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Build a for styrofoam, foam rubber & Are you into modelling or upholstery? Then you probably have often wanted a hot-wire cutter for foam plastics – but didn’t know how to build one. Wonder no more as we show you how to build a very cheap hot-wire cutter from odds and ends. By LEO SIMPSON M ANY PEOPLE have a need to cut expanded polystyrene foam (you probably know it as “styrofoam” or “coolite”), foam rubber (which isn’t rubber at all!) and many other soft plastic materials. Generally they resort to using a Stanley knife, handsaw, bandsaw, jigsaw or even an electric carving knife . . . and the result is generally a lot of mess for not much cut! The ideal way to fashion this material is with a hot-wire cutter. Actually, that’s a bit of a misnomer. It should be called a hot-wire melter because that’s what it does – but invariably they’re called hot-wire cutters so we’ll stick to that name. The cutter will go through most types of soft plastic just like a hot knife through butter (same principle really) and the result is a very smooth cut with no debris to sweep up afterwards. You also can do the most intricate cuts which would be impossible using any other method. And the cuts can be angled. In addition, you can also cut very 48  Silicon Chip thick material. Our prototype cutter would easily cut foam over 400mm thick and it’s not hard to make a much larger one if you wished. A hot-wire cutter is made from a length of resistance wire which is held taut and heated to just below red-heat. At this temperature you can slowly feed the material through and you will get a very smooth cut. As you can see from the photos, we made our hot-wire cutter from a variety of materials we had lying around the place. The baseboard was made from Laminex-coated chipboard left over from a kitchen installation. The vertical element was made from a scrap of 16mm Formply while the horizontal 5/16-inch threaded rod came from a cable reel. The hinge, screw eyes, spring and other bits were also hauled out of the junk box. The important point to note about this project is that it does not have to look good; it just has to work. For example, we could just as easily have used some raw chipboard for the base and a piece of hardwood decking for the vertical element. Or we could have used a piece of 3mm steel strap bent at rightangles and hinged from the base to carry the vertical cutter wire. No doubt you have other bits and pieces which could be pressed into service just as effectively. But where do you get the resistance wire? Fortunately, that is easily answered as it comes in small packs of 28 B & S Nichrome or Cuprothal from Dick Smith Electronics and Jaycar. Three types are available and just which type you use will depend on what power supplies you have available and how big you want the cutter to be. We made our cutter quite big because we envisaged using it to cut quite thick styrofoam for use in scenery for a model railway layout. If you want yours to cut thinner sheet materials then you may opt for something smaller. The active length of wire used in our cutter is 430mm and is probably just a bit longer than we need. But let’s say you want a similar length, other plastics 400mm. What we found is that you need about 40-45 watts to heat the wire adequately. More than 50 watts will make the wire glow brightly and that is not want you want as it could set fire to some materials. Anyway, if you have a power supply capable of around 50 watts you are in business. Your power supply could be a conventional adjustable DC power supply such as the 40V 3A supply we described in the January & February 1994 issues of SILICON CHIP. Alternatively, if you have a computer power supply capable of 200W or more, it can probably be pressed into service. Or you could even use a 12V car battery. Either way, if the supply you use is not adjustable, you will need a means of adjusting it. After all, the amount of heat for effective cutting will depend on the type and thickness of material so you do need to be able to adjust the available voltage over a small range. Now you need to consider how much voltage and current your power supply can provide because that determines what type of resistance wire you need to use. OK. Let’s consider the easy approach first and that involves using an adjustable power supply such as the 40V 3A unit referred to above. Since this unit can only supply a maximum of 3A, that meant that the resistance would have to be reasonably high. OK, so it’s not pretty – but it works! We scrounged the baseboard from an old kitchen cupboard, the spring from an old bed, the screw eyes and hinge from the junk box . . . we’re sure you get the picture. Here we’re about to cut through this thick block of polystyrene foam in just a second or so. The result: a beautifully clean, straight cut with no mess! April 2000  49 Like a hot knife through butter . . . that’s exactly how our hot wire cutter works. On the left we’re cutting a complex shape from a sheet of polystyrene foam, with a hunk of foam rubber waiting its turn. To prove the point, on the right is that same hunk of foam rubber being cut. Notice how straight, clean and mess-free the cut is? You can’t do that with a Stanley knife! For this example, we decided to use some 28 B&S Nichrome from Dick Smith Electronics. This comes in a small pack with a few metres of wire (Cat. W-3205). This has a nominal resistance of 13.4Ω ±5% per metre and so a 400mm length will be 5.4Ω. We jury-rigged up a 400mm length of this wire under tension and found that we needed about 40W to get it to satisfactorily cut a range of styrofoam in various thicknesses. That translated to a voltage setting of around 15V at 2.7A, comfortably within the 3A limit of the power supply under discussion. If you don’t have an adjustable power supply of sufficient current capacity, you might consider using a computer power supply or perhaps even a 12V car battery. Either way, you will need some means of adjusting the voltage fed to the cutting wire. We have a simple solution for that problem too and we’ll discuss that later. Diving into our junk box again, we came up with a 200W PC power supply that could deliver +12V at up to 8A and +5V at up to 20A. Such power supplies can be picked up very cheaply these days or salvaged from computers tossed out for council Fig.1: modified from the Glow-Plug Driver last month, this “power supply” enables you to heat the wire to just below red heat. 50  Silicon Chip cleanups. If we elect to use the 12V option (from a computer supply or car battery), it is appropriate to use the 28 B&S Cuprothal resistance wire pack from Dick Smith Electronics (Cat. W-3200). This has a nominal resistance of 6.09Ω per metre and so a 400mm length will be just under 2.5Ω (2.44Ω to be more precise). With 10V DC applied, the current will be just over 4A and again we have the right result of between 40W and 45W to achieve a clean cut with this length of wire. How do we get 10V from a 12V supply? Patience, now; we’ll come to that in a moment. But perhaps the computer supply you have scrounged cannot supply 4A from the +12V – some of them are a bit skimpy for this rail. The answer is to go to the 5V rail which even in a fairly modest machine will typically be able to supply 12A or more. So if we’re going to use the +5V rail, we need lower resistance wire again and in this case the 28 B&S wire from Jaycar could be the answer (Jaycar Cat. WW-4040). This has a nominal resistance of 3.77Ω per metre. A 400mm length will have a resistance of 1.5Ω. In this case we are in trouble because 5V across 1.5Ω will result in a current of only 3.33A and a power dissipation of 16.7W; not enough cutting power for a 400mm length. Clear­ly, we have to make other arrange­ments. One possibility is to double up, or better still, triple the wire. With three 400mm lengths paralleled up, we get Fig.2: the modified PC board component layout with the PC board itself at right for comparison. Both are reproduced same size. a total resistance of 0.5Ω. With 5V applied we’ll get 10A (in theory), or a power dissipation of 50W, more than enough for the job. Shorter cutting wire Alternatively, you could always compromise and go for a unit with a shorter cutting wire. We know that we need around 40W for adequate cutting from a 400mm length of resistance wire. That translates to 1W per centimetre. So if we decide on a 250mm cutting wire, we’ll only need 25 watts. Going back to that 28 B&S wire from Jaycar, a 250mm length will have a resistance of just under 1Ω (0.94Ω to be more precise) and when 5V is applied across it, the current will be around 10A and the power dissipation around 25W, right on the money for a 25cm cutting length. We could also repeat the exercise for the higher resistance wire. Using a 25cm length of the 28 B&S Nichrome wire from Dick Smith Electronics, we get a resistance of 3.35Ω. With 9.5V applied, we get just over 2.8A and a total power dissipation of 27W, which is OK for this cutting length. By now you should see how you can choose the length of the cutting wire and its resistance to suit the capabilities of your power supply. Of course, if you want a 1-metre cutting wire, you will need a cutting power of 100W and your power supply will need to be beefed up accordingly. (Hint: if you used the 6.1Ω/m Cuprothal wire, you would need a supply capable of about 25V and just over 4A). computer supply. It just so happens that we published a suitable circuit in last month’s issue under the guise of a “GlowPlug Driver” (see SILICON CHIP, March 2000, page 72). With a few minor changes, that circuit is ideal for our purpose. Fig.1 shows the modified circuit while Fig.2 shows the component overlay. If you compare the circuit of Fig.1 with the GlowPlug Driver circuit on page 73 of the March 2000 issue, you will notice that we have made three modifications. First (and second), we Adjusting the voltage We mentioned the need to adjust the voltage to the cutting wire if you are using the +12V or +5V rail from a This photo gives a good idea of the construction and in particular the tensioning method. The resistance wire “cutter” must be kept under tension to achieve a good straight cut. A healthy “twang” when plucked means the tension is about right! April 2000  51 swapped the positions of resistors R1 & R2 to change the output duty cycle. Whereas before the desired duty cycle was around 17% to obtain around 2V from a 12V supply, the modified circuit will give a range of duty cycles from around 75% to 85%. This is about right, if you want to use the examples quoted above and want around 9.5V to 10V from a 12V PC power supply or car battery. Our third modification was to remove the 0.1Ω 5W wirewound resistor R5 and replace it with a link. This resistor will otherwise cause too much voltage drop when you are using it from a 12V or 5V supply. Note that if you are using it on the 5V supply you probably will need to fit a small flag heatsink to the BUK453 Mosfet. You may also want to replace the 10kΩ trimpot with a conventional potentiometer if you want to easily adjust the wire temperature from time to time. It would also be a good idea to fit an in-line 5A fuse if you are going to power your hot-wire cutter from a 12V car battery. Foot-operated switch Another refinement to our circuit could be the inclusion of a suitable foot-operated switch to apply power to the hot wire at the appropriate time, leaving both hands free to guide the work. We’ll leave that part to you – just make sure any switch you use has high enough ratings (say 10A at 30V WATCH THE FUMES! When heating or melting any type of plastic (eg, with a hotwire cutter!), beware of the fumes which are given off. Always use the cutter in a well-ventilated area (preferably fan assisted) and avoid breathing the fumes. DC) and use heavy-duty connecting cables (again, at least 10A). Building the cutter While you can see the construction details from the photos, there are few points that need to be covered so we’ll briefly describe how our prototype was made. First, we made the baseboard from a piece of Laminex coated chipboard measuring about 700 x 300 x 18mm thick. The precise measurements are not important but ours was quite large so that it would have a large “throat” for cutting big slabs of material. Laminex or melamine coated pyne­board is ideal as it easily cleaned and suitable for sliding the material through the cut. It is also fairy heavy which means that the cutter does not move about when you are pushing material against the wire. We used an ordinary 100mm steel butt hinge for the vertical support which was made of 16mm Formply measuring 450 x 70mm. For the hori- zontal wire support, we used a 500mm length of 5/16-inch threaded rod. This has the advantage that it is easy to make the wire connections to it. The wire connection to the baseboard can be via a countersunk screw with the external wire connection underneath the board but we took the simpler approach with our prototype, as can be seen in the photos. The 28 B&S wire is held under quite a bit of tension by the small spring attached to the screw eyes on the baseboard and vertical support. Our spring came from an old wire bed frame. The vertical support needs to be hinged and under spring tension for two reasons. One, you need a fair amount of tension so that the cutting wire is not deflected as you push the foam onto it. Second, the resistance wire expands by about 10mm from cold to hot and the spring tension needs to take this up. A look at the photos will show that the threaded rod is under a fair amount of tension and can be seen to be noticeably bent against the load. Mind you, the wire should not be too tight otherwise it will tend to break. Ours made a pronounced “twang” when it was plucked. Inevitably though, the wire can be expected to break from time to time, so make sure you keep the leftover resistance wire in a safe place. Incidentally, the hotter you run the wire, the more likely it is to break. Finally, by using the threaded rod and the hinged vertical support, the hot-wire cutter can be easily dismantled and stored as a flat package. Where do you get it? It mightn’t look like the best cut in the world but hey, it was our first-ever attempt – and it’s a darned sight better than you could get with a knife! One trap for young players we found was to cut too slowly or leave the work in the one place too long. If you do this, the polystyrene starts to melt (as you can see happening near the hot wire). Adding a foot-switch to turn the power on and off could help prevent this. 52  Silicon Chip We’ve told you where to scrounge all the bits from in the cutter itself but so far haven’t said where to get the power controller. As we mentioned, this was published last month as a Glow-Plug Driver and the kit is exclusive to Oatley Electronics. So if you want to build the controller, simply buy the Glow-Plug Driver kit from Oatley Electronics (it sells for $14.95 including a case). They can be contacted on (02) 9584 3563, fax (02) 9584 3561, email sales<at>oatleyelectronics.com or visit their website: www.oatleyelectronics.com.au Just don’t forget to swap resistors R1 & R2 and leave replace R5 with a SC wire link.