Tolerance is the allowable range of dimensional deviations of a component based on part shape, fit and function. The term is used to define the accuracy of component measurements. It defines the amount of variation or deviation allowed in the base measurement, especially for dimensions of CNC machined parts.
To minimize these deviations, standard tolerances are used for CNC machining. Unless otherwise specified by the designer, tolerances are +/-.010‘‘ for plastic parts and +/-.005‘‘ for metal parts. Nonetheless, the higher the accuracy of the part required, the tighter the tolerance (e.g., +/-.0004‘‘).
A recurring question is whether tighter CNC part tolerances affect part production and design. The answer is a resounding "yes." The biggest impact on design and production may come from tighter tolerances.
Tighter tolerances lead to higher production costs
Tolerance levels affect machining turnaround times, and tooling affects its cost. Here‘s why. Tighter tolerances are often more likely to lead to increased scrap, the use of special measuring tools and additional fixtures. Requirements for tighter tolerances may also reduce machining speeds and thus increase production cycle times.
Therefore, depending on the level of tolerance required and its geometry, seeking tighter part tolerances may double design and production costs compared to achieving standard tolerance levels.
Three Ways Tighter Tolerances Increase Costs
There are three main ways in which tighter tolerances on manufactured parts may increase production costs. They include;.
1. Using specialized and expensive tooling
If specialized tooling is required to achieve the required tolerances and accuracy, then the use of such tooling will increase production costs.
2. Conducting expensive tests
CNC parts with tight tolerances need to be thoroughly inspected. Whether the testing is done by humans or automated, this requires more time and work. Sometimes, due to tight tolerances, every part must be inspected and any part that does not meet specifications must be rejected, even if it could have been used. As a result, this high sorting and scrap rate also affects the price of the part, making it more expensive.
3. Additional Processes
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.
Tighter tolerances require modifications to the manufacturing process
The optimal manufacturing process for a CNC part may change if you choose tolerances that are tighter than industry standards. 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 programs can also produce different surface roughness or characteristics. Grinding or lapping may be required to meet tighter tolerance standards.
Tolerance Tightness Depends on 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 due to the fact that the material will flex 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 aluminum, 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 because of 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 these marks can be visually inspected, this requires sophisticated inspection tools to obtain accurate data such as mark depth and length. 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.
