Hot wire foam cutter

A power supply, bow and table for cutting foam with a hot wire

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I want a hot wire cutter, mostly for cutting wings and fuselages for RC airplanes, but it will be useful for other projects as well.

The power supply will be unregulated, variable, and suitable for wide range of wire sizes and lengths. It needs a current and voltage display and adjustment. It can output up to 140W, with up to 22V and up to 20A.

The bow is needed for very thick cuts. I mostly need this for cutting endwise into foam sheets to cut wings. This needs to be capable of cutting 3 feet, so the bow will probably be about 42 inches.

The table is the more traditional format, with the wire coming straight out of the table for making perpendicular cuts in foam up to several inches thick.

The supply can also be used with a rigid wire cutter. That is, a thicker wire bent into a cutting profile and mounted on a wand for carving.

A hot wire foam cutter is a way to cut through foam.  It uses a resistive wire with current running through it to heat up above the melting point of the foam.  The wire can then be slowly pulled through the foam to make cuts.

The wire most often used is nichrome, a common wire used in heating.  This can be ordered, or pulled out of a forced air electric heating element, such as a hair dryer or some space heaters(look for the coils of wire as some heaters embed this into ceramic for safety).  Others have used thin braided stainless steel wire, which they say is stronger.  This is available as fishing line leader, making it perhaps more available in retail stores.  The most available is steel wire, which can be found at the hardware store.  I've used this in the past, though it seems to degrade pretty quickly.

The power supply needs depend on the use case.  A simple table type cutter is simplest as the voltage and current needs are moderate.  To support long cutting wires as in a bow, somewhat higher voltages are needed.  To support rigid wire cutters, low voltage but higher current is needed.  The supply developed here is to be capable of all these.  A bench power supply can be used directly for this, with the added benefit of constant current, though I don't have a more portable one.  A 3 or more cell hobby lipo along with a brushed motor speed control and servo tester is another simple and portable option.

Logs with more information:

  • 1 × Supply: Case and handle
  • 1 × Supply: AC inlet/cable
  • 1 × Supply: AC input breaker/fuse
  • 1 × Supply: Variac
  • 1 × Supply: Main transformer

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  • Handheld cutter

    Quinn04/04/2021 at 18:56 0 comments

    I also wanted to test out using a handheld cutter.  As a first test, I wanted to cut a slot into the back of the wings to insert a carbon fiber strip.

    As a general tool, I used a strip of scrap bamboo wood.  I pre-drilled and attached several screws to use as attachment points.  The wire can be sandwiched between the screw and the wood, and alligator clips on the other side.

    For this first one, I needed a cutter that would cut down 1/4", and 1mm wide.  The wire I'm using is 0.7mm, so pretty well sized already.  I created a loop that would hang off the wood to slide down a channel.  Because the wire is so short, I added an extra loop above just to bleed off a bit more voltage.  Even with this, it only takes 2.3V to get to 4.5A.  In the second picture you can see the extra loop, and how the cutting part shows a very thin profile when lined up right.

    In trials, I found that the wider loop would create too much heat concentration at the bottom, resulting in a wider slot than desired.  I reshaped the loop to be more pointy.

    To make the cut perfectly straight and narrow, I created a guide.  I found 2 very straight 1x2's, and clamped them together with some scrap of the carbon fiber as spacers.  This set the space between them at precisely the 1mm I needed. 

    To make the cut, I measured out where I wanted it on the bottom of the wing, and placed the guide over it.  I used a couple dead SLA batteries as weights to hold it in place.  To support the wing so it didn't get crushed, I set it in the top of the foam block it was cut out of.  Saving these pieces is great support for when working on the wings.

    The test cuts showed the issue with the wider cutting at the bottom, followed by a better one when I made it more pointed. 

    The setup for cutting the slot in the wing:

    The result:

    With the CF strip inserted:

    With the guide, this worked way better and easier than expected, which was fantastic.

  • Testing

    Quinn04/04/2021 at 18:34 0 comments

    For a first project I wanted to cut out some wing profiles for a DLG.  For this initial project, I only wanted 20" span, so a big easier to handle.

    I cut out some wooden guides to pin to each end.

    These are the wing profiles from the model plans I'm basing it from.  See (tbd) project for more details.

    I tried simply drilling some holes and pinning them to the ends with trial cuts.  I found the lead in and lead out  were awkward and didn't allow smooth motion, so wasn't a clean cut.  I glued on a steel pole to act as a lead in, and it worked much better.

    Here is that, pinned to an edge, post cutting.  (I cut two out of the block, so the other is also visible.)

    Note that I've left the cut pieces in provide support in future cuts.  

    Cutting creates a lot of very thin hairs, though they are easily sanded off.  You can see the all over the surface as well as out the end of the cut.

    I did several practice cuts before working to make the final pieces.  

    Tricks I learned:

    • The higher tension (20-30lb) was very helpful
    • Marking off 1" sections on the top at both ends is very helpful for making sure you are cutting evenly
    • It was best to set the cutting wire on the far side guides, then hold it only from the back middle(on the elastic)  This allowed the weight of the bow itself to trace the pattern pieces
    • I needed more pins than I'd expected to prevent the pattern from shifting.  I used 5 through each piece, as well as 2 more under the thinner side at the end
    • For this wire, it worked best around 4.3A.  Higher and it was very dependent on cutting speed.  Lower and it cut super slowly

    Overall it works fantastic, though takes a little practice.  The last set I did turned out fantastic, with very little sanding needed.  This will probably be my go-to method of making wing profiles in the future.

  • Bow

    Quinn03/23/2021 at 17:59 0 comments

    The bow is a simple affair, just a frame which can mount and keep under tension the cutting wire.  It will have a switch that can remotely turn the supply output on and off.

    I used some 1x2 scrap lumber, cutting slots in the ends of the central piece to make hinges and an H frame.  Some long machine screws with knobs make for easy adjustment.

    Tension is provided by an elastic cord on the back side, pulling the outer pieces together.  I used a couple old drawer knobs as hooks.  I didn't have long enough/strong enough elastic cord, so for first tests I folded what I had into 4 parallel strands, and used twine with loops tied into it to make up the distance.  The loops, about 1 every 2 inches, allow for easy tension adjust.

    After some testing, I found that while it worked, more tension was desired.  Thicker elastic cord would be good, though using what I had on hand, I made a rubber band rope, and tensioned it around several times.  This resulted in about 20-30 pounds of tension on the wire, which worked great.

    To easily attach the cutting wire, I used a pair of old ground binding posts.  These are all metal, so generally only useful in electronics for a ground point.  But here, mounted in wood, and not concerned about shielding, they can easily be put to use.  

    One note about using binding posts is how you attach the cutting wire.  While I did line up the wire holes in the posts, I'm not going to use these.  Using that and tightening the post creates a somewhat sharp edge the wire is pressed against.  This makes that point weak and more prone to snapping the wire.  Instead, I wrap the wire around must under one turn and tighten.  This prevents the possible breaking point.  However, if doing this, you need to attach the wire in the correct direction. From the main length to the tail, it needs to go counter clockwise.  This way, tension on the wire will tend to tighten the post, not loosen.

    For wire, I had some old heating coils from a scrapped thermal chamber.  I estimate this to be about 21 gauge nichrome wire.  It measures 0.81ohm/ft.  It was wound into coils, and mounted in a frame on ceramic bushings.  To mostly straighten it, it was a simple matter of sliding a rod through the center of the coil, then pulling perpendicularly at an end.  This unwound the coil as I went, and now have an easy bundle to work with.  Alternatively, nichrome wire can be easily ordered online for not too much.

    For the power leads, I terminated then with round crimp connectors that the binding posts mounted through.  Then simple drew then together and did a natural twist to keep them paired.  For convenience, I put a dual banana plug on the end.

    The remote control is a simple round pushbutton, with a pair of wires attached and protected with heat shrink.  This also got twisted around the power and a banana plug on the end.  I might mount the switch on the bow once I determine the best way to hold it while cutting.

    I made two central poles.  One with a nominal 24" span, and one with 42".  You can use the larger one for smaller cuts, but it is much easier to manipulate the smaller one.  The above picture shows the smaller one.

  • Supply Finished

    Quinn03/22/2021 at 20:24 0 comments

    After sorting out the issue with the current meter I had everything working.  This shows the internal layout, with the variac set in place(it is normally attached to the case top).  The end of the connection pcb was cut off and relocated underneath.  The end result is one of the more densely populated cases I've done, with barely any more space than could possibly be used.  Given that it weighs just over 12lbs and only measures 7x7x3.5, it is also one of the most weight dense too.  On the flip side, it feels extra solid.  It's weight density is about the same as brick.

    The variac was mounted in the case top, with a plastic shield around it.  The shield is mostly so that the wiper on the bottom doesn't get snagged on any of the wires.  I found the front to back balance point, and mounted a large handle on the top as well.

    Overall, works great, solid, easy to use, and easy to move around.

  • Wiring and test

    Quinn03/16/2021 at 19:18 0 comments

    I wired it all up, which is a bit messy.  The thicker 14 gauge stranded wire is part of it.  But it'll work.  I might need to put a divider in to keep the wiper on the bottom of the variac from tangling in the wires.  The schematics as before reflect what was implemented.

    This all got assembled, only to find that when I changed the pcb size, I forgot to take into account the variac which is mounted on the top. As a result, it won't fit. So it'll need some adjustment. Cutting off part of the pcb with the relay and it's terminals will resolve this, and I can relocate these below.


    While the variac doesn't currently fit, I could wire it and leave it off to the side to test.

    Testing it out using an HP 6060A DC load, the output works as expected.

    The only issue seen is that the current meter reads lower than real.  I may have modified the board incorrectly, or it may require an increased smoothing capacitor.  I measured the voltage off the shunt and it's correct, so there is something wrong with the meter or it's wiring.  After some further debug I found there were 2 issues.  There was an extra resistor on the meter circuit board which was causing a divider with my series 1K.  Removing that fixed this.  Also, the caps on the meter circuit were not sufficient, and a larger one was required to smooth the ripple.

    The case bottom can get quite hot at higher current outputs due to the bridge.  The thermal pad seems to work well, though the case bottom ends up substantially hotter than desired.  A minute at max current and it becomes too hot to touch.  I'll proceed as is, and will make a note of it on the case to take care with during use.  Rarely will the high current be needed, and even then the will be breaks between cuts to allow cool down.  The only other options would be to add forced air cooling, or relocate it to support an external heat sink.

    Heat from the shunt doesn't seem to be an issue.

    Edit: turns out it wasn't the cap on the current meter input that was the problem.  It was much trickier than that.  Hidden on the meter circuit board, underneath the meter module was a trace connecting the IN- to analog common.  The name of the latter is misleading and in most situations this pin must be left no connect.  It's ok to connect in this way only if the input is completely isolated from the meter supply.  In my use, I was connecting IN- to the low side of the shunt, which was in turn connected to the supply negative.  The manufacturer even has an application note detailing the trouble with this.  

    As the trace was under the meter, I was able to carefully find it by shining a light under the edge and seeing the trace through the pcb.  I then drilled carefully through the pcb to sever the trace, but not drill into the meter module.  I had to use a fly wire to wire it as needed.  Then, finally, the meter worked correctly.

  • Case and Mechanics

    Quinn03/14/2021 at 05:35 0 comments


    For a case, I found one I could reuse.  It was originally an old parallel printer port switcher.  I was never going to need that, so pulled the electronics out to use.

    I laid out the front panel with all the connectors, switches and meters, as well as spaced things inside.  It's going to be a tight fit, but should work.  The variac control will need to go on top, and because it is going to be pretty weighty, I will add a large handle on top as well to easily move it around.

    Front Panel

    As the front panel already had printing and various holes on it, to make it look nice, I cut an aluminum sheet to overlay it, then cut all the openings through both of them. 

    I finished the metal with a simple brushed appearance, by using 150 grit sandpaper all in the same direction.

    To label everything, I went to the quick solution I've used before and used dry transfer lettering

    I sprayed several layers of clear enamel over the whole front to protect the lettering, otherwise they would scrape off.  The panel is attached simply with the mounting of all the items.

    AC/DC Power supply

    To protect the AC/DC supply for 12V and 5V, I cut and folded a simple box out of clear plastic.  This is a common method used in many devices to offer a little extra shielding.

    Component layout

    To make everything fit, I needed to carefully piece it all together.  This required 3D placement as well, because the variac is attached to the top of the case, and yet other parts sit underneath it.

    Case Bottom and Back

    The main transformer is bolted in the back corner, sharing one of the screws for the rubber feet. 

    The AC/DC is in the other back corner, sitting on standoffs, and also sharing one of the rubber feet screws.

    I mounted an IEC socket and the 1.5A breaker in the back panel.

    For grounding, I used one of the studs on the back panel to attach the ground from the IEC socket and to the AD/DC supply input.  I needed to scrape the paint off the inside around it to make this work.

    For the bulk capacitors, I mounted them on a cut piece of perf board, and mounted it on brackets between the transformer and AC/DC.

    The back panel has a bunch of openings that I decided to leave alone.  This allows some ventilation, and there isn't really anything live exposed that would be an issue if a finger accidentally reached in.

    Up front of the capacitors I mounted the bridge to the case bottom, using a piece of thermal conductive pad to help with dissipation.  The steel case isn't going to be the best heatsink, but hopefully will be enough.

    Near the front I added a simple lever bracket to clamp the shunt resistor to the case bottom.  Again I used a thermal pad to help dissipate heat.  This has 14 gauge wires for the main current, but also a twisted pair for the sense lines.

    Lastly,  I ended up needing a small terminal strip as a couple of the connections were going to have just too many lugs on the same point, so this splits it up, and lightens the load on the PCB as well.

    Front panel

    The front panel has the various switches, connectors and indicators.  These are all panel mount with bolts.

    At top left is the input power switch, along with a lamp to indicate it is on.

    Below this is a switch which selects for the output to be on, or off/remote.  When in the off/remote position, the output will be off, but will turn on whenever the remote switch is turned on.  The lamp next to the switch indicates the output is on, regardless of where it is switched.

    Top middle is the voltage meter and top right is the current meter.  The board I reused didn't have any mounting for attaching to a front panel, so I made a simple clamp with a scrap piece of fiberglass behind it that traps the circuit board between it.

    Next along the bottom are the main outputs as binding...

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  • Schematics

    Quinn03/13/2021 at 06:04 0 comments

    Here are the intended final schematics.

    Things to note:

    • I added a switch to select if the output supports higher voltage or higher current.  This changes the rectifier circuit.
    • Because the switch I have for that which supports this current is DPDT, I'm using the other pole to change the transformer input winding as well.  The transformer has 2 taps on the high side for adjusting the voltage.  This bumps the ratio up(and output voltage down) when in the high current mode, allowing a bit more current.
    • For the output caps, I found and am using 2 @ 6800uF 80V caps in parallel.  These have two 4.7kohm resistors to bleed the voltage when powered off.
    • The circled numbers reflect connection points on the circuit board terminals.
    • The output/low voltage side is floating from ground.  To avoid building up a static voltage on it, the output common is tied to the chassis by a 1Mohm resistor and a 0,022uF cap.
    • Because I didn't have any leaded 1% 10k and 10 ohm resistors for the voltage meter divider, I used a 10k and a 20ohm multi-turn pot instead of the 10 ohm.  This will let me dial in the divide by 1000 needed to scale V to mV for the meter.

  • High current output option

    Quinn02/22/2021 at 23:26 0 comments

    It occurs to me that if I pull the output of the transformer using half the secondary, it increases the ratio to 15:1.  This would allow the 1.5A input to result in 22.5A output.  This would limit the voltage to be 0-7.5 VAC RMS, though the higher current would only ever be needed with thicker gauge wire, so lower voltage required.  

    I could add a switch to control which transformer output to use as one option.  I'd need a high current switch for that, and to increase the bridge and relay current ratings.  And if measuring the current, the existing shunt would be dumping 5W of heat at max current, which is it's max rating.  I suppose I could bond it to the chassis as a bit of a heat sink. (That configuration also means the meter would be maxed at 19.99A). I don't have a 20ADC breaker, so it would rely on the input breaker; I'd need to switch out the 10A one.  Or maybe get rid of the output breaker as redundant.

    Alternately, I could add a separate parallel rectifier and separate outputs, though it would then bypass the metering.  Might be easier to switch it's inputs though.

    After some thought, I'm going to implement this.  I have a bunch of 35A bridge rectifiers, and a 25A switch and relay.  When using steel or other lower resistance wires which I may want to do at some point will require lower voltage and higher current, so this will give extra flexibility.

    For switching modes, the switch will be move the output side common between the bridge negative and the transformer center tap.  Doing this will reduce waste heat in the bridge, and more evenly load the transformer output windings.

    Switching this way changes the rectifier circuit between these two configurations:

    Full wave center tapped rectifier:

    Full wave bridge rectifier

    For the center tapped version, I can use the same bridge rectifier. Only 2 of it's diodes will be in use which will reduce it's power dissipation; good given the higher current in that mode.  The bridge is rated 35A, however the heat becomes a limiting factor as I will only have the steel case as a heat sink, and no airflow.

    I'll remove the output breaker as redundant as the weak link in the chain really is the variac at the input, with the output side having margin.

  • Protection and isolation notes

    Quinn02/22/2021 at 22:58 0 comments


    When working with an AC input, it's important to have isolation to prevent shock.  A variac doesn't provide isolation, but the main transformer does, so that is covered.  

    The switching supply I confirmed is isolated.  In its previous use, the output ground was connected to chassis and earth ground, though I'm going to remove this.

    By doing this, all of the outputs are isolated not only from mains, but also from ground.  This means either of the outputs, or the remote switch lines can be connected to earth without causing a short.  Given that the output wires are open and exposed, this is very important.  If one were to contact ground from a piece of equipment or a metal table, no issues will occur.

    However, this allows a static charge to build up on the output/circuit side compared to ground.  I'll connect them with a 1M ohm resistor to bleed that off, as well as a higher voltage capacitor so that any esd can be routed to ground.

    Of course if the outputs are sorted, then the breakers offer protection, but current won't flow through an item being contacted to ground.

    Breaker protection

    I wanted to confirm how well these breakers will offer protection, especially the input breaker. This is an E-T-A 41-06-P30 breaker, rated 250VAC/48VDC, 1.5A.

    I did some testing with multiple samples.  I allowed them to cool between tests.  I tested at DC, I'm not positive if this makes a difference vs AC, but it doesn't show a difference on the datasheet.  I used an HP 6643A supply, and an HP 6060A DC load for testing.

    CurrentTime to trip
    1.75A-, -, >60min, >60min
    2A 290, 527, 187, 224sec
    3A  (2x)22, 24sec
    4.5A  (3x)8, 8sec
    6A  (4x)4, 4sec

    Here is the trip time/current chart from the datasheet

    We can see that at 2x, it should be between about 12-40sec, which matches data.  Likewise with 3x between 5-15 and 4x between 3-8.

    The chart shows that at 2A (1.33 times rated), it may technically never trip, but at minimum would be 60sec.  At 1.75A, none of the samples ever tripped.

    I'm not sure if I'm satisfied with this level of protection.  It'll certainly protect against an output short where current will spike, but for low level overload, I'm less impressed.  I'll try a second breaker, but maybe if I just use the third one I tested above it'll be fine.

    I have a number of E-T-A 1110-F112-P1M1-1A, which are rated at 1A.  They have a similar profile as above, but with the lower rating, will trip earlier.  They are a nicer format, as a push-push actuator, and could be used as the power switch. Additionally I have a matching 10A one, so both input and output would look the same.  I had passed them by because 1A was lower than I wanted, but seems worth trying.

    CurrentTime to trip
    1.25A>30, >60, min
    1.5A (1.5x)70, 66, 65sec
    1.75A (1.75x)38, 36, 33sec
    2A (2x)20, 18, 18sec
    3A (3x)
    4A (4x)

    Using this would limit output current to about 9A. (Or with a half secondary output, 18A)

  • Finding Parts

    Quinn02/22/2021 at 22:52 0 comments

    Following the DIY route, I started digging through stock for what I needed.  I already had the variac transformer and meters.


    I'm going to use this 1.75A variac, wired for 0 - 130V output.


    I'm going to use the second ups transformer, with 17.5VAC center tapped output.


    I mentioned the meters before.  I'm going to use this module I pulled out of some old electronics.  I need to use an external 0.01ohm shunt for the current, and a resistive divider for the voltage, but otherwise super easy to use.

    Input breaker

    Because I want to make this robust in case of accidental abuse such as short circuit, I need some protection.  The variac is probably the first thing to go in a short circuit situation as it's limited to 1.75A.  The transformer has a current limit probably about 23A, but with a measured ratio about 7.5:1, the variac can be exceeded.  1.75A input means a max output about 13A.  I did find a 1.5A AC breaker which will work well in this application.  This limits output to 11.25A, but gives a bit of margin.  It's important to note when using breakers that they have a time delay aspect to them, which varies by how much the current is exceeded.  In this case as the device being protected is a transformer, it matches well, because it's failure also has that ratio because it's failure mode is generally overheating causing windings to melt.

    Output breaker

    I didn't have a 12A low voltage DC breaker, but did have a 10A 28VDC breaker that I'll use.  Gives further margin, which is plenty for this use case.

    I considered fuses for both of these, but having the breakers decided they are more convenient.  I could have omitted the output breaker as the input should always trip first, but decided to include as I had it.


    I needed 3 switches for this project.

    The first is simply an input power switch. SPST 1.75A is sufficient.

    The second controls the output, and selects between turning the output on, and enabling the remote output switch.  Functionally this is just in parallel to the remote switch, and only controls the current to the relay coil, so a SPST, low current switch is sufficient

    The last is a remote switch, which can be located on the bow/table to make it easier to use.  I needed a momentary SPST pushbutton for this, and it is just wired in parallel with the main output switch.

    Continuing in the retro industrial robust design, I pulled out a pair of large toggles.  For the remote, I found a round one, so wires can be soldered to it, and heat shrink applied around the button body and wire make an easy handheld button.


    The variac takes a bit more force to turn than a normal pot, so needed at least a moderate sized knob.  I pulled out three options fitting the more retro look.  I'll decide which to use when I get to the end.

    Output connector

    I pulled out a ganged pair of binding posts/banana jacks for the output.  I expect I'll mostly use the banana jacks, but the convenience of the binding posts in case I need a custom setup is nice.

    Remote connector

    For robustness, and because I like the connectors, I went with a pair of banana sockets.  I have plenty of dual banana plugs, so will just mount one on the end of the switch wire.

    Power input


    For a bit more safety to indicate it is on, I wanted a couple indicators.  To keep in the styling, and because I didn't really care about the power use, I pulled out a matching pair of red and green incandescent bulb indicators.  

    The green will be for the input switch on(though this is also indicated by the meter being in), which will simply be wired to the 12V supply output.

    The red will be for output on, and simply wired across the relay coil.  I'll add a 33ohm in series to knock the voltage down a little to extend the lifespan.  They are plenty bright regardless.

    Control supply

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