TABLE FOUNDRY | MODEL CONSTRUCTION

Somehow in the course of working on one invention I needed a melting device with a wide range of heating, which would be convenient to use at home. I tried various options and electrospinning gas heaters and made sure that neither one nor the other suited to the task: they were either bulky and inconvenient to use, or didn’t provide enough heat. Here, I think, would be the electric arc melting, but operating at a slower pace!

So came the idea to use for this purpose carbon-graphite powder that is poured between two same carbon-graphite working electrodes to which is supplied a voltage in the range from 25-50 In a sufficiently powerful (such as welding) transformer. Due to the existing ohmic resistance of the graphite powder is gradually intense heat. The temperature in such furnaces can reach up to 3000 °C, which gives the ability to melt almost all metals (in small portions). Despite such an impressive heat inside the oven, the outer layer of carbon and graphite powder remains dusky or reddish in color, so blinding glow, as is the case for arc welding, furnace is not coming. The heating time of the furnace varies in the range of 3-5 minutes, which makes it easy to monitor and control the smelting process, including disconnecting from the mains transformer. As the metal melts a little, it especially does not spread inside the furnace and the powder well enough to keep its shape.

The furnace is made from simple and readily available materials: graphite, mica and asbestos tiles. Due to the fact that the asbestos on medical grounds is prohibited and is a rarity, it is possible to replace a tile or cement tile.

The kiln dimensions are not strictly defined. It all depends on the capacity of the mains and the output voltage of the transformer. The more the output voltage, the wider must be the distance between the electrodes. When the size of the furnace, as indicated in the drawing, it is sufficient to apply electrodes 25-30 volts: the oven is heated in a smooth mode, but pretty intense. In the case of the welding transformer industrial design, which usually gives 50-60 volts, the distance between the electrodes should be increased by about half, 150-200 mm. In volume of the furnace shown in the drawing (100x65x50 mm), you can melt 60-80 grams, for example, silver, which is already considered a good result.

Step-down transformer is 25 volts, and connection diagram of the furnace. The network contains a winding of 620 turns of copper enameled wire with a diameter of 1 mm. Decreasing winding has 70 turns of period rectangular seonie 4,2×2,8 mm in Steklovolokno isolation.

An electric melting furnace:

1 — powder pregrevica; 2 — the place of metal smelting; 3 — wire hookup of the furnace body; 4 — a lining of mica; 5 — tile asbestos; 6 — carbon-graphite electrode; 7 — wire contact.

As electrodes for furnace suitable brush from the powerful motor. They are convenient because you have a good contact patch. If it is not possible to get such electrodes, they are easy to cut yourself from a piece of graphite, for example, used the rod electrode used in furnaces tugoplavkikh. In a makeshift electrode it is only necessary to drill two side holes with a diameter of 5-6 mm, insert them stranded copper wire with a thickness of 5 mm and seal carefully to score here any nail. On the inner side of the electrodes is a mesh-notch with a file to improve contact with the graphite powder.

As the inner lining layer of the furnace wall is applied mica: thanks to its webbing, it serves as a good heat-insulating screen. The outer walls are further reinforced or asbestos cement tile with a thickness of 5-10 mm. For the extreme simplicity of Assembly of the wall tie or soft copper binding wire. Insulating stand for oven is an ordinary brick; the bottom is still fit enameled metal pan with sides.

Carbon and graphite powder can be obtained from the end terminals by using a coarse file or multiblade saws for metal. We must remember that during the melting process graphite powder still gradually fade and it is necessary periodically to pour.

Assembled oven is connected to the transformer thick enough copper wire (7-8 mm) with mandatory outer insulation to avoid during operation, accidental short circuit.

Ready to work first as oven should be heated to allow to burn out organic inclusions (while ensuring proper ventilation in the room). In the future the furnace works virtually without the presence of soot and fumes.

Melting of metals is carried out as follows. First, using the small blade in the middle of the furnace in powder, make a hole, put it in the first portion of the metal and buried. If used scrap of various sizes, first place the largest piece, and only after melting it add small pieces.

To make sure that the metal is melted, the furnace may be lightly shake — the powder surface in this case also begins to sway. After cooling of the metal turned over and melted again. This is repeated several times until the workpiece will not accept more or less spherical form, certifying the quality of the melt.

When it is necessary to melt small chips or sawdust simple metals, they fall asleep right in the hole and melt as usual. More precious metal for safekeeping, placed in a glass vial under medication and melt along with it. Formed at the melt crust of glass sprinkled easily when cooling in the water.

Low-melting metals tin, aluminum and the like — is better placed in an iron Cup.

‘ To get the alloys first put in the powder, a refractory metal, and then melting is injected fusible. For example: copper + tin, copper + aluminium.

In the electric furnace to smelt tin, aluminum, iron, Nickel, copper, silver, gold, palladium. After melting the resulting blank are forging. They need to rashlebyvat on the anvil slowly, especially in the beginning, a small hammer. And as often as possible to heat the workpiece on a gas stove red-hot, then cool in cold water and then rashlebyvat to the desired size.

It is categorically impossible to melt magnesium, lead, cadmium, zinc and zinc alloys (zinc brass, cupronickel) as well as silver contacts from various types of relays, instruments, starters — they contain up to 50% of cadmium, which burn out, forming a yellow toxic smoke.

If you can not buy a powerful transformer, it can be replaced with composite. You need to take a slightly less powerful single-type transformers and in parallel to combine their output windings (assuming they are all designed for the same voltage). Possible and a homemade transformer. It is assembled from l-shaped permalien plates with an inner cross section 60×32 mm. Its network coil is wound enameled wire 1 mm thick and contains 620 turns. The reduction ratio of winding is wound with wire of rectangular section 4,2×2,8 mm and contains 70 turns.

With regard to safety when working with this oven, it is necessary to remember that welding transformer requires very careful handling. We can not allow there is a short circuit in the wires or between the electrodes in the furnace itself. The mains switch of the transformer should be placed close to any second it can be turned off. It is also a moment to leave a working stove unattended. There always must be a container of water, where we chill in the hot workpiece.

POPOV, Krivoy Rog, Ukraine

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