Tolerances are the allowable range of dimensional deviations of parts based on their shape, fit, and function. This term is used to define the accuracy of component measurements. It defines the amount of variation or deviation that is allowed for the foundation measurement, especially the dimensions of the CNC machined part.
To reduce these deviations, standard tolerances are used in CNC machining. Unless otherwise stated by the designer, the tolerance for plastic parts is +-010'', the tolerance of the metal parts is +-005''。Nonetheless, the higher the required accuracy of the part, the tighter its tolerances (e.g. +-..)0004 inches).
A recurring question is whether tighter CNC part tolerances affect the production and design of parts. The answer is a resounding "yes"! Probably the biggest impact on design and production comes from tight tolerances.
Tighter tolerances lead to higher production costs
Tolerance levels affect machining turnaround times, and tools also affect their costs. Why is that? Tighter tolerances are often more likely to lead to increased scrap, the use of special measuring tools and additional fixtures. The need for tighter tolerances can also reduce processing speeds, increasing production cycle times.
Therefore, depending on the required tolerance level and its geometry, seeking tighter part tolerances can double the design and production costs compared to achieving standard tolerance levels.
Three ways tighter tolerances can increase costs
Tighter tolerances for manufacturing parts can increase production costs in three main ways. They include;
1.Use dedicated and expensive tools
If specialized tools are required to achieve the required tolerances and accuracy, then the use of such tools increases production costs.
2.Conduct expensive tests
For CNC parts with tight tolerances, a thorough inspection is required. Whether the test is done by a human or controlled automatically, this requires more time and effort. Sometimes, due to tight tolerances, every part has to be inspected, and any part that does not meet the specifications must be rejected, even if it could have been used. Therefore, this high sorting rate and scrap rate will also affect the quality of the part, making it higher.
3.Additional processes
Tighter tolerances for CNC parts often require more processes, equipment, and tooling, increasing production costs. For example, when it comes to the surface finish required for metals, secondary operations such as grinding, honing, and refining are costly and should only be done when necessary.
Tighter tolerances require modifications to the manufacturing process.
If you choose tolerances that are tighter than the industry standard, the optimal manufacturing process for CNC parts may change. The reason for this is that the choice of production process may affect tighter tolerances.
For example, a vertical mill may apply one set of tolerances when machining holes, while another set of tolerances may be applied when boring with a lathe, resulting in longer lead times. In addition, the basic tolerances of CNC machines with different axes may vary.
In addition, the type of parts that can be machined by a CNC machine depends on its tolerances. In some cases, the part requires further manipulation to achieve tight tolerances. If you want your products to have small or very fine finishing features, you may need to perform several different machining operations on them. Different machining procedures also produce different surface roughness or properties. Grinding or grinding may be required to meet tighter tolerance standards.
The tightness of the tolerances depends on the material
Depending on the material, the complexity of manufacturing a product with specific tolerances can vary greatly. Because materials bend as they cut, it is often difficult to maintain set tolerances for softer materials such as thermosets and thermoplastics.
The degree of resistance achieved by a given material depends on its quality. These characteristics include:
Thermal stability: Non-metals such as plastics often experience thermal stability issues. These materials deform with the accumulation of heat during CNC machining, affecting the tolerance of the part.
Abrasiveness: For some CNC machined materials with high carbon content, such as carbon steel and titanium, its 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 replaced.
Hardness: Harder materials such as aluminum, stainless steel, brass, and copper are easier to machine precisely to tolerances than softer materials. The reason is that it is easier to change dimensions when the machine is in contact with softer materials. As a result, milling more delicate materials requires patience.
Complexity and design of the part
CNC part design is the most critical factor in controlling tight tolerances. Making adjustments during the design phase not only results in consistent, tight tolerances, but also improves quality, increases customer satisfaction, and reduces expenses.
Tolerance control can be affected by part geometry, overall dimensions, and wall thickness specifications. Thick walls can experience different shrinkage rates within their thick sections, making it difficult to maintain precise tolerances due to the different shrinkage rates within different sections.
Tighter tolerances for impact parts inspection
Verifying tighter tolerances is difficult and takes longer. The reason for this is the need for better measuring equipment and inspection technology. In addition, the cost of producing parts will also rise.
For example, vibrations caused by milling can cause chatter. While these markers can be visually inspected, this requires sophisticated inspection tools to obtain accurate data, such as mark depth and length. These markings can be disruptive, resulting in noise and vibration in the assembly (especially if the tolerances are tighter).
Select only when necessary
Many people tend to be too specific when choosing tolerances. Select 0001mm tolerance instead of 001mm can cause *** two to three times. Tighter tolerances require more meticulous manufacturing and further testing to guarantee the accuracy of the parts.
Tighter tolerances are required in some cases. However, part tolerances should be determined primarily by how they work together, what they are used for, how they are manufactured, and how sensitive the feature is to change.
CNC machining tolerances