Friday, August 7, 2009

Lost Foam Casting - 1 Foam Cutter

All of this treatise on lost foam casting of larger pieces will begin with the same title so, if you are not interested, you can skip these blogs.

The first thing one needs is a foam cutter. A hot wire is superior to cutting, sanding, or grinding foam to a desired shape. It leaves a clean, sealed edge The only downside are foam vapors which can not be good for one's health so I wear a mask when I cut.

There are tons of plans for cutters on the web and the prevailing aesthetic seems to be to make them as much as possible with scrap from your shop. You need a power source, a wire that can get hot, and a way to hold it together.

Power source: A/C is dangerous although some folks use bell transformer. I had a computer power supply that had enough power but shut off as soon as I connected it. My theory is that it was smart enough to sense a short circuit and shut off to not damage the computer. I wound up buying an inexpensive battery charger that puts out 10 amps at 12 Volts. Beware, the more expensive automatic shutoff models do not work because you can not control them. Get the less expensive fixed charge model that produces at least 8-10 amps. I have seen very inexpensive models that put out only 2 or 4 amps and they are not enough.

Wire: Since I weld, I have MIG wire available. Inexpensive, easy to replace, lasts a surprisingly long time. If you do not weld you can get a small spool at Home Depot. I use .023 or .025 wire; .030 works but needs more power to get hot enough so I prefer thinner wire. You do not need expensive wire such as nichrome.

Hear Control: A light dimmer works fine with the battery charger.

Assembly and wiring: As the wire gets hot, it will stretch. In order to keep it tight, the top arm of the cutter is on a pivot (a bolt) and there is a spring at the back to keep it pulling upwards. The bottom picture shows this assembly and a black alligator clips from the battery charger. The red clip is connected to the bottom of the post to a piece of lamp cord that runs underneath the base.

Lamp cord runs from the top out across the arm, and is wrapped around a bolt - you can see it in the top picture. The MIG wire is twisted on to the other side of the bolt. At the bottom there is a hole in the base for the MIG wire to pass through to connect around another bolt to the lamp cord.

To provide clearance for the wiring on the bottom, I glued some wood strips to raise it about an inch.

Bill of Materials: scrap wood; four bolts and nuts; battery charger; light dimmer; outlet box for light dimmer; about 4' of lamp cord; plug; MIG wire.

Design Feature and Flaw. In the middle picture you can see that the top arm is angled. The spring is off to the side so it tilts the arm. As a result the wire is not perpendicular to the base. I will fix this some day but it is only an issue when cutting a very thick piece of foam.

The feature can be barely seen in the bottom photo - a line of holes extending at 1/2" intervals from the wire to the front of the base. These holes are just big enough for a small finish nail. When I want to cut a circle, I run the nail through the center of the foam and put it in the appropriate hole. Turn on the power, rotate the foam, and you have a nice disc of foam.

1 comment:

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