For sand-casting patterns, what is the typical range of draft (taper) allowance specified per metre of pattern height to enable easy and clean withdrawal from the mould without damaging the sand?

Introduction / Context:

Draft (taper) on patterns prevents the mould walls from scuffing and collapsing when the pattern is withdrawn from the sand. Correct draft reduces rework and improves surface quality.

Given Data / Assumptions:

• Green-sand moulding with typical permeability and strength.
• Average surface finish and standard stripping techniques.
• Moderate pattern height and no severe undercuts.

Concept / Approach:

Draft allowance is commonly specified as millimetres per metre (or degrees). For general work, a draft of about 1–2 mm per metre (roughly 0.06°–0.11°) serves as a baseline, with larger values for rougher sands, deeper pockets, or vertical surfaces without rapping.

Step-by-Step Solution:

Verification / Alternative check:

Foundry handbooks list similar ranges, often converting to degrees. The exact value depends on process capability, but 1–2 mm/m is a widely cited general allowance.

Why Other Options Are Wrong:

• 2–5 mm/m and 5–10 mm/m: possible for difficult geometries but higher than typical general practice.
• 10–15 mm/m: excessive for ordinary sand casting surfaces.
• 0.1–0.5 mm/m: usually insufficient to guarantee clean withdrawal.

Common Pitfalls:

• Confusing degree values with mm/m; both are used but must be consistent.

Final Answer:

1 to 2 mm/m