The Pattern Is Not the Part: Draft, Shrinkage and Machining Allowance

Every surface that runs perpendicular to the parting plane must be tapered so the pattern lifts cleanly. A vertical wall drags sand with it and leaves a torn mould, and that tear becomes a lump on the casting. One to three degrees is usual for sand casting, more for deep pockets and less for shallow ones. The rule of thumb is that a deeper draw needs more draft, because a small angle over a long distance still leaves the sand gripping. Draft always ADDS material away from the parting line, never removes it. A part that must be a precise size at the bottom of a deep feature has to be machined there, because draft is not optional and it is always in the direction that makes the casting thicker.

A cast surface is rough, slightly scaled, and not accurately placed. Any face that has to be flat, parallel or dimensionally exact must be machined, and the pattern has to carry extra metal there for the tool to remove. Two to five millimetres is typical for sand casting, and more on large surfaces and on the cope side — the upper half of the mould collects more inclusions, so critical machined faces are placed in the drag where possible. Do NOT add it everywhere. Extra metal is extra weight, extra cost, extra shrinkage to feed, and extra machining time. Deciding which faces are machined and which are left as-cast is a design decision made before the pattern is started.

Every scratch, dent and grain on the pattern is reproduced in the sand and then in the metal. A pattern sanded smooth and sealed gives a smooth casting; a rough one gives a rough one, and rough castings are harder to clean and harder to machine. Seal and wax wooden patterns. Bare wood absorbs moisture from green sand, swells and changes size, and it also grips the sand instead of releasing. Fillet every internal corner. A sharp internal corner in the pattern makes a sharp sand edge that breaks off into the casting, and it makes a stress concentration in the finished part — the same argument as a fillet in a printed part or a forged one.