could anyone help me out and tell me what kind of wire they use in their jotwire. i’ve tried just about everything and they all break or burn through after about 30 seconds,
nichrome wire, 26 gauge I think. $5 for like 40ft on ebay.
guitar string, non-wound.
if nichrome and superfat unwound guitar strings won’t last longer than 30 seconds, sounds to me like you’re running too much current. get a variac.
Exactly- a variac would be ideal or mebbe a rheostat in the circuit to cut down the current.
Some tables here: http://www.wiretron.com/nicrdat.html
Design formulas to use here: http://www.wiretron.com/nicrdat.html
If you don’t know the gauge wire you’re using ( and I didn’t, wasn’t marked on the spool that was given to me ) , a machinist’s caliper can be very handy. Just found out that mine was 23 gauge ( AWG) .
Now, to check my little setup, which is 15" between terminals and has a 3 amp 6.3 volt DC doorbell transformer for power;
1.25 feet x 1.35 ( close enough, as I dunno if it’s Nichrome A or Nichrome C, plus the resistance of the wires and terminals from transformer to nichrome wire ) ohms per foot gives me a total resistance of about 1.7 ohms.
Using the current vs temperature tables mentioned above and multiplying the overall amperage by 1.25 ( it’s not specified, but I believe those tables are per foot )
For 23AWG the tables read
400°F- 2.58 watts/ft
600°F- 3.3 watts/ft
multiply by 1.25 ft, 'cos that’s how long the wire is;
For 23AWG
400°F- 3.23 watts
600°F- 4.13 watts
So with my little transformer I am probably not getting a whole lot more than 350 degrees F, which seems reasonable considering how it goes through foam; sloooowly.
For the transformer setup I am using 25AWG wire would be a better choice, giving me temps of around 500°F.
A couple of design hints:
If you tension the wire a little, that’s good, but if you tension it too much it’ll neck down at some spot, which decreases the wire diameter and increases the temp…a lot, whereopon it overheats, gets even skinnier and breaks.
Shoot for a gentle tension, just enough to keep the wire from curving as you cut through foam.
What temperature to use or shoot for? Well, one article suggests (http://en.wikipedia.org/wiki/Hot-wire_foam_cutter ) 200°C, so that you’re vaporising the foam before it hits the wire but not making vast craters along your cut if your cutting speed changes a lot.
Too cold ( less than about 300°F ) and you get a crust of polystyrene solid on your wire, which isn’t real helpful. Feeding the cutter through the foam slowly with my ~350°F cutter seems to work okay, call it no more than an inch every couple seconds. Too fast and it drags and forms that nasty ol’ crust.
hope that’s of use
doc…
wow man, i was just looking for a few pointers but you just gave me a lifes worth of research and developement on hot wire cutters. amazing!!! i really appreciate it.
surf’s up!
shaper-d
Naw, not at all. Actually, I should be thanking you for giving me a good excuse to think through my setup, what it is and what it oughtta be doing…and maybe why it isn’t doing that.
Best regards
doc…