Nichrome wire gauge for hotwire?

[peterwSUPr]

Hi, I have a couple hotwires from non-SUP projects, but it looks like the width I'm going to be cutting for my board is pushing the limits of my hotwire.  My wire setup currently uses a battery charger with 12V, set to 2A.  I have 2 different types of wire that I got a long time ago.  One is nichrome I think, the other stuff is a mystery.   I see lots of different sizes of nichrome wire on ebay for quite cheap.  Anyone know, with my power setup, what my best wire size might be for EPS (cutting a 27" block)?  I'm guessing 28 or 30 gauge or so, but am open to suggestions.

Thanks,
Peter

[Dwight (DW)]

Www.mcmaster.com is where I buy my nichrome wire. The part number I order is 8880K19

.025"

Nichrome cuts smoother with more even temp

No idea whether your 12v will work. I use variac

[PonoBill]

It would be pretty tough to calculate that. there are formulas for determining the temperature rise, but they're difficult to work backwards, and you probably don't know the optimal temperature for the wire anyway. The easiest way to do what you're after is to use a variable power supply--either a variac or a variable solid state supply that ranges probably from 0-24 volts and at least 5 amps, or something like 24 volts (two car batteries in series) with a heavy duty rheostat to create a voltage divider. The challenge is that the longer your wire is, the more resistance it will have and the less it will heat with your fixed 12V supply. Using a larger gauge will help a little bit since it will have less resistance per foot, but you'll probably bump into the limitations of that 2amp charger. And heavier wire won't cut as smoothly--I use 24 gauge which is about .020"

You can do something funky, like use the wire as the variable controller by connecting one side of your bow or stretched wire to the power supply, and then using an alligator clip to attach the other side of the power supply so you can vary the length. But it will be clumsy and will probably get too hot. Cheap way is probably two 12V batteries in series with a big rheostat.

[peterwSUPr]

Actually, the charger I have has a 2A and 10A setting, and I have used it before for 24" wide, so the extra width is not an issue for cutting, it's just that I think if I start messing with my setup and need a few more inches of wire I have a good risk of things getting broken where the wire was attached before and kinked. 

I just don't have something fine enough to measure the wire that I have so I can get more of the same, that's my only issue.

DW, that .025" wire is 22 gauge, roughly what voltage do you use for a full width board?  When I was a teen, in the 80's my dad could borrow a hotwire and variac from his work and we ran it at about 22 volts I think, but I think the wire I have now is a bit finer.  I also cut a bit slower.   (No, he did not work for a board builder, but he did build a windsurfer in 1977)

BTW, if anyone is trying this and you need to vary the power, I have used a section of the same wire mounted to another frame in an out-of-the-way place, hooked up in series for greater total resistance.  Where there is a will, there is a way!

Thx, Peter

[Dwight (DW)]

This is my variac http://www.frys.com/product/4638758?source=google&gclid=CLLzlfvm2r0CFUYV7AodWU0Agg

It's the one recommended on Swayloks by lots of people. I think I paid a lot more for mine, than this one at Fry's.

I really don't advise doing hotwire work without precise temp control. Every cut requires a different temp. You want to cut slow and cool as possible. That's how you get flawless cuts. I've gotten to the point where I no longer even use a planer. I can hotwire precision rockers, decks, and rail bands. Then I'm just scrubbing the blank out with no more effort than the guys paying for CNC cut blanks.

[PonoBill]

Peter, the current output doesn't much matter (unless you exceed the capability, and then the voltage will droop), if you use a longer wire you will increase the resistance and the temperature will drop. Using another piece of wire and moving the connection point across it is the same as changing clip location--or using a rheostat for that matter. You get a voltage drop across the wire, but you need a higher voltage to begin with so you can set the temperature properly. If it's too low the wire will drag and bow making inaccurate cuts, if it's too high the kerf will be too wide and you make a hard melted surface on the plastic that's hard to sand off.

If you have a 12V lead acid battery kicking around you can use your charger to top it off, then connect the charger in series to get 24 volts (actually, more like 28V), then use either an auxiliary wire with an adjustable connection or a rheostat to get your temperature just right. You can use thicker or thinner wire with not a lot of difference in the cut. The kerf of the cut is much more a matter of how hot the wire is than how thick the wire is.

[55NSup]

http://www.jacobs-online.biz/nichrome/NichromeCalc.html

These guys have all the wire and tables. Even a dummy like me figured it out. Took over 10 hrs of online research to end up at this site.

[PonoBill]

http://www.jacobs-online.biz/nichrome/NichromeCalc.html

These guys have all the wire and tables. Even a dummy like me figured it out. Took over 10 hrs of online research to end up at this site.

Very cool. sometimes searching online is really a matter of just choosing the right phrase--and then recognizing the wheat among the chaff. The complex information I tried to use to calculate wire gauge and supply voltage is the second listing--a pdf. The calculator you found is the second from the bottom. I never looked at it until your post. I have no idea how or why I missed that.

[peterwSUPr]

Nice calculator, thanks.  I hadn't found that one.  It says above the calculator that 600F is the normal foam cutting temperature, and that suggests that I need 26 gauge, rather than 28 to 30 as I thought.  Then again, I'm not sure what a 600F cutting heat feels like.  I would think the 6'6" surfboard blank I cut last year was easily over 5 minutes to cut the length of the board.  How long are you guys taking to cut the rocker the length of a board?  (and how long a board?) 

It's weird to ask this question having been cutting for so long, but I've never had his discussion before with anyone and am not surrounded by others doing the same thing. 

Good deal on the variac, although exchange rate, broker and cross-border shipping would ramp the price up for me.  But, having used 12vDC numerous times before I know I can stick with that, even if it might not be the ideal solution for someone just tooling up for the first time.

Thx,
Peter

[PonoBill]

That is WAY too long. the wire should move through the foam steadily and smoothly with minimal bowing, at the speed of any smooth, precise tracing movement. For a 6' blank probably 30 seconds or less. I've watched Bill Foot cutting blanks from a huge block of EPS with a two-person rig (wooden handles on a wire) instead of a bow and he was doing each 12' X 36" cut in less than 30 seconds. The surface of the cut area was almost identical to the outside of the block.

[peterwSUPr]

Maybe my time guess was a bit high, I wasn't timing, but 30 seconds sounds really fast.  When I've done that I'm only pulling quite gently.   

I guess with a perfect smooth template and lots of shop space that works.  But when I had access to a variac we found that the faster the cut the more potential there was for a small wobble by hand to show up in the quality of the cut.  Having a big enough area for lots of movement on both sides of the blank is not always a luxury I've had.  Plus if the wire caught on a splinter or imperfection on the template that was not seen the wire would hang up and burn in a bit.  If the template was built for production it might have a more perfect surface I suppose.   

I assume there must also be something used to take up slack in the wire too, since the hotter wire seemed to sag more.  Also with some boards we had to cross glue joints too, as I will have to with this board.    I should add that the surface of the cut does come out quite good even at the low speed.  For anyone who has to cross a glue join when hotwiring, (which I don't recommend) here is a tip we learned the hard way about 30 years ago.  If you are cutting at a shallow angle the middle of the wire will ride along the glue join, so make sure that any cuts done like that will leave extra foam on the blank for shaving off after, rather than digging in deeper than planned.

Thanks for all the help.  Sounds like I am not doing this like some folks out there, but I think I have got enough info from this discussion to help me go from here.

Peter

[PonoBill]

I've had the problem with templates being too rough--you need a smooth template that doesn't get affected by the hot wire. Masonite works well, and then I sand the edges round and smooth. The first use sometimes reveals a soft spot that the wire digs into. I patch those with a little epoxy with masonite dust mixed in. I've experimented with a lot of different purchased tools, but the one I like best is a four foot bow I welded up from scrap aluminum. It's light enough to one-hand but stiffer than the commercial tube bow I bought, and I can set the bow depth from an inch to about a foot with extension arms, and even offset one arm from the other. If I remember I'll shoot a picture of it when I get back to Hood River in a month.

I've been playing around with an idea of making a computer-controlled cutting system that would work with both a mill (a router really) or a hotwire. One of those projects that will probably never happen, though I've gotten to the pencil drawing stage and I've figured out how the various CAM file formats work.

[Dwight (DW)]

Plus if the wire caught on a splinter or imperfection on the template that was not seen the wire would hang up and burn in a bit. 

Been there done that a few times. Patched up like Bill does it too. But those were the days when I cut with stainless steel wire.

Nichrome holds a more even temp. It cuts much smoother too. I cut cool enough, that I can pause for several seconds and not burn the Masonite. Cutting slower and cooler, gives me higher quality finished cuts. Flawless actually.

[55NSup]

My 16-5 x 70cm  took about 10 min. Iabout 2 min per meter. Is that too slow?

 thought my wire was too cool. Wire bowed, but cut was pretty good.

[Dwight (DW)]

..... but cut was pretty good.

That's all that maters.

Wire always bows.

But, there is one web site claiming no stretch when using inconel wire. I'd love to try it. Pricy!