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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.

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.

  • Power supply options: unregulated diy

    Quinn02/15/2021 at 19:46 0 comments

    I do have sufficient parts on hand to build a suitable supply, though it's likely to be bigger and heavier than I'd like. This would be unregulated, transformer based.

    Main transformer

    I have a number of higher current output ones.

    A 6.3v, 20A, which isn't terribly large. This won't work great for the bow however with such a low voltage. It would be great for a rigid wire cutter though.

    From a large UPS, I have a transformer that I expected to be 120v to 24v, probably 50A. The backup used a 24v battery, though I don't remember if the drive was a full bridge or single bridge.  I ended up testing it.  There is a 120v coil, and a lower current secondary 14v coil. The main output is 14v however.  Given the UPS power, this must be 70A.  It's huge and heavy, overpowered and lower voltage than desired.

    From a second large UPS, I tested it's transformer to be 13.6 or 15.7V output depending on which 120 input tap is used.  I don't have a way to know the output current, but based on the wire, it's probably 90+A.  Given that this is a 24V battery system, I'm again surprised the low voltage side is this low, but I guess it must work better for these high power systems.

    From a smaller UPS, I have a transformer I tested to be 120 to 17.5v, center tapped, probably 17A. I know this circuit switched the 12v back and forth between each side of the center tap in order to get sufficient voltage from a 12v battery.

    Another one from a UPS also has 17.5vac output, center tapped, likely 23A.  There is also a smaller 24V output coil.  It also has a second tap on the higher voltage end, likely used to get a higher output boost when on battery.

    Variac

    To adjust the output voltage, I have a number of variable transformers that could be used. These could feed the input of the transformer for smooth voltage adjustment.  All of these can be wired to output 0-100% or 0-110% of the input.

    I have a bunch of 1.75A variacs. These are reasonable size, and would handle up to 210W at the maximum transformer output voltage. Equivalently, with that smaller UPS transformer, output up to 12A, which is sufficient for the direct needs. With the larger transformer, it would be lower current.

    I have a pair of 3A variacs, a little bigger, and would handle a bit more power.

    Past that, I have a couple stand-alone variacs about 7 or 8 amps.

    And I have a pair of 20A variacs, which are huge and heavy.

    DC

    I think regardless, I'd like to rectify the transformer output and put some caps on it to smooth it out. I should have a handful of 15A bridges, but also some much higher power diodes if I wanted to go higher.

    Metering

    To help be more consistent with cutting, I'd like voltage and current metering on the supply. At first I dug through my analog meters, but I didn't have a good matched pair.

    Instead I found a set of these modules I unsoldered from some power product. Thankfully I worked out the pinout and wrote a note so it was pretty easy to get going.

    These use 0-200mV voltmeter modules, Datel/Murata DMS-20PC-0-YS. The volt side directly connects to the module input pins, so I simply need to scale with a resistive divider.

    The current side has a OPA234PA op-amp to drive the meter input. Input pins feed the op-amp. I didn't work out the values in the op-amp feedback, so I'm not certain the gain. It was originally set to measure over a 600A/50mV shunt, so 83uohm. Based on the original application, this implies it's configured for a 1.2x gain. There are a couple multi-turn pots, one in the feedback loop which can adjust gain, and one as a zero adjust. The op-amp I assume is in this circuit in order to scale based on the odd value current shunt.

    I was able to find a 0.01ohm 5W resistor in stock I can use as my shunt, which can directly feed the meter (without the op-amp). Given that the existing circuit gain could not reach a useful value, I will just disconnect the op-amp output from the meter...

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  • Power supply part 1

    Quinn02/15/2021 at 15:59 0 comments

    For a power supply, I'd considered several options.

    A bench supply can be directly used, with advantage of using constant current mode.  While I have several, they are all larger and set up at my bench, not convenient to move around.  As cutting needs to be done in a well ventilated area, moving one of these to the garage every time would be a nuisance.

    There are quite a few lower cost switch mode power supplies which could be used.  I would prefer to use things from my stock however.  This model for  $50 seems like good capability, though I'd still need to dig up a ac-dc supply with sufficient voltage and current and a case.

    A battery powered solution would be to use a moderate or larger size hobby lipo battery(3 or more cell, 2000+mah).  A brushed motor speed controller can be used to regulate the power, and a servo tester used to control the speed controller.  

    I could also make my own, which I've also decided to explore, given all the stock of parts I have.

    Needs

    This table from http://hotwirefoamcutterinfo.com/Introduction.html is super helpful for estimating the current and voltage needed for various wires.  Unfortunately it doesn't go out to the wire length I need for the bow, but having the resistance table and needed currents are the key, and can easily sort out the voltage for a given wire length.

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