A tolerance defines the acceptable deviation from a nominal dimension. In CNC turning, CNC milling, and gear production, it is not practical to apply the same tight tolerance to every dimension.
The key is to separate functional dimensions from supporting dimensions. A functional dimension can affect bearing fits, mating faces, concentricity, hole spacing, threads, gearing, or safe part operation. A supporting dimension may allow more freedom if it does not affect use.
On drawings, mark critical holes, fits, mating faces, positions from reference edges, and dimensions that decide installation. For shafts and geared parts, call out functional diameters, splines, gearing, and surfaces that require the strictest checking.
Overly tight tolerances can change the manufacturing route. A part may need slower machining, additional operations, grinding, more specific measurement, or a different process. Accuracy should be focused where it creates functional value.
Which dimensions need tighter tolerances?
| Dimension type | Recommended approach |
|---|---|
| Fits and functional diameters | Specify the tolerance and surface requirement when they decide function. |
| Gearing and splining | Include module, tooth count, spline type, and inspection requirements. |
| Mating faces | Define flatness, concentricity, or relation to a reference face. |
| Mounting holes | Mark spacing, position, and reference edges. |
| Supporting dimensions | Use a general tolerance if they do not affect function. |
Drawing checklist
- Separate functional and supporting dimensions.
- Mark reference edges, axes, fits, and mating faces.
- For gearing, include module, tooth count, and core gear parameters.
- Do not use a tight tolerance where it adds no value.
- When unsure, describe the installation or operating context.