○ √ No machining allowance
No machining allowance · Estado superficial · ISO 1101
Example in plan
How to interpret it
On the drawing, “√ symbol with circle” states a no machining allowance requirement.
What controls
A variant of the surface texture symbol indicating that the face must be left in the condition produced by the previous process (casting, forging, rolling, extrusion) and that material removal is not permitted. It is a process requirement with dimensional consequences: that face has no machining allowance and its dimensions are governed by the general tolerance of the originating process, not the machining tolerance. It does not mean the surface can come out any old way: the symbol may carry a roughness value that must also be met.
How to verify
Visual inspection with grazing light to confirm there are no material removal marks or local recesses. Dimensions in that area are checked against the general tolerance of the originating process, with calipers, templates or, on large parts, a scanner comparison against the model. If the symbol includes a roughness value, it is measured with a profilometer on a representative area, bearing in mind that a coarse texture requires a matching cutoff length.
Common mistakes when interpreting it
- Touching up or sanding the surface to improve it, which is exactly what the symbol prohibits.
- Applying machining tolerances to it: those dimensions fall under the general tolerance of the originating process.
- Using an unmachined face as a datum or as a repeatable clamping locator.
How to get it in the workshop / if it comes out of tolerance
It appears mainly on castings or forgings where the original skin is wanted: for strength, for corrosion resistance or simply for cost. Don't use it as a precision locating surface when clamping, because no two parts are the same in the as-cast state. And don't touch it up with a grinder to make it look nice: that is removing material. If such a face does not fit in the assembly, the problem is the general tolerance or mold shift, and it is resolved with the supplier.
If the face comes with flash or embedded sand, the fix is deflashing and shot blasting, not grinding the surface. If the as-cast geometry does not allow the assembly to go together, measure several parts: if they all deviate the same way, it is the supplier's tooling. And if machining there is really necessary, that is a drawing change: machining allowance has to be added and the symbol changed.
Related symbols
Other symbols in Surface texture
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