Casting a replacement part: pattern, draft, shrink allowance and machining stock

When you cast a replacement for a broken part, the pattern has to be bigger than the part. Aluminum shrinks about 1.3% (roughly 5/32" per foot) as it cools, so scale the pattern up. Every face that pulls out of the sand needs 1 to 3 degrees of draft. Any surface that must be accurate gets an extra 1/16" of machining stock to clean up later. Fillet every inside corner with wax or putty; sharp inside corners crack.

I 3D print patterns now: model the part, scale 1.013, add draft in CAD, print at 0.2 mm, sand and prime. Petrobond pulls perfect detail from a printed pattern.