Pattern Materials — Which Allowance Is Typically Smaller for Metal Patterns than for Wooden Patterns? Considering surface finish and strength of pattern materials, which allowance is generally required to be less when using metal patterns instead of wooden patterns?

Introduction / Context:
Pattern material affects dimensional allowances because of surface finish, strength, and wear resistance. Wooden patterns are more flexible and rougher, while metal patterns (aluminum, brass) are stronger, dimensionally stable, and smoother. This influences how much taper (draft) is needed for clean withdrawal from the sand mould.

Given Data / Assumptions:

• Comparison is for identical casting design, same moulding sand and process.
• Metal patterns are stiffer and maintain sharper edges.
• Allowance definitions: shrinkage (compensate metal contraction), machining (post-cast material removal), draft (taper for pattern draw), distortion (for warpage tendencies).

Concept / Approach:
Smoother surfaces and precise geometry of metal patterns reduce friction against sand during draw, allowing smaller draft angles while still ensuring clean release. Shrinkage and machining allowances depend on the casting alloy and finish requirements, not on pattern material. Distortion allowance relates to casting behavior under solidification, again independent of pattern material selection.

Step-by-Step Solution:
Identify which allowance depends on pattern surface and rigidity: draft.Note that shrinkage and machining are casting-alloy/finish dependent, not pattern-material dependent.Conclude: metal patterns require less draft than wooden patterns for the same mould.

Verification / Alternative check:
Foundry handbooks list recommended draft values; metal patterns consistently allow smaller draft angles owing to reduced abrasion and better finish.

Why Other Options Are Wrong:
Shrinkage: set by metal being cast (e.g., grey iron ~10 mm/m, steel ~20 mm/m).Machining: determined by surface finish tolerance of final part, not the pattern material.Distortion: influenced by geometry and alloy solidification behavior.

Common Pitfalls:
Confusing rapping/shake allowance with draft; rapping can slightly enlarge the cavity but is a separate practice.

Final Answer:
draft allowance