m Degree of machining m
Degree of machining m · Fundición (ISO 10135) · ISO 1101
Example in plan
How to interpret it
On the drawing, “Grade m on bores and sealing faces” states a degree of machining m requirement.
What controls
Degree of machining stating that only some surfaces of the part are machined (the functional ones: bores, seating faces, seats, holes) while the rest is delivered as cast. It is the most common case for machine castings. It means the drawing must make clear which faces carry machining allowance and which do not, and that two tolerance worlds coexist on the same part: the general casting tolerance for as-cast features and the drawing tolerance for machined ones.
How to verify
Two measurement plans on the same part: the machined faces with the instrument their tolerance requires (micrometer, bore gauge, CMM) and the as-cast ones against the general casting tolerance grade. The tricky point is dimensions that relate a machined face to an as-cast one, such as the remaining wall thickness: these are measured with a thickness caliper, with ultrasonics, or by comparing the part scan against the model.
Common mistakes when interpreting it
- Applying the drawing tolerances to dimensions that fall in as-cast areas: those are governed by the general casting tolerance.
- Assuming that machining the functional faces corrects the position of the internal cavities, which stay where the core left them.
- Not checking the resulting wall thickness after machining the outer faces.
How to get it in the workshop / if it comes out of tolerance
The first thing when you receive the part is to identify which faces are machined and how they are referenced to each other: the datums are almost always machined faces, and the relationship to the as-cast faces is governed by the general tolerance. Probing the part before cutting is mandatory: the allowance is distributed differently in every pour. Keep in mind that machined areas expose sound material and the adjacent as-cast areas do not, so they will look very different, and that is correct.
The typical failure is that, after machining the functional faces, a wall ends up below its minimum thickness because core shift ate the material from the inside. Before production machining, measure the thickness in the critical areas of the first part. If the assembly does not close up, check whether the problem is in the machined portion (correctable on the machine) or in the as-cast portion (correctable only at the foundry).
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