The studs would then be welded into the the slots, so the gap is held together by flexible mild steel studs rather than a weld with a crack in it.This is the door to the tool compartment and seemed more than strong enough after the studs had been welded in place.Preparing the gearbox casing for welding took the best part of an afternoon.I made studs to hold the major parts together, then prepared the joints in a V shape.Cast iron can be welded perfectly well using a stick welder and nickel rods, or with pre-heating by a gas welder using cast iron rod. It can be done, and without too much difficulty, although the resulting join won't be as strong as the more accepted methods. It's a tough project for a first cast iron weld as the alignment has to be spot on for all the gears to mesh correctly. It had been dropped from a forklift and a couple of the castings were broken. This is the gearbox used to motor the bed backwards and forwards for thread cutting.
In fact the two parts of the casting fell apart when the weld had cooled.Cast iron is stiff and brittle so either the casting or the weld will tend to crack.I tried MIG brazing wire instead of mild steel wire.Braze is much softer than mild steel welding and hence is less likely to crack on cooling.Even so it has to be done in small lengths and then allowed to cool enough to touch before welding any more.The casting needs to be hot enough so that the flux flows ahead of the braze (the flux is visible as a silver shimmering on the surface). There is more about brazing on the MIG brazing page.With the braze ground down the finished job looks quite neat.It was really cold in the workshop, so I pre-heated the casting with a blowtorch to about 60 degrees Celsius before welding.Here the studs have been welded in place, and two triangular pieces have been mig welded on two sides. I had been tempted to complete the welding using mild steel wire.I left the original fracture for about a third of the thickness to align the broken parts.If the weld between the stud and the casting bridged the casting then the weld would crack.