Adjustable Hotwire Foam Cutter With Fence




Initially starting with Creativeman’s Mighty Goliath I decided to create a foam cutter that was adjustable for various angles, had a fence to facilitate straight cuts, made removal and installation of the hotwire easy, and folded for taking less space when stored. Additionally, I wanted the table to be an element unto itself – the idea was to use a battery charger as my transformer, along with a separate control box and foot pedal (that are used in other projects) so that I would not have to have a single power source and controller for the three tools I use them with.

Step 1: The Table

1.     As pictures explain quite a bit I won’t go into to details of the construction but I will point out a few of my reasons for building it the way I did. For those not familiar with hotwire cutters there are several other folks with instructables that explain the theory and even include electrical diagrams. I am no electrician (although I have been known to provide them with employment if not entertainment) but a basic knowledge of wiring is all I needed for this project.  For the table (please excuse the Jackson Pollack design – I reused my children’s design table which I painted this way to hide their messes - if you can’t beat ‘em join ‘em) I decided that a round shape would give me more space and made it easier to place my fence at any location in regards to the wire. The elongated slot gives enough room for the wire when the arm is rotated and is probably wider than it needs to be. Four legs form the base and the arm is made from three separate wood pieces, two horizontal and one vertical. At the pivoting section of the arm, provided by a 6 inch bolt, there is enough friction between the two wood pieces to hold the arm tightly without shifting. The fence is just a smooth piece of rectangular aluminum and has a small bolt at one end with a round wood washer to clamp or pinch the table to hold it tight. At the other end I use a cheap plastic spring clamp since it is easy to place at any point on the length of the fence.

Step 2: Arms

1.    A small spring at the top of the arm puts some pressure on the wire and will take up slack as the wire heats up – I use #10 guitar string and also bought #12, 14 and 16. This #10 cuts well even with multiple layers (up to 4 inches thick) so I haven’t tried anything else. The wire arm is adjustable past 45 degrees however you have to keep in mind that you must cut in-line or parallel to the axis of the arm – meaning the fence has to be parallel to the bolt on which the arm rotates to have a true 45 degree cut. I put witness marks in ink on the back of the lower support that the vertical arm is bolted to. The lower attachment is a smooth shank bolt, screwed into the lower vertical arm that the wire wraps around and continues over to a small cable clamp. The clamp makes it easy to adjust the tension of the wire and bolt gives the wire something smooth to curve around and provides good contact for electricity.  The lead is attached with a small bolt to the head of the smooth shank bolt after drilling and tapping it. I attached it to that instead of the aluminum bar as the bolt is not tightened down and doesn’t have good electrical contact to the bar – again it’s purpose is to give the wire a smooth radius to pull around as it goes to the clamp. This bolt should be on the same axis of the adjustable arm but even if it isn’t the spring on the top arm will stretch to compensate for the wire being pulled tighter when you reposition the arm.  

Step 3: Battery Charger Connections

1.     Two bolts at the ends of the leads (white wires that follow the two horizontal arms) are where the leads for my battery charger are attached. This is the same spot you could attach your configuration of a power supply and control box (transformer and dimmer switch for example).

Step 4: The Box

1.     This old wine box is the most useful part (to me anyway) of this system. I have two other tools in which I use this box - a chestnut roaster made out of an old washing machine and an old hair dryer that is used on a metal melting blast furnace. Both systems require that I adjust the voltage to them to modify their speed – since I only had one switch (used to control an old ventilation system) I needed it to be interchangeable between my uses. My battery charger also sees double duty for de-rusting metal parts (i.e. electrolysis). Sometimes I even use it to charge batteries. In any case I didn’t want to dedicate these tools to any one use. Hence, I created “the box”. Basically it has two outlets (in Italy they have 4 different types of plugs – no one agrees on anything here) so I had to make it adaptable to various plugs. It has an on/off switch and a separate variable speed switch. The typical dimmer switch (found in Home Depot) incorporates both of these (push on/off and turn to vary the voltage) although they might not have the same amp/voltage rating.  The three pronged plug (old phone system connection) goes to my foot pedal.

Step 5: Foot Pedal

1.      A very beneficial addition to any hotwire system is a foot pedal – credit for this idea goes to this website; - I got some great information from these folks. For any of you working with foam I highly recommend a foot pedal, I cannot emphasize just how handy it is. For the pedal to work I have a light switch (common in older Italian homes) that is a simple spring contact switch – push and hold to turn it on. To disconnect the pedal from the rest of the system I used an old lever-type switch (see inside box in previous step) that my grandfather had amongst his bits and pieces collected over many years in refrigeration (Depression Era folks didn’t throw anything away, luckily for me). When the switch is closed electricity flows through it but when open electricity can only pass through the foot pedal. A regular plug could be used in place of my three-pronged telephone plug, but I wanted something that couldn’t be accidently plugged into with anything else. Again, I am not an electrician and am not including any schematics here.  A basic idea of wiring is all I used and you would need to adapt your materials to your use. My stuff is a bit different as I live in Italy.

Step 6:

1.     This is how the system looks when connected. The battery charger is plugged into the box and the box is plugged into a regular wall outlet. The foot pedal is plugged into the box and the lever switch inside the box is opened. The leads to the battery charger are connected to the bolts on the arms of the cut table, and I am set. Before actually cutting any pieces I make a few test cuts to make sure the wire isn’t too hot for the type and thickness of foam I am cutting. As far as storage, with the wire loosened from the cable clamp and the arm resting all the way down against the table this table (hanging vertically) takes up about 6 inches of wall space. 
I welcome any corrections, comments, or alternate ideas!



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    17 Discussions


    7 years ago on Introduction

    Nice project!
    Do you have an electric wire diagramm?

    Thanks in advance


    8 years ago on Introduction

    So, let me see if this is right. You are using the complete #10 guitar string length, and 12 volts DC for cutting. Do you know what the string resistance is? While the foot pedal is a good idea, is it necessary to use the pedal on and off to keep the wire at cutting temperature, or is that just a safe way to insure that it is not left in the on position? BTW, I like the fact that you can pour complex parts with the foam variation of the "lost wax" method of making jewelery. Very clever! How small (diameter) must you maintain to get a guaranteed good pour? I too, am pleased with the nice workmanship of this project.. One last question. Other than sand, what material would you use to make smaller aluminum castings if you had to make a small part?

    1 reply

    Reply 8 years ago on Introduction

    I do not know what the resistance of the string is - I bought several sizes, #10, #12, #14, etc. but so far have only used the #10. I have burnt through it (snapped the string) once so far, with (really rough estimate here) about 20 hours of cut time. All of this is variable - I can increase the voltage (amperage) of the charger which results in a slighty higher temp on the string; this allows you to push the foam faster or cut 2 to 3 layers of foam at the same speed. Or, if you push the foam slow, you will cut a wider kerf in the foam. I found the pedal to be handy because I found many times that in the middle of a cut (like doing inside cutting instead of an outside form) I needed to turn off the charger so I could reposition the piece. However, it never fails that keeping the foam still with one hand AND turning off the charger with the other results in melting something I didn't want melted such as an otherwise smooth clean edge. The pedal keeps both hands on the foam at all times and the wire cools immediately - I can actually touch it after one second. Most of my pours have had a sprue of at least an inch in diameter. In my pours I am trying to feed the aluminum in the mold ASAP before it starts cooling. Most of my parts are not very small, i.e. not jewelry sized so I have not needed a small sprue. It always ets cut off of my forms anyway. In any case, the process is the same for large or small parts. While there are other methods to use for casting aluminum, I certainly do not have the expertise to talk about them. I read somewhere that certain types of production casting may use steel forms but that kind of work is well beyond home casting. I know that some home casters have created molding sand from regular beach sand and kitty litter as the "binder". In my case I just found a local aluminum foundry and they let me buy a couple of 40-pound bags of Petro-bond. This is a reddish colored oil-bonded sand and well worth the slight expense if you decide to persue metal casting as a hobby. My initail forays into casting were from reading Dave Gingery's series of books "Build Your Own Metal Working Shop from Scratch". Thanks for the comment!

    Nice instructable !! It is good to see someone who takes pride in his work, EVERYTHING is so neat  well constructed ... Love the wine box idea !!

    4 replies
    oompaLectric Wizard

    Reply 9 years ago on Introduction

    Thank you - I got the idea for the wine box (I work in a bar so I have a few of these) from another instructable about a "mad scientist's light".  I can't say I take the time to do all my projects nicely, but this one turned out very well and I like to use it - that makes a HUGE difference in my opinion.


    Reply 8 years ago on Introduction

    May I ask, what do you exactly use this for? I know cutting foam, but, what are you making the foam into? Do you have any pictures of things you made?


    Reply 8 years ago on Introduction

    Hi Tcase,

    I built this foam cutter to supplement my metal-melting hobby.  Basic pattern making for aluminum casting generally involves making a pattern out of wood then putting this pattern in a 2-part sand mold, packing the sand around the pattern, then opening the mold and removing the pattern.  Then you pour molten aluminum in that empty space and once it cools you have a copy in aluminum of your original pattern.  The big benefit of foam however, is that you do not have to remove the form from the sand - you just pour the molten aluminum on top of it and it vaporizes instantly (molten alum being around 700 degrees), filling that space with the metal.  It makes it MUCH easier to create complex shapes (I also use a hot glue gun to attach foam-to-foam pieces, the glue melts/vaporizes also) that might prove very difficult to remove from the sand without ruining the sand mold.
    I included a picture of a couple of things I have made in aluminum, they are exact replicas of what the foam pattern looked like.  Regarding the metal gear blank - I used adhesive spray to glue a white paper gear blank to a foam circle and followed that outline with the hotwire cutter.  The gear was a missing piece in a set of compound gears on my metal lathe.  I wanted to cut a particular thread on my lathe and of course Murphy decided I needed that one missing gear to cut that particular thread.  While generally one would machine-cut the teeth on a blank metal circle, I didn't have that setup.  It wasn't perfect (I made three patterns, used the best one, and still did a bit of hand-filing to get that good one to run smooth), but in the end it worked.  Yes, I could have bought one and saved myself quite a bit of time but what fun would that be?  The main reason is that, though - to create patterns for metal casting.  An added benefit is that this cutter keeps my 6 year-old son occupied cutting three scrap pieces of foam into 30000 totally useless little pieces while I work on other projects in my shop :)  It is pretty easy for my kids to manipulate too!

    I just downloaded it myself without problems - are you right-clicking and choosing to "save as . . . " to your computer?


    Reply 9 years ago on Introduction

    Eh si, ce ne son ancora una ventina!!  Sono forte, devo trovare un uso per loro . . .  (translation - I still have 20 or so more!!  They are cool, I need to find a use for them . . .)


    9 years ago on Introduction

    Thanks for the nod, oompa. Excellent 'ible, and your design looks well thought out.  A little more complicated than mine, I think, but we work with what we have so I understand.  They are a lot of fun to have and to work with for sure. Good job. Cman


    Reply 9 years ago on Introduction

    Thank you - I am a bit surprised after so many views to not have any questions.  I didn't explain as much as I thought I should, but I know I usually don't have questions if enough pictures are provided of the process.