CNC machining tolerances
Tolerances define the permitted variation of a part's characteristics. They should be chosen according to the part's role in the assembly and communicated clearly in the technical documentation.
Tolerance guides
Dimensional tolerances
How to specify permitted deviations for dimensions and how to read the limits of a dimension.
Dimensional tolerancesGeometric tolerances
How to describe the form, orientation and location of part features.
Geometric tolerancesSurface roughness
What roughness requirements mean and how they relate to the functional surface.
Surface roughness
Which type of requirement to specify
| Requirement | What it describes | Details |
|---|---|---|
| Dimensional | The limits of a size, such as a diameter or thickness. | Dimensional tolerances |
| Geometric | The form, orientation, location or run-out of a feature. | Geometric tolerances |
| Roughness | The characteristics of the evaluated surface texture. | Surface roughness |
Roughness is included here so the drawing can be prepared completely, but it is not a dimensional tolerance. An Ra value does not replace requirements for size or flatness.
Why there is no single tolerance for every part
The same numerical value can involve different difficulty depending on the characteristic, size and material. The assessment must also consider workholding, the final condition after treatments and the inspection method.
How project requirements are confirmed
Send the revised drawing, the standards referenced and the critical dimensions. The quotation should state the accepted requirements and any clarifications. Values presented as examples in the guides are not capabilities guaranteed by BSK.
Questions for quotation
- The achievable tolerance is confirmed after assessing the part, the material, the requested dimensions and the inspection conditions. Send your documentation for an answer that applies to your project.
- State the standard, edition and class required in the documentation. If they are missing, the requirements must be clarified before the order is accepted. A general tolerance class should not be assumed just because the part is CNC-machined.
- The tolerance should support the function of the part. Unnecessarily tight limits can require additional operations or inspections. The designer sets the functional requirement, and feasibility is discussed with the manufacturer.
- Positioning accuracy describes a machine characteristic under defined test conditions. The result on the part also depends on material, tooling, workholding, temperature and the measurement process. The part tolerance must be confirmed separately.
What to send for assessment
- The drawing and any available model, with the same revision.
- The exact material, the quantity and the stock dimensions if you supply it.
- Dimensional, geometric and roughness requirements, with the surfaces concerned.
- Treatments and the condition of the part at acceptance.
- The measurement documents required and any special acceptance rule.
Examples and confirmed values
The calculation examples in this guide are for explanation. For an order, the requirements of the drawing and the values accepted in the quotation apply.