Patterns, pattern making and core boxes
Domain 3 - Patterns, pattern making and core boxes
This domain tests pattern allowances, pattern types, pattern materials and core-box design. The exam expects you to convert a finished casting dimension into a pattern dimension and to choose a pattern for a given shape and production quantity.
Allowances
- Shrinkage or contraction allowance: added because metal contracts on solidification and cooling; grey iron about 1%, steel about 1.5–2%, aluminium about 1.3%.
- Machining allowance: added where surfaces will be machined; depends on casting size and finish.
- Draft or taper: 1–3° on vertical faces so the pattern draws without tearing sand.
- Distortion allowance: for U- or V-shaped castings that warp.
- Shake or rapping allowance: negative, because rapping enlarges the mould cavity.
Pattern types and core boxes
Solid, split, match-plate, cope-and-drag, loose-piece, sweep and skeleton patterns suit different shapes. Wood is easy to shape but wears; metal patterns last longer for production. Core boxes include half-core, split, dump and gang boxes. Core prints locate the core; core-box dimensions must include core shrinkage.
Worked example
A steel casting is 400 mm long and needs 4 mm machining on each end. Shrinkage allowance is 2%. Add machining first: 400 + 8 = 408 mm. Apply shrinkage: 408 × 1.02 = 416.16 mm. The pattern length is therefore about 416 mm. Applying shrinkage only to 400 mm and then adding 8 mm gives 416 mm as well here, but the correct principle is to shrink the final casting size, including machining allowance.
Traps
Draft is added to the pattern, not subtracted. Do not apply a positive shake allowance. A core box is not made to the core’s finished size; it must allow for core shrinkage and core-print clearance. Match-plate patterns need the plate thickness and guide pins considered. Record the allowance direction on the drawing.