a brushed speed controller and a servo tester to set the temperature.

 

2. Connected my power supply through a Dimmer switch this would reduce the mains voltage feeding the power supply.   

 

Steve.

[FlyinFlynn]

The problem with 'controlling' the AC is that you are going to be feeding that regulated voltage into a transformer. Transformers are designed to be fed with sinusoidal AC and if you go feeding it with a chopped up sine wave it will most likely bite you..... So... controlling the secondary is your best bet. The voltage of your secondary winding is pretty much irrelevant providing you have enough to push the maximum current you will be using through your hotwire, in my experience, for my bows, I needed something over 12V and I bought an 18v secondary transformer at 60VA (so about 3 Amps) so that is really the first bit of info you need - how much current are you going to be needing?  After deciding that you can buy a CURRENT regulator to set the required current to achieve the temperature you want. You can get inexpensive current regulators with a nice volts/amp/watts display like this one.

[adrian garnham]

I use DC power supply function on the following battery charger from hobbyking with my CNC 4 axis foam cutter with great results.

https://hobbyking.com/en_us/turnigy-accucell-c150-ac-dc-10a-150w-touch-button-smart-balance-charger-us-plug.html?queryID=b702e9426399e20e10d7f75d661d8b10&objectID=79273&indexName=hbk_live_products_analytics

 

The find the approx Voltage and amps I need for the length wire I am using I use this online calculator https://www.easycalculation.com/engineering/electrical/nichrome-wire-calculator.php

 

I have attached a short video showing the power supply and setup on a piece of scrap

[Martin McIntosh]

CNC cutters are way above what the average modeller would have available for a one off. A simple set up as I described above is all you need. Any ridges are easily sanded off so you do not need to be too fussy. I have in the past managed to cut 2x 1m tapered wing halves single handed, not easy but they were very accurate.

You can of course send the templates to someone who will cut the cores for you, but that is just not me.

[Piers Bowlan]

I think I am of the same school as Martin; keep it simple (and cheap!). In my youth I connected my simple cutting bow to an old car battery and when I found it rather too hot for the foam, I just connected a car tail light bulb in series. It worked a treat.

[kc]

One further piece of advice is that it's often a 2 person job guiding the hot wire and it might be better to ask a friend to help rather than asking your wife.   It's rather stressful and divorce ( so they tell me ) is very expensive!

 

[Martin Gay]

My iCharger 106B+ has a setting that allows it to act as a power supply for a hot wire foam cutter.

 

Works well with my Tekoa foam cutting bows.

 

Martin.

[Chris King 3]

Some useful (and amusing!) advice, many thanks. I've got a few things to try now. Working it out is half the fun tbh.

[David Hall 9]

I discharge my 3s LiPo's into a bank of car headlamp bulbs at up to 25A DC, controlling the output current by using a brushed 30A ESC.   It works a treat from barely on to full power.   I guess that a similar setup would lend itself well to your application on the DC side.   A brushed (non-sensored)  ESC is pretty much a simple variable pulse width output device.  

 

( I should add that this setup will attempt to provide pulses at full power but of a shorter duration, so not so good for discharging batteries that can't accommodate the full output current.  For smaller batteries the load is reduced, reducing the pulse current.)       

Edited July 20, 2021 by David Hall 9

[Chris King 3]

Just as an update and to let anyone else know who's going through a similar process, I thought I'd just quickly see what the more powerful transformer would do without any way of reducing the power and it barely warms the wire!

It doesnt matter how powerful it is at 12V it still isnt going to get the wire hot.

Based on what Piers said above about a variable voltage supply for laptops etc I dug out an old one I had that goes up to 19V. Works like a dream and you can vary the power very nicely by selecting the different voltages. Perfect.

[Piers Bowlan]

I am glad you found this simple solution satisfactory Chris, welcome to the wonderful world of foam wing cutting! ? It is a great way of quickly producing light and inexpensive wings, especially when covered with brown paper/PVA, which makes them very stiff. Easy to add more reinforcement where necessary with thin ply webs or long carbon strips let into cuts in the foam. 

[Outrunner]

With the price of balsa a bit scary now I'm going to have a go at foam wing cutting too. Question for Piers, your above photo showing a lovely pair of foam wings, are they cut freehand or using a weight and pulleys etc? Where is a good place to buy polystyrene foam these days as blue foam seems a bit tricky to buy now?

 

I already have a laptop PSU, lots of brown paper, several plans that only show root ribs and Rene 41 wire is available on Amazon.

 

Phil

[Andy Stephenson]

I fully recommend Rene wire it's much like Inconel the alloy they make jet turbines out of, it can get red hot without loosing any strength or stretching much. It tends to have a higher resistance than NiChrome wire because it can be used in a much thinner gauge so you will need more voltage to drive it. Being thinner it also makes a cleaner cut with less drag.

[Piers Bowlan]

Yes Phil, they were cut freehand or at least I didn’t bother with weights and pulleys. I placed heavy weights on top of a short plank to spread the load so that the foam wouldn’t move. At either end of the foam blanks were my profile aerofoil sections made of thin ply. These were ‘pinned’ to the foam with sharpened dowels to stop them moving. The aerofoils had lead-in sections so I could rest the wire on them prior to switching on the power. Once ready to cut, switch on, wait a few seconds for the wire to get hot and then drag it across the aerofoil. Just let it cut, applying steady pressure. I made a 90 degree guide for trimming the foam cores to length with my cutting bow. The guide is just aluminium strips screwed 90 degrees onto the ends of a length of conte board.
I bought my 8x4ft sheet of 50mm polystyrene from B & Q - £17ea. I roughly chopped the foam into four 4x2ft pieces with a knife to fit it in the car in the car park! I found a piece without any of the ‘black bits’ in.

I use Rene41, definitely better than the nichrome wire of old.

Not just wings, I thought I would try cutting a fuselage out of polystyrene foam too, suitably reinforced and brown paper covered. ?

[Outrunner]

Piers, thank you for taking the time to reply. Your cores shown above look really good and done without weights and pulleys makes it even more attractive for the amount of wings that I'm likely to cut.

I must get the bits together and make a cutting bow, just need to nip down to B&Q and find some polystyrene.

Have you made a foam fuselage yet? Please share some pictures when you do, it sounds and interesting project.

Phil.

[Chris King 3]

I can cut foam very effectively now but haven't had a huge amount of success with my chosen wing. I'm trying to build a scaled down Scimitar Slo Motion so I can use some batteries and brushless motor and controller I already have. The problem is with the taper of the wing which makes it hard to get the wire moving evenly over the distance of the formers.

I'll either 1) Get someone to help or 2) Go for a plank wing without taper. Happy with my setup though, works really well. I'm using 32swg  Nichrome wire and bought EPO foam from Wickes for about £15 and chopped into two pieces in the car park!

I'm the mean time I got impatient and bought a Crash Test Hobby Pinata (flying wing) from the States. A friend who flys out of the same local field as me has bought one too so we can fly combat for a laugh. They are EPP foam which you cant seem to get in the UK anyhow. Will probably take a careful note of the dimensions and airofoil section so I can reproduce it in EPO in the future though! ?