Pattern allowances — shrinkage practice: During pattern making for casting, the shrinkage allowance is applied by adding to external dimensions and subtracting from internal (hole) dimensions. Evaluate this rule of thumb.

Introduction / Context:
All cast metals shrink on solidification and cooling to room temperature. Pattern allowances compensate for this predictable contraction so that the final casting meets drawing dimensions without excessive machining or fit issues.

Given Data / Assumptions:

• Shrinkage is uniform and predictable for the chosen alloy.
• Pattern dimensions must be adjusted before moulding.
• Machining and draft allowances are considered separately.

Concept / Approach:
External dimensions on the casting will contract from the mould size; therefore, the pattern must be made oversize so that after shrinkage, the casting matches the nominal dimension. Internal features (holes, recesses) will also shrink, tending to close up; to achieve the correct finished size, the pattern for the core or internal cavity must be made slightly smaller (i.e., subtract the allowance) so the resulting hole ends up at the intended nominal size after shrinkage.

Step-by-Step Solution:
For an external length L, pattern length = L + shrinkage allowance.For an internal diameter D, core print/core size = D − shrinkage allowance.This opposite application keeps both external and internal surfaces true to drawing after cooling.Hence, the rule stated is correct.

Verification / Alternative check:
Foundry handbooks list tables of shrinkage allowances and illustrate adding on externals and subtracting on internals to counteract contraction trends.

Why Other Options Are Wrong:
Limiting to aluminium or steel is unnecessary; principle applies across alloys with proper coefficients.

Adding on all dimensions would oversize holes after casting.

Common Pitfalls:
Confusing shrinkage allowance with machining allowance; ignoring directional solidification that can cause nonuniform shrinkage if risering is poor.

Final Answer:
Agree