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The Construction

 

Once I had the crucibles I could work out the internal size I required for the furnace. I found that an old 10 litre paint tin was the ideal diameter. From that I could then work out, allowing for 3" thick walls, the size of container that I needed. After much searching I found an old metal dustin was ideal. I cast 3" of refractory cement in the base, then cut a hole about 2" above this for the gas inlet. The paint tin was then placed in position and the cement packed around this and the gas inlet tube. As I worked my way up I was careful to keep moving the tin and the tube so that I could finally remove these when the cement was dry. I cast a lid in a plastic tube, again I found a suitable tube to form the vent and cast metal rods to give this strength. I used the last of the cement to cast some small fire blocks which would fit into the base of the furnace to stand the crucible on.

Once the cement was packed in it was left for several days to dry out and work continues on the heating system. It was decided to run the furnace on high pressure propane. A tube was inserted through the hole cast in the side of the furnace and measured. The gas jet was made out of a length of 1/4" brake pipe. The end was slightly hammered to create a jet, the pipe bent to angle the jet towards the furnace and brazed into the larger tube. The gas supply from a high pressure regulator was connected by flexible hose. It was originally planned to use an old vacuum cleaner motor to provide the "blow" but this could not deliver enough air. Experiments were made using the workshop compressor and the best result was found by creating a venturi effect by placing a jet of compressed air at about 40psi about an inch from the end of the pipe.

The furnace was first lit and allowed to burn for short periods without the forced air, this was to assist in the drying and curing of the refractory cement. The lid was placed on after a few firings to speed up the process. This allowed the furnace to eventually to be brought up to operating temperature without cracking or spalling.

The tools necessary for handling the crucible were manufactured. These were lifting tongs, made from two flat bars bolted as scissors and then twisted with two curves welded to the end with appropriate radii to grip the crucible. These were also found to be suitable for removing and replacing the lid. The pouring ring was made from a flat bar, bent to a circle to fit under the shoulder of the crucible and welded to a long T bar.

The furnace was finally fired up and a charge of aluminum was added to the crucible. This was scrap alternator bodies that I had been given. It took about 15 minutes to melt enough aluminum to cast 3 ingots 1"x2"x8". These was quite a lot of dross that had to be removed from the surface of the melt before I could start the pour. I put this down to using scrap.

I have found that on subsequent melts that I can get the charge melted in about 10 minutes and that if I use the cast ingots the metal is much cleaner and there is less dross to be removed.

 

The next stage is to experiment with brass, again I will probably start with scrap, plumbing fittings and such like. With the speed that this furnace can melt aluminum I see no reason why it should not reach the higher temperature required for brass within a reasonable time. I have heard reports that this type of furance can attain the temperature required for cast iron, maybe one day I may try that.

One important thing to remember is safety... leather apron, gloves, eye protection and boots are a must before starting work. No matter how careful you are, accidents can happen.