Home made Aluminum Furnace

In order to be able to build face plate discs (to hold the bearings of my H3 Robot wheels), while avoiding some heavy routing work (6 wheels = 12 plates) and lots of wasted aluminum during the process (if done from a solid block), I decided on building an aluminum furnace, so that I can make a mould and then just poor aluminum into it, to roughly produce my face plate discs.

I based my furnace on an old gas container shell. Using a gas container, can be dangerous due to residual gas that is still inside. In order to remove the pressure valve, I had to first remove any remaining gas and oil, very carefully. I let remaining gas escape for a whole day, by just forcing the valve to open slightly, making sure to not produce any spark during the process, due to the risk of explosion. In order to determine when all gas had efectivelly escaped, a small amount of water or other similar liquid (oil for example), can be used, to cover the valve openning. As long as we can see gas bubling through the liquid, it's not done yet. Simple, but efective.

After this process was complete, any remaining oil inside the container was removed by just unscrewing the pressure valve, and letting it flow out.

Once the shell was free of gas, and hence safe to work with power tools, it was time to cut it. To make a lid, the top was cut flat, as high as possible to maximize useful internal work volume. An extra hole was done near the bottom, to provide a viable source for heating the contents of the future furnace.

In order to be easy to close and open the furnace, a large hinge was adapted to the outer shell, allowing to operate the cut out lid.

In order for the furnace to withstand the heat required for melting aluminum (around 600ºC), and to make the melting process more eficient, the interior was filled with a heat refracting clay. Heat refracting concrete is also available and can be used instead of clay.

To accomplish this, first we need to define the required work volume so that this volume can be isolated, so that the clay can be poored between the outer shell and this inner shell that will limit the useful working volume available.

The inner shell, must be made from a material that can withstand the heat that will be generated. Any iron based metal sheet will do, since iron has a much higher melting point (above 1100ºC) than aluminum.

Before pooring any clay into the device, the inner shell needs to also have a hole where an iron tube will connect it to the outer shell, so that heating can be supplied from the outside. The main ideia here, is to slant the heat/flame admission tube, so that it will be relatively tangent to the surface of the inner shell cylinder. The ideia behind this setup is to force heat to spiral it's way up, hopefully providing a better distribution of heat.

The heat admission tube, must be soldered, since it will receive lots of heat when the furnace is working. I arc welded mine, but it was somewhat hard to make it stick and weld well, because the tube was a very different metal from the one used in the container. But In the end it got welded good enough to withstand the furnace working conditions.

The ideal way to apply the clay is to make it as much liquid has possible, without becoming completely fluid. After pooring the clay, it takes some time to dry, since there is a lot of water in it. A possible option is to turn on the furnace, and hope that the clay won't crack, while cooking, which I didn't try because I expected it to form air bubble pockets, that would ruin part of the intended thermal isolation. Instead I opted to let it dry by itself, in the sun. Concrete might be better in this situation, since it "cooks" itself slowly while setting.

However, while pooring clay or soldering the tube, the inner metal sheet, shaped like a cylinder, got a little bit off center. But not enough to make the furnace fail. Something to account for, and correct if I ever refill the heat refracting material

It worked ok, apart from the fact, that all the clay that was placed in the lid, to protect it from furnace heat, just crumbled apart. I somehow expected this to happen, but didn't think much of it. To prevent this, I should have soldered a simple metal net/grid or similar to keep the clay in place.

When it crumbled, I just used the pieces to fill the remaining space that wasn't completely filled before.

Next time, I will show what happened when I fired it up.