Casting Pattern Allowance Worksheet: Shrink and Machining Stock per Feature
A pattern is larger than the finished part because the metal shrinks on cooling and the casting needs stock for machining. This worksheet applies both to every feature, so you can send a clear pattern dimension table to the pattern shop.
| Feature | Type | Finished size (mm) | Machining stock per side (mm) | Machined sides | Shrink (%) | As-cast size (mm) | Pattern size (mm) | Pattern minus finished (mm) | |
|---|---|---|---|---|---|---|---|---|---|
426 | 430.26 | 10.26 | |||||||
216 | 218.16 | 8.16 | |||||||
28 | 28.28 | 3.28 | |||||||
184 | 185.84 | 5.84 | |||||||
96 | 96.96 | 0.96 |
Enter finished size as drawn. For bores formed by cores, an inside dimension shrinks the opposite way so the core box needs the allowance reversed in practice; confirm with your pattern shop. Add draft angles, rapping allowance and distortion allowance separately.
Tooling and revision control
Optional: a short review of how patterns and drawing revisions are controlled in your ERP.
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How to use the worksheet
List each feature to be controlled on the pattern, such as overall length, flange thickness and boss height. Enter the finished size from the drawing, the machining stock per side, how many sides are machined, and the shrink percent for the alloy. The sheet calculates as-cast size and pattern size. Use the shrinkage table linked here for typical percentages, and replace them with your foundry's own rule.
The calculation
As-cast size equals finished size plus machining stock multiplied by the number of machined sides. Pattern size equals as-cast size times one plus shrink divided by 100. The order matters, because the machining stock is metal that must exist in the casting after shrinkage. Applying shrink to the finished size alone leaves the casting too small on machined faces. The last column shows how much larger the pattern is than the finished part.
- Machining stock first, then shrink
- Count machined sides, not machined faces in total
- Use one shrink value per direction if the casting is restrained
Other allowances to record
Draft is added to vertical faces so the pattern withdraws cleanly. Rapping allowance slightly reduces pattern size on faces parallel to the draw direction and compensates for mold enlargement when the pattern is rapped loose. Distortion allowance counters known warping in long thin castings. Machining stock grows with casting size and with the roughness of the process, so large steel castings carry much more than small investment castings. Record these separately from this sheet.
Common mistakes and ERP notes
Common mistakes are mixing finished and as-cast dimensions in the same column, using a shrink rule from a different alloy or foundry, and not recording the revision. Pattern equipment is an asset: in the ERP it should link to the item, the drawing revision and the foundry that holds it. A changed shrink factor or machining stock is an engineering change, not just a note on a sheet.
Frequently Asked Questions
Do I apply shrinkage before or after machining stock?
Add machining stock to the finished size first, then apply the shrink allowance to the total. That ensures the casting still has the intended stock after it contracts.
How much machining stock should I use?
It depends on casting size, alloy and process. Small precision castings may use 1 to 2 mm, while large sand castings often use several millimetres or more per side. Follow your foundry guidance or the applicable casting standard.
Does the sheet include draft angle?
No. Draft is a geometry feature added to faces parallel to the pattern draw. Record it separately on the pattern drawing, or add extra rows with the average added size.
Can I export this to the pattern shop?
Yes. Export to Excel, CSV or PDF and attach it to the pattern request. Make sure the drawing revision and shrink value used are written on the sheet.
If tooling revisions and casting drawings are not tied to items in your ERP, we can help set up a simple link so changes are controlled.
Related Tools
Casting Shrinkage Allowance Reference Table
Look up typical linear shrinkage allowances by alloy for sand casting patterns, with conversions to inches per foot and millimetres per metre.
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Calculate casting yield from casting, gating and riser weights, then see the metal you must actually charge per good casting after scrap and melt loss.
Quality & InspectionCasting Defect Troubleshooting Checklist
A practical checklist of what to check when castings show gas porosity, shrinkage, inclusions, sand defects, cracks or dimensional problems.
Costing & FinanceFoundry Melt Loss Calculator
Convert your melt loss percentage into kilograms, cost per heat and annual cost, then test the saving from improving loss by a few tenths of a point.
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