Master Pattern Allowance — Wooden Master for Casting an Aluminium Pattern (to be Used Later for Grey Iron) An aluminium pattern is to be produced by casting it from a wooden master pattern. What shrinkage allowance should be provided on the wooden master (considering the aluminium pattern casting step)?

Introduction / Context:
Patternmaking often uses a wooden master to cast a durable aluminium pattern. The allowance to be applied on the wooden master must correspond to the shrinkage of the aluminium pattern casting itself, not the final grey iron casting that will later be produced using the aluminium pattern.

Given Data / Assumptions:

• Step 1: Wooden master → Aluminium pattern (casting process).
• Step 2: Aluminium pattern → Grey iron castings (later production use).
• Typical linear shrinkage values (approximate): aluminium ~13–16 mm/m, grey iron ~10 mm/m, steel ~20 mm/m, magnesium ~26 mm/m.

Concept / Approach:
The allowance added to the wooden master must offset the contraction that occurs when molten aluminium solidifies to form the aluminium pattern. Therefore, choose the value appropriate for aluminium. Many shop tables round to convenient values; 16 mm/m is a commonly adopted allowance for aluminium patterns to ensure the produced aluminium pattern meets nominal dimensions for later service.

Step-by-Step Solution:
Identify target: compensate shrinkage of the aluminium pattern casting (not grey iron).Consult typical allowances: aluminium ≈ 13–16 mm/m.Among choices, the correct practical value is 16 mm/m.Therefore, select 16 mm/m for the wooden master.

Verification / Alternative check:
Patternmaker charts often specify aluminium at ~1.3–1.6%; selecting the higher standard value ensures safe tolerancing in production.

Why Other Options Are Wrong:
10 mm/m suits grey iron, which is not the metal of the pattern casting step.20 mm/m is typical for steel and would oversize the aluminium pattern.26 mm/m corresponds to magnesium alloys, not aluminium.6 mm/m is too small for aluminium and would under-compensate shrinkage.

Common Pitfalls:
Confusing allowances for the pattern metal with those for the final casting alloy.

Final Answer:
16 mm/m