Patternmaking data — shrinkage allowance for cast iron When laying out a pattern for a gray cast iron component, what approximate linear shrinkage allowance per metre should be provided to compensate for solidification and cooling?

Introduction / Context:
As molten metal cools from pouring to room temperature, it contracts. Patternmakers add shrinkage allowance so that the resulting casting achieves the specified final dimensions. Knowing typical allowances by alloy helps avoid systematic undersize castings and costly rework.

Given Data / Assumptions:

• Alloy: gray cast iron.
• Linear shrinkage considered; no special chills or unusual section thickness effects.
• Standard patternmaking practice with wooden or metal patterns.

Concept / Approach:
Gray cast iron exhibits relatively low linear contraction compared with steels and some nonferrous alloys because of graphite expansion during solidification. A common rule-of-thumb allowance is about 1% on linear dimensions, i.e., roughly 10 mm per metre. Actual values may vary slightly with composition, section size, and mold material, but 10 mm/m is a widely taught nominal figure.

Step-by-Step Solution:

Verification / Alternative check:
Patternmaker's scales for cast iron are commonly graduated at 1.01 actual to automatically provide 10 mm/m during layout.

Why Other Options Are Wrong:

• 16, 20, 26 mm/m: closer to steels and some bronzes; too high for gray cast iron.
• 5 mm/m: too low; risks undersize castings after cooling.

Common Pitfalls:
Mixing up shrinkage allowance with machining allowance; using one value for all alloys regardless of graphite effects.

Final Answer: