Hotwire

Anyone got some hot wire tips? I bought some nichrome from Dick smiths today. Its 0.32mm, gives 13 ohms per meter. I am going to put it on a 12V battery.

What current is ideal before I start experimenting?

Hilly,
I am an electrican. I just finished an advanced dip in electrical enginering. Amps an volts are not my issue, I am looking for some one who has some practical experience with nichrome and ot wire of EPS. Thanks for the laugh but I am not going to hook myself up. Happy new year in WA when if finally gets there.....

Gday Scotty Mac,

OK, this is what I've done. Started off reading a whole lot of stuff on the net ranging from car battery to full on electronic control with feedback.

Ended up making a board with three old halogen light transformers in parallel, I can switch them through the power adaptor that has switched outputs so that I can have one two or all three running at once. This is a bit over the top but didn't know how much grunt I'd need. I made it all up on a cheapo chopping board, just countersunk all the mounting screws from the underneath.

Vary the 240 volts with a high current dimmer - neat thing about this is that pressing the dimmer knob in turns the thing on and off, you can do it with your foot. The thing I've found about wire cutting is that you need more than two hands, great to have a mate with you. I also ran an LED so that I could see if it was on or off - you can just see it on the dimmer plate.

Bnaccas made the plywood wire cutter frame, you can change the angle of the wire in relation to the flat reference timber - works a treat.

Started off with stainless steel fishing leader wire, pretty sure it was 0.5mm, the big fishing joints have a full range and it's good quality single strand wire. Trouble was that the wire flexes more when it heats up and it's a pretty fine balance to get it hot without it getting red hot and going all over the place. Finally ended up cutting some wire off a roll of stainless 0.9mm mig welding wire - works really well because it doesn't expand too much, you just need the grunt to run it.

I've seen the wire carrier Dean made for cutting out blanks - maybe he will post a photo. It's simple and effective, pretty much the way Dean does most things IMO!

If you are stuck for wire I'll cut off a bit from the roll and send it to you, would seem pretty silly to buy a whole roll.



This is how it works! Bnaccas and I cut this shape with the hot wire. We cut three different angles in the front middle and tail of the board and just blended them in - super quick



This is what you end up with instead of mountains of dust

Hi JonathanC,

Thanks for posting up that info.

Any reason why you have used 3 transformers instead of just 1? How do you have them wired up?

PS: Nice re-purposing of the cutting board...

The posts are speaker cable terminals - you unscrew the knob then pass the wire through a cross hole then re-tighten the knob. They are a bit crappy but work OK. We started with a spring to tension the wire but put the crocodile clip onto the start of the spring instead of the wire by mistake, instant super stretched red hot spring DOH!

Because there is some flex in the plywood with a little pre-tension it acts like a spring and the 0.9 wire doesn't expand enough to be a problem. On a set-up with a longer wire you would need a spring for sure to keep tension in the wire when it's hot. I really like the stiffer wire because even if its a little cool it doesn't get pulled out of shape by the foam. I reckon it will last forever too. Hopefully Dean will post a photo of his set-up - if he doesn't see this I'll do a sketch to give you the idea if anyone wants it.

The three transformers were used to get enough grunt to heat a longer piece of the 0.9 wire if we start cutting full blocks instead of just rails. I did some sums to work out the current we would need and came up with 10 amps so used three to give me a total of about 12 amps at 12v. To buy a 12 or 15 amp transformer is very expensive - I had these lighting ones already so it was a no brainer. I think the old heavy iron core halogen lighting transformers are a little more tolerant to the kind of abuse this process involves than the lightweight switch-mode transformers, but this is just a guess - I'd try whatever you have, worst case you cook it and have to chuck out the tranny.

Because I can switch the transformers individually that is a coarse power adjustment and the dimmer gives fine adjustment. Because I had over 10 amps I went for a 15 amp dimmer which was pretty expensive (can't remember $70?), you really need to be able to vary the power, if it gets too hot it melts big holes in the foam and you have to move too fast for comfort! When you get it right you can easily follow a line drawn on the foam, that's how we did that displacement nose in the photo. A property of the stainless wire is that when it gets hot the resistance changes so then it gets even hotter! With the thin wire it can start off OK then suddenly get hotter and hotter, that's one of the reasons it helps to have two people, if you are in the middle of a long run you can get the other person to crank it up or down as you go. I think the nichrome wire is different - the resistance stays pretty much the same so you don't get this feedback effect, its the stuff inside you old kettle or the toaster. But most of the readily available nichrome is very fine.

The fishing leader wire (0.5mm) only needed one transformer running from memory, but don't forget we have a set-up with relatively short hot wire, the longer the wire the more power you need.

The outputs of the transformers (low voltage) are wired in parallel, to explain imagine there is a + and a - output from each transformer, you would connect all the +ve cables together and that would go to the red crocodile clip, then you would connect all the -ve cables together and that would go to the black croc clip (there isn't really positive and negative because it is AC not DC like a battery).

If anyone wants the whole circuit drawn out I'll do that for you. It's a very fast and clean way to cut the foam and you can cut rail shapes with a few passes along different lines drawn on the top of the blank with the wire set on different angles, hard to explain but the cuts are just as you would do with the a plane. There is a heap of info about it on Swaylocks if you have the time and energy to plow through it all.

Newcastle SUP is dead right about practising on foam scraps to start with, if it gets too hot it can all go wrong real fast!

Thanks mate - makes sense.

It goes without saying if you are not 100% confident that you know how this works then please seek the help of someone who does.

Either way both 240v and hot wires are very dangerous beasts so take care and happy shaping! In that order of course...

Thanks all for the info everyone, much appreciated.
Just happens I changed out a heap of downlight transformers at home with new electronic ones so I have a box full of spares.
0.9mm seems a big wire, NC surfer, what size wire are you using? I am cutting 40cm wide foam for full length rocker?

NC, 0.55mm sounds good, not too thick. Do you know what current and voltage you are applying?

I don't know. I use the Variac everyone on Swayloks recommends and uses. It's the one seen in Greg Leohr's shaping video. It's made in China and probably available world wide.

Mine is 0-130 V 500VA and I run it near the lower 1/4 of the dial. So it's more power than needed. It's very easy to adjust and get right.

http://www.electroniccity.com/shopping/pricelist.asp?prid=1147 It looks like this one, only it's 500VA instead of 300VA

I make 30" wide cuts with it.

Cheers Fellas. Much appreciated.