Shrinkage scaler
Scale a 2D DXF drawing or a 3D STL model by an exact percentage. Built for casting: oversize the pattern so aluminium, iron or steel shrinks to the size you want — or just use it as a precise resizer.
How casting shrinkage works
When molten metal cools and solidifies it contracts, so a mould made to the finished size would produce an undersized casting. Pattern-makers fix this by making the pattern slightly larger — the standard linear contraction allowances (about 1.3% for aluminium, 1.0% for grey cast iron, 2.0% for cast steel) are added to the pattern, i.e. the model is scaled by 1 + shrinkage%. This tool applies that factor uniformly to every point of your DXF or STL and shows the before/after dimensions so you can check them. It only scales the geometry — nothing is re-meshed or distorted — and everything runs in your browser.
Which factor does it use — 1 + s or 1/(1 − s)?
It uses the standard pattern-maker's rule, scaling by 1 + s/100, because published casting-shrink percentages are already the linear pattern oversize. For small percentages the difference from 1/(1 − s) is negligible (a few thousandths of a percent); if you need that exact form instead, switch to “Scale to a percentage” and enter it directly.
Does it keep my units and exact geometry?
Yes. STL and DXF are unitless, so a 100 mm feature simply becomes 101.3 mm at 1.3% — the numbers scale, the units are whatever your CAD uses. The geometry is only multiplied, never re-triangulated, so there's no loss of accuracy or detail.
Origin or centre — which should I pick?
For a cast pattern it usually doesn't matter, because only the size matters, not the position — Origin (scaling about 0,0) is the simplest and keeps the model anchored there. Choose Centre if you want the model to grow outward from its own middle and stay roughly in place.
From model to pattern
Scaling for shrinkage is the last step in preparing a pattern, not the first. Work at the finished size for as long as you can — that is the size on the drawing, the size the customer checks, and the size every other decision refers to. Only once the geometry is settled do you scale the whole thing up by the contraction allowance and send that to the pattern maker or the printer. Keep the unscaled model as well: if the casting comes out wrong you want to change the part, not reverse-engineer it out of an oversized pattern.
Shrinkage is not the only allowance
Contraction is one of three things a pattern carries, and this tool handles only that one. A machining allowance — extra material on the faces you will cut afterwards — is added locally, on those faces alone, not by scaling the whole model. Draft, the slight taper that lets the pattern release from the sand, is local as well. Uniform scaling is right for contraction because the metal contracts everywhere; the other two are decisions per surface and belong in your CAD model before you come here.
Useful outside the foundry too
The same exact percentage scaling solves other problems. A 3D print that comes out consistently undersized can be compensated once you have measured the error. A drawing supplied in the wrong unit — inches read as millimetres — is fixed by a single factor. A laser or waterjet part that has to allow for kerf, or a component that needs a deliberate interference fit, can be scaled rather than redrawn. In every case the geometry is only multiplied: nothing is re-meshed and no detail is lost.
