The photo 10 shows the worm gear after cutting all the teeth. Because of the chuck overhang, the gear was supported on the underside, with a clamp bolted down onto the milling table to resist the downward pressure of the cutter. Notice a feeler gauge inserted under the clamp to take up the slack between the top of the damp and the periphery of the worm wheel. The next item tackled was the base casting. In the original article, Mr Tardrew's casting was made of aluminium, but I didn't have enough aluminium for such a casting. My castings were made of zinc with approx. 10% aluminium added to hopefully improve the quality of the casting and to reduce shrinkage cracks forming as the metal cooled. The source of the zinc was from old car wheel balance weights from the local tyre fitting centre. Some of the weights were zinc marked Zn, some were marked Zk which are the ones alloyed with aluminium, commonly called Zamak. Technically my alloy should also contain some magnesium and copper to be called Zamak. This alloy melts around 390°C so I used a stainless steel saucepan as my crucible and a 2" propane burner to melt the metal. Don't be tempted to mix lead with the zinc as the castings will slowly degrade over time. In case you are wondering how you melt two metals together with widely differing melting points the answer is simple. Melt the zinc - don't get it so hot, that it starts to vapourise and gives off white smoke, and definitely don't breathe in said white smoke (zinc fumes are dangerous, please take all necessary precautions if you do this - Ed.). An electronic foundry thermometer is useful here. Mine reads up to 1,000 C°. With the zinc molten, stir it with strips or pieces of aluminium. You will discover that the aluminium dissolves into the molten zinc, like sugar in your coffee! Simples!!
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