Tighter CNC part tolerances often require additional processes, equipment and tooling, which raises production costs. For example, when it comes to the required surface finish of metals, secondary processes such as grinding, honing and refining are costly and should only be performed when necessary.
The optimal manufacturing process for a CNC part may change if you choose tighter tolerances than the industry standard. The reason for this is that the choice of manufacturing process may affect the tighter tolerances.
For example, one set of tolerances may be used when machining holes with a vertical milling machine, while another set of tolerances may be used when boring holes with a lathe, resulting in longer lead times. In addition, the basic tolerances of CNC machines may vary for different numbers of axes.
In addition, the type of parts that can be machined on a CNC machine depends on its tolerances. In some cases, parts require further manipulation to achieve tight tolerances. If you want products with small or extremely fine finishing features, you may need to perform several different machining operations on them. Different machining procedures can also produce different surface roughness or characteristics. Grinding or lapping may be required to meet tighter tolerance standards.
Tolerance tightness depends on the material
Depending on the material, the complexity of manufacturing a product using specific tolerances can vary greatly. Maintaining set tolerances for softer materials such as thermosets and thermoplastics is often difficult because the material flexes when cut.
The degree of tolerance achieved for a given material depends on its qualities. These characteristics include:
Thermal stability: Non-metals such as plastics often experience thermal stability issues. These materials deform as heat builds up during CNC machining, affecting part tolerances.
Abrasiveness: For some CNC-machined materials with a high carbon content, such as carbon steel and titanium, their abrasiveness makes it difficult to achieve specific tolerances while maintaining accuracy. This difficulty is mainly due to the number of tools that need to be changed.
Hardness: Harder materials such as aluminium, stainless steel, brass and copper are easier to machine accurately to tolerance than softer materials. The reason for this is that it is easier to change dimensions when the machine touches softer materials. Therefore, milling more delicate materials requires patience.
Part Complexity and Design
CNC part design is the most critical factor in controlling tight tolerances. Adjustments made during the design phase not only produce consistent tight tolerances, but also improve quality, increase customer satisfaction and reduce costs.
Tolerance control can be affected by part geometry, overall dimensions and wall thickness specifications. Thick walls may experience different shrinkage rates within their thicker sections, making it difficult to maintain accurate tolerances due to the different shrinkage rates within different sections.
Tighter Tolerance Impact Part Inspection
Verifying tighter tolerances is difficult and takes longer. The reason is that better measuring equipment and inspection techniques are required. In addition, the cost of producing the part goes up.
For example, vibrations caused by milling can lead to chatter marks. Although it is possible to inspect these marks visually, this requires sophisticated inspection tools to obtain accurate data, such as the depth and length of the marks. These marks can be destructive, leading to noise and vibration in the assembly (especially with tighter tolerances).
Choose only when necessary
Many people tend to be too specific when choosing tolerances. Choosing a tolerance of 0.001mm instead of 0.01mm can result in a two to threefold increase in price. Tighter tolerances require more detailed manufacturing and further testing to ensure the accuracy of the part.
Tighter tolerances are required in some cases. However, part tolerances should be determined primarily by how they work together, their use, how they are manufactured, and how sensitive the features are to variation.
