Pattern and core box dimensions for a machined cast iron bush
A pattern shop receives an order for 200 grey iron bushes. The finished drawing gives outside diameter 160 mm, bore 90 mm and length 150 mm. Both the outside diameter and the bore are machined: the drawing note allows 4 mm machining allowance on the outside diameter, 3 mm on the bore diameter, and 4 mm on each end face. Grey iron shrinkage is taken as 10 mm/m. The foundry will use a one-piece cylindrical pattern with the bore formed by a core; core prints 40 mm long are provided at each end. The patternmaker proposes to make the pattern at 160 mm diameter × 150 mm long and the core box at 90 mm, arguing that 'shrinkage only affects the outside, and the machining allowance will cover the rest'. He also intends to use the same pattern if the customer later changes the material to 0.35% carbon steel, since 'the shape does not change'.
1. Calculate the correct pattern outside diameter, pattern length and core box diameter, and show each step.
Read the worked answer
Rule: finish size → add machining allowance → then apply shrinkage, because the casting itself shrinks after solidifying.
Outside diameter Finished 160 mm + 4 mm machining allowance (given on diameter) = 164 mm casting size. Pattern = 164 × (1 + 10/1000) = 164 × 1.01 = 165.64 mm, say 165.6 mm.
Length Finished 150 mm + 4 mm at each end = 158 mm casting size. Pattern = 158 × 1.01 = 159.58 mm, say 159.6 mm.
Bore / core box The bore must be smaller in the casting so that machining opens it to size: 90 − 3 = 87 mm casting bore. Core box = 87 × 1.01 = 87.87 mm, say 87.9 mm.
Core prints The print length on the pattern is also a casting dimension and shrinks: 40 × 1.01 = 40.4 mm each end, and the print diameter must match the core box figure plus the print taper.
The patternmaker's 160 × 150 mm pattern and 90 mm core box are finished sizes with no allowance at all, so every dimension would come out undersize after shrinkage and machining. The proposed dimensions are wrong in the direction that produces scrap, not rework.
2. Explain the logic for adding allowance to the bore and print dimensions rather than leaving them at drawing size, and note one practical reason the bore may not shrink exactly 1%.
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Allowances are applied to the casting, not to the pattern, and the pattern must be larger than the casting in every direction the metal contracts. That applies to internal features too: the metal around the bore contracts on cooling, so the bore itself becomes smaller than the core. If the core box were made at the finished 90 mm, the cast bore would be about 87 mm before machining and about 84 mm after boring 3 mm - 6 mm undersize, scrapped. Making the core box at 87.9 mm means the solidified casting bore is about 87 mm, and boring to 90 mm removes the allowance as intended. The same argument applies to the core prints, which are part of the pattern and must be proportionally enlarged so the core sits correctly in the mould.
One practical caution: shrinkage is not uniform. The bore is restrained by the core, which does not contract with the metal, so the bore often shrinks less than 1% and can even be slightly oval. For that reason the core should be collapsible (well vented, low strength, or a shell core), the shrinkage figure should be confirmed by a trial casting, and the pattern should be marked with the shrinkage rule used (a 1.01 shrink rule) so that every dimension is treated consistently.
3. Recalculate for 0.35% carbon steel at 20 mm/m and explain what would happen if the grey iron pattern were used for the steel casting.
Read the worked answer
Steel shrinkage factor = 1 + 20/1000 = 1.02.
- Pattern outside diameter = 164 × 1.02 = 167.28 mm
- Pattern length = 158 × 1.02 = 161.16 mm
- Core box diameter = 87 × 1.02 = 88.74 mm
Using the grey iron pattern (1.01) for the steel casting: the mould cavity is 165.64 mm and the steel contracts 2%, giving a casting of 165.64 / 1.02 = 162.4 mm. After 4 mm machining the outside diameter is 158.4 mm, which is 1.6 mm under the 160 mm requirement - scrap. The bore is worse: cavity 87.87 mm gives a casting bore of 86.1 mm, and after boring 3 mm it is 89.1 mm, undersize. The length is short by roughly 1.5 mm as well.
The conclusion is that a pattern is tied to the alloy, not to the shape: changing from grey iron to carbon steel requires a new pattern and core box (or at least a new set of shrinkage allowances), and the steel casting also needs a different gating and feeding design because of its much larger solidification contraction. Quoting the same pattern for the new material to save tooling cost would produce a whole batch of undersize castings.