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Crucible

The next item required is a crucible.

I decided to machine one from solid bright mild steel stock with overall dimensions of about 80 mm OD and 100 mm high with a wall thickness of about 4 mm.  While the steel will not melt in this furnace repeated firings will cause mill scale which will thin the walls over time.  Thick walls also retain heat while transferring the melted charge to the mould.

The only piece of steel I could find for the crucible was 100 mm in diameter and 170 mm long without purchasing a stock length.  This is enough for two slightly smaller capacity crucibles with the dimensions adjusted to suit the stock.  This reduced the crucible capacity to 0.25 litres (about 0.65 kg of aluminium).   While I could increase the crucible diameter this will restrict the flow of hot gas between the crucible and the furnace walls.

 

Crucible6

Figure 1.  Steel Crucible
(Note the flame impingement mark on the crucuible.)

 

An alternative (and somewhat simpler) construction technique is to weld a plate on the end of an appropriate diameter steel pipe.  I might try this technique in the future because machining from solid is right at the limit of by boring bar and turns an awful lot of perfectly good steel into swarf.

Some folk are using stainless steel containers as crucibles modified with lifting eyes.  While these are relatively low cost they have very thin walls.  This increases their charge capacity but they will not retain heat so require relatively efficient pouring operations.

The crucible needs to be securely lifted from the furnace and tilted for controlled pouring.  I settled on two 5 mm diameter holes near the top edge of the crucible as lifting eyes and pouring pivots.  Tilting the crucible was achieved with a simple steel ring attached to a wooden handle which allows good pouring control and an appropriate degree of safe handling for molten metal.  I also made a simple dross lifter.

 

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Figure 2.  Handling Tools

 

Now we need some green sand.  Find out a bit about sand by clicking here.

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