pattern

In foundry work, a pattern is a full-sized wooden model built to the exact external form of the final metal casting. The pattern sits in a moulding flask filled with compacted sand, leaving a cavity when withdrawn. Molten metal poured into this cavity reproduces the pattern's shape in iron, brass, aluminium or other alloys. The pattern itself never touches the metal; it is purely a temporary shaping tool, sacrificed through removal rather than consumed in the pour.

Patterns are almost always made from wood, typically softwoods like yellow pine or mahogany selected for ease of carving and modest cost. Hardwoods or maple appear where fine detail or wear resistance matters. Larger castings require patterns split into multiple pieces, each registered to the next by wooden dowels and matching parting surfaces. A pattern for a 200-pound casting may itself weigh 50 to 60 pounds and come as three or four separate sections that fit together precisely before moulding begins.

The pattern must be slightly larger than the desired finished part to account for metal shrinkage as it cools. This shrinkage allowance, typically one-eighth inch per foot for iron, is built into the pattern dimensions from the start. Bosses, ribs, and thick sections of the pattern may need to be undercut or fitted with loose pieces called drawback cores to permit withdrawal without tearing the sand mould. A skilled pattern maker anticipates how the mould will separate and designs each piece to pull cleanly from the sand.

The Pattern Maker's Role

Pattern making is a specialised trade requiring deep knowledge of metal behaviour, sand moulding mechanics, and woodworking. A pattern maker must read a casting drawing and add allowances for shrinkage, draft angles (tapers that aid withdrawal), and core prints (projections that locate the sand cores inside the mould). The drawing alone does not tell the story; experience determines whether a particular cavity will tear, whether a section will hold together, and where extra thickness or reinforcement is needed in the wood.

Once a pattern has been used to produce its first metal casting, it enters a cycle of wear and repair. Repeated pressing into fresh sand causes splintering at edges and erosion of fine details. Patterns are refinished, reinforced, and sometimes faced with strips of harder wood or thin metal sheets where sand abrasion is heaviest. A production pattern for a common part may produce dozens or hundreds of castings before it is finally retired, making its initial cost a minor fraction of the total metal output.