Machining allowance is the extra metal left on a casting’s to-be-machined surfaces so the finishing cut can take off the as-cast skin and hold size and finish. On a drawing it is the required machining allowance (RMA): one value in millimeters with an ISO 8062-3 grade letter. On the ferrous sand castings we pour it runs from about 1.5 mm on a small machine-molded part to 18 mm on a large hand-molded one, and it is not the same thing as shrinkage allowance.
What Is Machining Allowance in Casting
Machining allowance is stock added to the pattern on every surface the drawing marks for machining, so the raw casting comes out oversize on those surfaces by a set amount. ISO 8062-3 calls it the required machining allowance (RMA); North American foundries still say finish allowance or machining stock. It exists on machined surfaces only — a face that stays as-cast gets none — which is why the drawing has to say which surfaces are which.
The allowance has two jobs. The as-cast skin — burned-in sand, scale, and the rough, sometimes porous outer layer — has to come off completely, and the working rule is that the first cut runs entirely below the cast surface by at least 1/16 in (about 1.5 mm). Beyond that, the feature can land anywhere inside its casting tolerance, so the raw dimension is set so that the full allowance is still there at the low end of that band.
A face machined on one side carries the allowance once; a diameter, or a feature machined on both sides, carries it twice. By default one value covers the whole casting, selected from the largest overall dimension of the finished part rather than the feature — a small machined pad on a large gearbox housing gets the housing’s allowance.
Typical Machining Allowances
For a ferrous sand casting, typical machining allowance is 1.5–4 mm on a small machine-molded part, 2.5–6 mm machine molded or 3.5–12 mm hand molded at 250–630 mm, and up to 18 mm on a large hand-molded casting; investment castings need 0.4–1.8 mm. The bands below are the ISO 8062-3 grades by process and largest finished dimension, with the grade letters a drawing would carry.
| Casting process | Largest finished dimension | Typical machining allowance |
|---|---|---|
| Die casting (non-ferrous) | — | 0.25–1.5 mm |
| Investment casting (ISO grade E) | up to 400 mm | 0.4–1.8 mm |
| Sand casting, machine molded or shell (grades F–H) – small parts | 100–250 mm | 1.5–4 mm |
| Sand casting, machine molded or shell (grades F–H) – medium parts | 250–630 mm | 2.5–6 mm |
| Sand casting, machine molded or shell (grades F–H) – large parts | 630–1,600 mm | 3.5–8 mm |
| Sand casting, hand molded (grades G–K) – medium parts | 250–630 mm | 3.5–12 mm |
| Sand casting, hand molded (grades G–K) – large parts | 630–2,500 mm | 5–18 mm |
For steel, treat the small-part row as the standard’s minimum, not a starting point. SFSA’s steel castings tolerance handbook never goes below 3/16 in (5 mm) on a flat face for steel castings, and no cope surface should carry less than 6 mm.
The cope face collects the slag, sand, and gas that float up during filling, so it needs more stock than the drag and side walls: on a large casting, 16 mm on the cope against 10 mm elsewhere is normal. The fix is to mold critical machined faces in the drag whenever the geometry allows.
Machining Allowance vs. Shrinkage Allowance
Shrinkage allowance makes the pattern oversize on every dimension so that the casting, after contracting as it cools, lands at nominal size; machining allowance adds stock on top of that, on marked surfaces only, so those surfaces land oversize on purpose.
| Machining allowance | Shrinkage allowance |
|---|---|
| Adds metal that is later machined away | Compensates for the metal’s contraction as it cools |
| Applied only to surfaces marked for machining | Applied to every casting dimension affected by solidification and cooling |
| The excess stays on the raw casting as visible stock | Extra pattern dimension only; nothing extra remains on the casting |
| Set by process, part size, molding method, mold position, and the machining requirement | Set mainly by the alloy’s contraction and the foundry’s shrink rule |
Because the foundry’s shrink rule is an estimate, machining allowance should not be shaved on a new pattern before first-article inspection. Machining stock is what absorbs whatever the shrink rule got wrong; a face with too little of it cleans up at one end and not the other.
Conclusion
Put three things on the drawing: which surfaces are machined, the RMA value with its ISO 8062-3 grade, and any surface that needs more than the general value. Pick the starting value from the table by process and largest finished dimension; for steel, hold 6 mm on any cope face or move that face to the drag.
Leave shrinkage to the pattern shop; it is the foundry’s correction, not your stock. Then confirm the allowance with the foundry that will pour the part — its shrink rule and its surface are what set it.