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The foam cutting lathe

Here is a homebuilt foam turning lathe!! This is made from a window motor adapted and appropriately painted for the application. The pillow blocks that the rotating shaft bear in were made from foam patterns and cast using the above mentioned techniques. The two dials on the electrical box are light dimmers with simple numerical 10 through 100 markings for remembering settings. One is for the wire heat, the other controls motor speed. The object that is being turned on the lathe is a step pulley pattern. The picture below shows a similar pattern and a resulting pulley casting!

Foam cutting table and bow

Here I am cutting out a pillow block pattern for a lost foam casting. The paper stencil was designed using DeltaCad 3.2 and simply cut out with scissors. The curved rod is a 1/4-20 threaded rod with brass nuts to position the top wire terminal for varying cutting angles. It is a bit tricky to set it perfectly square with the cutting table. The transformer on the right is controlled by one of the light dimmers, center. The transformer is a modified microwave oven transformer with several taps for different lengths and sizes of cutting wire. The wire is not easily visible in the photo, it is a fairly fine wire, a violin "e" string.

Better view of foam cutting table

Note the knob for loosening the wire.

Pillow block template was made using DeltaCAD

Here the pattern is done except for the hole in the center for the shaft. To cut the hole, I loosen the knob at lower right to release the cutting wire. Then a reamer or punch is used to make a hole in the foam pattern, the cutting wire is fed through this hole, and re-tightened. The power is turned back on and the hole cut out. Then the wire is again released and the plug is removed from the hole. The pattern is ready to cast. Instead of making a core to form a hole in the casting, I simply put a 5/8" shaft through the hole and cast the metal right on it. After cooling, the shaft is driven out. This forms a nice smooth hole without ever machining it.

Step pulley pattern cut from foam

Notice the rough round foam blanks in the center background. These get glued together to form a blank large enough to turn a foam step pulley pattern from. Also, note the mold lines on the aluminum step pulley casting. This casting was not done with lost foam techniques, but by traditional molding techniques using a split foam pattern! Foam as you can see is a very versatile pattern material. It is, of course more fragile than wood and is harder to get a good surface finish like with wood patterns.

TECHIE STUFF !! (what you really came for...)

Now, here is the more technical notes concerning hot wire foam cutting techniques. These notes were inspired in part by email communications that I had with David Wimberley, the author of the David Wimberley Forge plans, available through Lindsay publications. (www.lindsaybks.com) David happened across this site sometime around August of 1999, and told me of his plans to begin experimenting with foam cutting. At that time, we had an inactive (coming soon) link that was bringing a few "foam cutting" hits, and after conversing at some length with him concerning this art, or sport, or whatever, decided it was time to address the matter.

Wire for hot wire cutting

First the wire. Any wire will do that can remain taut under heat. If the wire is low grade steel, for example galvanized picture wire, it won't last long because it gets soft with the heat -- even the low temp of foam cutting -- and snaps. David's choice of guitar wire is probably good. I have used stainless MIG welding wire, and I've used violin "e" string wire. I don't know what guitar wire is made of, but I do know that it is a very springy type of wire, and made to handle tension. I have used mandolin wire also (practically the same thing as guitar) and it worked quite well. The size of the wire is also noteworthy. The smaller wire diameter, the finer the cut you can make.

Schematic for dimmer, doorbell transformer foamcutter

Power supply

Getting the power from your 110 volt receptacle to a suitable foam cutting format presents several problems. A standard light dimmer can be successfully used to vary the heat, but it can't heat the wire in a direct circuit. With a light dimmer powered by straight line current, you are taking a great risk of electric shock. Also, a guitar string is not going to be longer than about 2-3 feet (unless you get it bulk). But then how long of a wire are you planning to cut with? If the wire is too short (like only a foot or two) the light dimmer won't be able to go low enough and may self destruct from excessive current, or it may light the wire up orange hot and snap it.

What I recommend for a power supply is a bell transformer. They are available at your local Kmart, Walmart, or Home Depot for about 15 dollars, and are intended for doorbells. They put out 24 volts nominal, and they can be operated with most light dimmers to vary the current. I haven't found a light dimmer yet that doesn't vary a transformer. This despite the warnings that you shouldn't use a dimmer with a transformer. (But don't try to vary the speed of your ac-adapter powered tape recorder with this arrangement.) The bell transformer also isolates the resistance wire voltage from line current so that if you do come in contact with the wire or it's terminals that you aren't a short circuit to ground. Also, there's less voltage there to hurt you should you happen to touch both ends of the wire at once. I don't mean to be simplistic about safety, just extra cautious, as I am not a professional, but a hobbyist.

An alternative to bell transformers: I use transformers that come out of old power supplies for radios, office equipment, etc. These can have varying voltages, allowing for longer wires for cutting, etc. If you do end up needing a higher voltage for longer cuts, just gang up two or more bell transformers in series. Or, need more current? Gang them up in parallel. Just make sure you get the polarity right. No, transformers don't have a plus or minus, but when you have two windings (four wires) they do have polarity with respect to each other.

Wire Temperature

The cutting wire does not get red hot to cut foam. It does get hot enough to smoke, however. The power supply ideally should be capable of heating up the wire well above the cutting temperature, and the reason is covered here. Getting the wire red hot isn't all bad. I find that being able to heat the wire up well beyond cutting temperature is helpful in cleaning the wire. When the wire gets gummed up with residue, it can easily be cleaned by heating up the wire till it starts to smoke, then wipe a cloth over it and it comes nice and clean. The trouble with residue is that it leaves little blobs on the finished cut. However, getting the wire red hot takes the temper out of it, and too much red heat on your cutting wire will fatigue it and cause it to fail, possibly at an important stage of a complicated cut.

Cutting Speed

The speed that your cutting wire moves through the workpiece is noteworthy. If you try to cut too fast, the wire will make a distorted cut because it is pulled into an arc in the midsection of the workpiece. If you cut too slow, the wire will cause the foam to shrink back from the heat, and also droplets of melted foam will adhere to the wire and leave hard little knots in the surface of the cut. It is important to understand what the wire is doing as it cuts foam.

Some important events take place when the wire initially heats up, when you start into the workpiece, and when you come to a corner or a detail in your stencil. One thing I have discovered is that after you turn on your power supply and dial it to the desired heat setting, the wire has already gotten too hot. But that very same setting is too cold once you've cut an inch or two and cutting goes SLOW. This is a problem that I have been thinking about for sometime, and am on the verge of an invention with.

The Bow

Taut-hot-wire foam cutting requires some kind of bow, or hoop, or frame, to keep the wire tight and straight. These can be any thing your imagination can cook up. It should meet a few simple requirements, however, to make the technique as trouble free and practical as possible.

The bow needs to allow wiring or other system of electrical conduction to the wire, to heat it, of course.

The bow needs to exert some tension on the wire to keep it straight.

The bow needs to have sufficient throat to accommodate desired depth of cuts.

The bow needs to be able to be handled, or to be fixed to a stationary surface in such a way to allow the workpiece to be moved across it.

If you do much foam cutting, you will find that soon you have an array of bows, each for a slightly different type of cut. The bow that I prefer to use for small lost foam patterns has a table that keeps the workpiece square with the cutting wire. This allows nice square cuts, perpendicular to the flat surface of the foam sheet. The next section will cover shaped wire cuts, which have their own special type of wire holder, different from the bow in that it does not exert tension to the wire, but rather holds it in a fixed position, maintaining the intended shape of the wire.

Shaped Wire Cutting

This technique was employed in the making of the step pulley patterns. Here, stiff, heavy wire was bent into a shape that was to be applied to the workpiece: V-grooves. The factors that need to be considered here are the actual shape desired from the wire shape (they're not equal), and the cutting resistance of the foam. There may be other factors, as well. Almost any hot wire cut will show signs of the foam shrinking back from the hot wire. This may mean that in order to cut a 1/4" groove, you may need to bend up a wire shape that reflects more of a 3/16" groove. Bending wire shapes to cut a predetermined shape in foam is an art, and I have not mastered it, but have certainly had a lot of fun experimenting with it. That's all for now, folks!