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My DIY hot wire cutter

I first needed a hot wire cutter when building my Carbon Fiber String Dobsonian Telescope. It was a really rudimentary device, constructed in about 10 minutes, of scraps that were on hand. But it worked just fine for its purpose, which was to cut EPS ("Finnfoam") cores to be laminated with carbon fiber.

I lost that awful device when moving (probably just threw it away), and I whipped up another similar device for some one-off project, before finally deciding I ought to have a properly adjustable wire also capable of cutting circular parts (to use as bulkheads in an amateur rocket project which is still ongoing).

The power supply: I used the same power supply through all iterations of the hot wire cutter. A simple toroidal transformer that outputs 24 VAC with certainly more than enough current capacity (I don't actually remember what I got this transformer for, originally) fed by a Lübcke Vario R52-260-T Variac which I've had in my electronics lab for quite a while, since rescuing it from a dumpster at my old university.
This is by no means a sensible way to power a hot wire cutter, but it was immediately available, and works just fine. I usually set the variac to about 30 V output (giving just over 1 V from the following transformer), but that will depend on the thickness of the wire in the cutter. Mine is probably on the thick side, as far as hot wires go. Some kind of low-voltage high-current SMPS should not be too difficult to design, if you don't have a variac on hand. Or I wonder if an old-school light dimmer might work in place of the variac.

The first generation cutter: This was made of particle board scraps and a clothes pin, clued together! Cobbled together in 10 minutes or so. But the wire was some "actual" resistance wire, for whatever that's worth... Rather thick, though, so it required some serious current at low voltage. Of course, any uninsulated wire would do, regardless whether it's Nichrome or Kanthal or plain old iron or steel (well, maybe not copper, though).
The wire is contacted electrically by alligator clips, placed between the physical connection points, so as not to needlessly heat those up—the plastic clothes pin, used to keep tension on the wire, would soon melt otherwise!
You can see the jaw is not terribly big, and there's precisely nothing at all to adjust. I mostly used it for cutting freehand, or clamped some simple guide rail onto this thing if I needed uniform, straight pieces of foam core.

The second generation cutter: Having misplaced the first cutter, and finding myself needing one again, I jury rigged this one in about the same time as before, again of materials immediately on hand. This one has a slightly larger jaw, but in most other respects it's only marginally less awful than the original. Still sporting the clothes pin and alligator clips.
I did connect the lower end of the wire to a moveable block underneath the base board. With the sizeable hole in the base board, this allowed me to fine tune the wire to make it actually vertical with some accuracy. Nothing holds that lower block in place, though (other than the tension on the wire, and friction against the base board), so it's also prone to losing its fine adjusted vertical setting.

The third generation cutter: I found myself needing rings cut out of EPS ("Finnfoam"), so I finally went to the trouble of actually designing a hot wire cutter for that purpose. (The same cutter will, of course, work just as well for freehand and straight pieces as the earlier two generations.)
I think the photo (and the following photos) says it all. There are two lengths of wood with a gap between them. An axle piece with four nails sticking up from it is pushed into the EPS sheet, and it provides a pivot point around which the sheet can be turned through the wire to make accurate circles (see this photo). That axle can move back and forth along the gap between the wooden pieces, and adjustable stoppers in the groove are used to set the radius of the circle. This way I can heat up the wire, push the raw EPS sheet into the wire until I hit the stopper, turn the circle (as in this photo), and pull it out again—this leaves no unsightly grooves at the starting point of the circular cut. I can also start from a pre-drilled hole on the inside of the disc, pull the raw sheet backwards to the pre-set radius, turn the inner circle, and push out again, to obtain a ring of EPS. (Of course this does require disconnecting the wire and threading it through that pre-drilled hole, and disconnecting it again after cutting.)
The jaw is bigger than in the earlier versions, and comprises a vertical block of wood, on which a horizontal beam is hinged, and tensioned by an adjustable compression spring, as you can see in this photo. The hinge is nothing more than two round-headed screws protruding from the vertical block, sitting in holes ground into the horizontal beam. The single screw and washer you can see in the photo only keeps the beam from falling off when the wire is unhooked.
The lower end of the wire is held by a moveable block (as in the second generation cutter), and by changing its position, you can even cut conical parts as in this photo. (If the angle is off 90° by very much, that block will try to slide back towards the 90° position, unless held by some makeshift cord arrangement—that orange cord you see in the photo—adjusted by adding or removing blocks of wood behind it. Not all too convenient, but workable.)
And yes, the power supply is still connected with alligator clips.

So, yeah, as much as modellers the world over love the Proxxon THERMOCUT hot wire cutters, I think this device will serve me for the immediate future quite well, thank you very much. And when I do finally find it lacking in some way, I'll just go ahead and jury rig a fourth generation version of it. Maybe that one will have something more sophisticated than alligator clips, and easier adjustments on the necessary axes. If you're looking to make a hot wire cutter, feel free to steal some ideas from me! :)

Don't burn your fingers!

Antti J. Niskanen <uuki@iki.fi>

	The power supply: I used the same power supply through all iterations of the hot wire cutter. A simple toroidal transformer that outputs 24 VAC with certainly more than enough current capacity (I don't actually remember what I got this transformer for, originally) fed by a Lübcke Vario R52-260-T Variac which I've had in my electronics lab for quite a while, since rescuing it from a dumpster at my old university. This is by no means a sensible way to power a hot wire cutter, but it was immediately available, and works just fine. I usually set the variac to about 30 V output (giving just over 1 V from the following transformer), but that will depend on the thickness of the wire in the cutter. Mine is probably on the thick side, as far as hot wires go. Some kind of low-voltage high-current SMPS should not be too difficult to design, if you don't have a variac on hand. Or I wonder if an old-school light dimmer might work in place of the variac.
	The first generation cutter: This was made of particle board scraps and a clothes pin, clued together! Cobbled together in 10 minutes or so. But the wire was some "actual" resistance wire, for whatever that's worth... Rather thick, though, so it required some serious current at low voltage. Of course, any uninsulated wire would do, regardless whether it's Nichrome or Kanthal or plain old iron or steel (well, maybe not copper, though). The wire is contacted electrically by alligator clips, placed between the physical connection points, so as not to needlessly heat those up—the plastic clothes pin, used to keep tension on the wire, would soon melt otherwise! You can see the jaw is not terribly big, and there's precisely nothing at all to adjust. I mostly used it for cutting freehand, or clamped some simple guide rail onto this thing if I needed uniform, straight pieces of foam core.
	The second generation cutter: Having misplaced the first cutter, and finding myself needing one again, I jury rigged this one in about the same time as before, again of materials immediately on hand. This one has a slightly larger jaw, but in most other respects it's only marginally less awful than the original. Still sporting the clothes pin and alligator clips. I did connect the lower end of the wire to a moveable block underneath the base board. With the sizeable hole in the base board, this allowed me to fine tune the wire to make it actually vertical with some accuracy. Nothing holds that lower block in place, though (other than the tension on the wire, and friction against the base board), so it's also prone to losing its fine adjusted vertical setting.
	The third generation cutter: I found myself needing rings cut out of EPS ("Finnfoam"), so I finally went to the trouble of actually designing a hot wire cutter for that purpose. (The same cutter will, of course, work just as well for freehand and straight pieces as the earlier two generations.) I think the photo (and the following photos) says it all. There are two lengths of wood with a gap between them. An axle piece with four nails sticking up from it is pushed into the EPS sheet, and it provides a pivot point around which the sheet can be turned through the wire to make accurate circles (see this photo). That axle can move back and forth along the gap between the wooden pieces, and adjustable stoppers in the groove are used to set the radius of the circle. This way I can heat up the wire, push the raw EPS sheet into the wire until I hit the stopper, turn the circle (as in this photo), and pull it out again—this leaves no unsightly grooves at the starting point of the circular cut. I can also start from a pre-drilled hole on the inside of the disc, pull the raw sheet backwards to the pre-set radius, turn the inner circle, and push out again, to obtain a ring of EPS. (Of course this does require disconnecting the wire and threading it through that pre-drilled hole, and disconnecting it again after cutting.) The jaw is bigger than in the earlier versions, and comprises a vertical block of wood, on which a horizontal beam is hinged, and tensioned by an adjustable compression spring, as you can see in this photo. The hinge is nothing more than two round-headed screws protruding from the vertical block, sitting in holes ground into the horizontal beam. The single screw and washer you can see in the photo only keeps the beam from falling off when the wire is unhooked. The lower end of the wire is held by a moveable block (as in the second generation cutter), and by changing its position, you can even cut conical parts as in this photo. (If the angle is off 90° by very much, that block will try to slide back towards the 90° position, unless held by some makeshift cord arrangement—that orange cord you see in the photo—adjusted by adding or removing blocks of wood behind it. Not all too convenient, but workable.) And yes, the power supply is still connected with alligator clips.