Building a Hot Wire Foam Cutter

Hot wire foam cutters work by heating special wire to the point where it vaporize foam it is brought in contact with. It is a technique commonly used commercially and hot wire foam cutters are available for any where to several hundred to several thousand dollars. Heck, I will happily sell you mine for a fraction of that but you should probably know that it only cost me $4 to buy the resistance wire or nichrome plus maybe $15 - 20 dollars in scrap stock I already had.

I leave it to the reader to supply their own power supply. I am using a supply from an old external SCSI drive. My cutter runs from the five volt supply and 25cm of nichrome wire pulls about 900mA or 4.5 watts. That is actually running a little hot and I plan on building a temperature control circuit shortly. I have read that the key to successfully hot wire cutting of foam is apparently "slow and cool". After my initial experiences I believe it.

My design is pretty simple and breaks down into four parts; base, top, shuttle, and cutting arm. Each piece is described below.

A small hook extending from the base of the shuttle bar holds one end of the heated wire. The shuttle is driven to a desired position along a lead screw. Adjusting the position of the shuttle bar along the screw allows for an adjustable cut angle. Recall from your Trig that the higher your support arm the father your shuttle will have to move to reach the desired cut angle. For the default bar on my cutter for example, which is 20cm high, a maximum cut angle of only 30 degrees can be achieved. That can of course be increase by lowering the cutting arm. With this design it is simple to have multiple interchangeable support arms with differing and heights.

The bar is driven along the lead screw while a piece of steel rod is used as a guide. The rod keeps the threaded rod from taking the entire load. Probably overkill for this application but it is simple to add and ensures the shuttle keeps working without binding.

I used 1/8 inch 24tpi threaded rod and 3/16 inch steel stock but only because that was what I had in my scrap box. The loads here are so light that any straight steel rod will work for the guide or lead screw. As a suggestion you should probably look for the lowest TPI on the lead screw you can find.

The screw that you see running into the block is used to attach a ground wire to the shuttle. This ground complements the positive terminal on the support arm.

Since I was using a thin top I cut out four blocks and glued them into the corners of the base. These blocks were then drilled and formed the seat into which the base's top gets screwed down.

Both the threaded and steel rod are seated in 5cm wide pieces of brass stock mounted to aluminium bar stock. Both brass and aluminium stock in this type are commonly available at most hardware stores. The steel bar is friction fit and trapped in the base by the walls. The threaded rod on the other hand needs to extend beyond the case walls.

I used a small piece of brass tube stock as an insert to the hole in the side of the box through which the lead screw extends. This keeps the action of the action of the screw from fraying the wood. The screw is held in place by a small clip on the inside of the brass support. If you don't have any cer-clips at hand you can saw cut the file and washer as I did. I would suggest that the aluminium bar stock should be screwed into the base as soon as possible in construction, even if it has to be removed later, in order to provide a fixed reference to work with.

If your are like me and don't have a lathe you can make the notches needed to clip in the threaded rod with either a warding or milling file and a triangular file and a hand drill. Set up the base and mark off where the clip is to fir the screw with felt tip pen. You will likely have to do this several times so keep things handy.

By chucking up the threaded rod you can *slowly* work the rod along the edge of a file. Make sure you move the rod back and forth along the file so that wear on the file is even. If you don't you will ruin the file. Start with the triangle file to cut the edges to your notch and then remove the bulk of the material between the notches with a milling, warding, or similar file.

Again if you don't have a lathe you can flatten the ends of round stock by chucking it in a hand drill and slowly working it against a file. Use wood blocks to sandwich your file in a bench vice. Then slowly work the rod across the file. The averaged out surface will be close to flat and tangential to the axis of the rod's rotation.

The last piece to add to the base is the "u" shaped support block that holds the support bar. The support bar on this design is friction fit. The fit needs to be tight enough to firmly hold the arm in place yet loose enough that the arm can be removed for storage or to change arms. Not hard to do just a lot of filing and measuring to get it right.

The base is cut to size to match the base. Then four holes are drilled to match the pilot holes in the corners of the base. Use counter suck screws and bevel the edge of the holes so that the face of the screw sits flush with the tabletop.

Since the support arm is friction fit once the screw holes have been added attach the top and mark arm. Make sure that you under cut the measured mark slightly and then repeatedly file and measure the opening to get a tight friction fit.

The slot for the nichrome wire needs to be measured, marked, and cut after the shuttle is finished. If you don't have a table saw or router to make this cut you can mark a line then drilled repeatedly along that line removing the excess material with a file. It is not ideal but it is what I did here and if you are careful it will yield reasonable results.

The base of the arm sits on the outside of the box with a toe extending through the top to be held in place by the "u" shaped support block. I used a mortise and tenon to quickly join the two pieces of the arm together. A spring loaded bold mounted on the arm provides wire tension.

The heated wire is wrapped around a screw on the tensioning clip. Tightening the screw locks down the wire. The spring loaded bolt in the arm is screwed into the clip providing tension. A washer and used below the support arm both to retain the spring loaded bolt and for use in setting the initial tension level on the clip.

So a friend of mine Ross immediately suggested that we try cutting one of those wooden model / toy dinosaurs out of foam. Actually he suggested that we cut it out of foam and then try pouring it in aluminium with a lost foam process this weekend. Ross is known for these sorts of cool and slightly eccentric sort of ideas.

So I used some packing foam that a monitor was shipped in. I the foam into blocks small enough to feed to the hot wire foam cutter with a hack saw. Those blocks were then measured, marked, and cut into sheets of foam. I then marked out the outline of the pattern of the various pieces and then and cut out and assembled the pieces. The total construction time was about 40 minutes and most of that just experimenting and playing. Another head could likely be built from scratch in about half the time.

So once we started cutting out the Dino from sheets of foam we quickly burned through all the sheet foam we had on hand. Since I built the hotwire foam cutter Ross nocked up a foam plane. This was one of his better ideas. By having the heated wire stretched parallel to the plane over which the foam is moved you cut a sheet of foam of uniform thickness with each pass. This lets you break up all the foam packing material that comes with modern consumer electronics and turn it into usable stock.