Fettling: What Happens After the Mould Is Broken Open

Breaking a mould open early to see the casting is the commonest impulse and one of the commonest ways to ruin one. A casting cooling in sand cools slowly and evenly; exposed to air it cools fast on the outside and can crack. Cast iron is the extreme case: cooled quickly it forms hard white iron at the surface that cannot be machined at all, and the whole point of a grey iron casting is lost. Slow cooling in the sand is what makes it grey. Wait until it can be handled with gloves. That is usually far longer than it feels, and the casting is not improving in any way by coming out early — it is only at risk.

Saw or cut the runners and risers off rather than snapping them. A break can run INTO the casting, taking a piece of the part with it, and that is not repairable on anything that matters. Brittle alloys — grey iron, some bronzes — can be struck off cleanly where a notch has been cut first to control where the fracture runs. Ductile ones must be sawn or cut. Leave a few millimetres proud and dress back to the surface afterwards. Cutting flush in one operation risks going into the casting, and a raised stub is far easier to remove than a gouge is to fill.

Flash comes off with a file or a grinder, following the shape the casting was meant to have. The temptation is to grind until the line disappears, which takes the casting's form with it. Work with the eye and the hand on the FORM, not on the defect. A flat file on a curved surface produces a flat, and on a cast form that flat is more visible than the flash line was. Sand is abrasive and it blunts everything quickly. Wire-brush the casting before filing it, and accept that fettling files are consumables — or use a grinder first and files only for the finish.

Measure the finished casting against the drawing. A systematic difference from the intended size is a shrinkage allowance that was wrong, and that is a pattern correction rather than a process one. Weigh the casting and the gating separately. That gives the YIELD — casting weight over total poured — which is the number a foundry actually optimises, and 50 to 70% is typical for sand casting. Then record everything: alloy, pouring temperature, sand condition, gating layout, what came out and what was wrong with it. Every rung in this batch ends at that notebook, because a casting process is reproducible only to the extent that it is written down.