Table smelter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / welding equipment

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Once, while working on an invention, I needed a melting device with a wide range of heating, which would be convenient to use at home. I tried different versions of gas and electrospiral heaters and made sure that neither one nor the other did not meet the task: they turned out to be either bulky and inconvenient to use, or did not provide sufficient heating. Here, I think, there would be electric arc melting, but operating at a slower pace! So the idea came up to use carbon graphite powder for this purpose, which is poured between two working carbon graphite electrodes, to which a supply voltage of 25-50 V is supplied from a sufficiently powerful (such as a welding) transformer. Due to the existing ohmic resistance in the graphite powder, gradual intense heating occurs. The temperature in such an electric furnace can reach up to 3000 ° C, which makes it possible to melt almost all metals (in small portions).

Despite such an impressive heating inside the furnace, the outer layer of carbon-graphite powder remains darkish or reddish in color, so that a blinding glow, as is the case with electric arc welding, does not come from the furnace. The heating time of the furnace varies in the range of 3-5 minutes, which makes it easy to control and manage the melting process, turning on and off the transformer from the network. Since the metal melts a little, it does not particularly blur inside the furnace and the powder holds its shape quite well.

The electric furnace is made from simple and quite affordable materials: graphite, mica and asbestos tiles. Due to the fact that asbestos is banned and becoming rare for medical reasons, it can be replaced with tile or cement tiles.

The dimensions of the furnace are not strictly defined. It all depends on the power of the existing electrical network and the output voltage of the transformer. The higher the output voltage, the wider the distance between the electrodes should be. With those dimensions of the electric furnace that are indicated in the drawing, it is enough to apply 25-30 volts to the electrodes: the furnace heats up smoothly, but quite intensively. In the case of using an industrial design welding transformer, which usually produces 50-60 volts, the distance between the electrodes should be approximately doubled, up to 150-200 mm. In the volume of the furnace shown in the drawing (100x65x50 mm), 60-80 grams of, for example, silver can be melted, which is already considered a good result.

Brushes from a powerful electric motor are suitable as electrodes for the furnace. They are convenient in that they have a good current-carrying flexible wire. If it is not possible to get such electrodes, it is easy to cut them out yourself from a piece of graphite, for example, from a used electrode rod used in arc furnaces. In a home-made electrode, you just need to drill two holes with a diameter of 5-6 mm from the side, insert a stranded copper wire 5 mm thick into them, and carefully drive another suitable nail here to seal. On the inner side of the electrodes, a mesh notch is made with a file - to improve contact with graphite powder.

Mica is used as the inner lining layer of the furnace walls: due to its layering, it serves as a good heat-insulating screen. The outer walls are additionally reinforced with asbestos or cement tiles 5-10 mm thick. For the utmost ease of assembly, the walls are tied with soft copper or knitting wire. An ordinary brick serves as an insulating stand for the furnace; an enamelled metal pallet with sides is laid under the bottom.

Carbon graphite powder can be obtained from spent rods with a coarse file or a multi-blade hacksaw. It should be taken into account that in the process of melting the graphite powder still gradually burns out and it must be added periodically.

Rice. 1. Electric melting furnace (click to enlarge): 1 - carbon graphite powder; 2 - place of metal melting; 3 - wire-tying of the furnace body; 4 - mica lining; 5 - asbestos tiles; 6 - carbon graphite electrode; 7 - current-carrying wire.

Rice. 2. 25 volt step-down transformer. The network winding contains 620 turns of enameled copper wire with a diameter of 1 mm. The lowering winding contains 70 turns of rectangular wire 4,2x2,8 mm in fiberglass insulation.

The assembled furnace is connected to the transformer with sufficiently thick copper wires (7-8 mm) with mandatory external insulation to avoid accidental short circuits during operation.

The oven ready for operation is first warmed up properly to allow the organic inclusions to burn out (while ensuring adequate ventilation in the room). In the future, the furnace works practically without the release of soot and burning.

Melting of metals is carried out according to the following scheme. First, with the help of a small spatula in the middle of the furnace, a hole is made in the powder, the first portion of the metal is placed in it and buried. If the scrap used is of different sizes, then the largest piece is placed first, and only after it is melted are small parts added.

To make sure that the metal has melted, the furnace can be shaken slightly - the surface of the powder in this case also begins to sway. After the metal has cooled, it is turned over and melted again. This is repeated several times until the workpiece takes on a more or less spherical shape, indicating the quality of the melt.

When it is necessary to melt small chips or sawdust of simple metals, they are poured directly into the hole and melted as usual. The more precious metal, in order to preserve it, is placed in a glass ampoule from under the medicine and melted along with it. The glass crust formed in the melt is easily sprinkled upon cooling in water.

Low-melting metals - tin, aluminum and the like - are best placed in an iron cup. To obtain alloys, a more refractory metal is first put into the powder, and after its melting, a fusible one is introduced. For example: copper + tin; copper + aluminum.

Tin, aluminum, iron, nickel, copper, silver, gold, palladium can be melted in an electric furnace. After melting, the resulting blanks are subject to forging. They must be riveted on the anvil slowly, especially at the beginning, with a small hammer. And as often as possible, heat the workpiece on a gas stove red hot, then cool it in cold water and rivet again to the desired size.

It is strictly forbidden to melt magnesium, lead, cadmium, zinc and zinc-containing alloys (zinc brass, cupronickel), as well as silver contacts from various types of relays, devices, starters - they contain up to 50% cadmium, which burns out, forming yellow poisonous smoke.

If it is not possible to purchase a powerful transformer, then it can be replaced with a composite one. To do this, you need to take several less powerful transformers of the same type and connect their output windings in parallel (provided that they are all designed for the same voltage). A homemade transformer is also possible. It is assembled from L-shaped permaloy plates with an internal section of 60x32 mm. Its network winding is wound with enameled wire 1 mm thick and contains 620 turns. The lowering winding is wound with a rectangular wire 4,2x2,8 mm and contains 70 turns.

As for safety precautions when working with this furnace, it must be remembered that the welding transformer requires extremely careful handling. A short circuit must not be allowed to occur in the wires or between the electrodes in the furnace itself. The transformer network switch should be located nearby so that it can be conveniently turned off at any second. It is also impossible to leave a working furnace unattended for a minute. There should always be a container with water nearby, where hot workpieces are cooled.

Author: A.Popov, Krivoy Rog, Ukraine

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