Tolerance is an allowable range of variations in part dimensions based on its shape, fit, and function. This term defines how accurate the measurement of a component must be. Defines the degree of variation or deviation allowed from the base measurement, particularly with regard to the dimensions of CNC machined parts.
To reduce these deviations, standard tolerances are used in CNC machining. Unless otherwise specified by the designer, tolerances are +/- 0.010 inches for plastic parts and +/- 0.005 inches for metal parts. However, the higher the part precision required, the smaller the tolerance will be (e.g. +/- 0.0004 inches).
A recurring question is whether the tighter tolerances of CNC parts affect part production and design. The answer is a resounding yes! The biggest influence on design and production will likely come from tight tolerances.
Tighter tolerances lead to higher production costs
Tolerances affect processing time and machining tools and therefore influence costs. And that is the reason? Tighter tolerances often lead to more scrap, the use of special measuring tools and additional accessories. Striving for tighter tolerances can also decrease machining speeds, thereby increasing production cycle times.
Depending on the required tolerance level and part geometry, aiming for tighter tolerances can double the design and production cost compared to achieving standard tolerance levels.
Three reasons why tighter tolerances increase costs
There are three main reasons why tighter tolerances in part manufacturing can increase production costs. That includes:
1. Use of specialized and expensive tools
If special tools are required to achieve the required tolerance and precision, purchasing these tools would increase production costs.
2. Carrying out expensive tests
Full inspection is required for CNC parts with tight tolerances. Regardless of whether the inspection is performed by humans or automatically controlled, more time and effort are required. Sometimes, due to tight tolerances, all components must be inspected and any part that does not meet specifications must be discarded, even if it could otherwise be used. Consequently, this high sorting and rejection rate also affects the price of parts and increases it.
3. Additional processes
Tighter tolerances of CNC parts often require more processes, equipment and tooling, which increases production costs. For example, when it comes to the surface finish required for metal, secondary processes such as grinding, honing and finishing are expensive and should only be carried out when necessary.
Tighter tolerances require a change in the manufacturing process
Choosing tighter tolerances than industry standards can change the ideal manufacturing process for your CNC parts. The reason for this is that the choice of production process can influence tighter tolerances.
For example, a hole with one set of tolerances can be machined with a vertical mill and a different set of tolerances when drilling the same hole with a lathe, increasing lead times. Additionally, basic tolerances may be different for CNC machines with different numbers of axes.
Additionally, the type of parts a CNC machine can handle depends on its tolerance. In certain circumstances, more machining is required before the part can reach tight tolerances. If you want small or extremely fine finishing features on your products, you may need to subject them to several different processing operations. Different machining processes also lead to different roughness or surface qualities. Grinding or lapping may be necessary to meet even stricter tolerance standards.
The tolerance limit depends on the material
The complexity of producing a product to a certain tolerance can vary greatly depending on the material. Because the material bends as it is cut, it is often difficult to maintain a set tolerance for softer materials such as thermosets and thermoplastics.
The level of tolerance achieved with a given material may depend on its properties. These properties include:
Thermal stability : Non-metals, such as plastics, typically have thermal stability issues. These materials deform during CNC machining as heat increases, compromising the tolerance of the part.
Abrasiveness : Some high-carbon CNC machining materials, such as carbon steel and titanium, make it difficult to achieve certain tolerances while maintaining accuracy due to their abrasive nature. This difficulty is mainly due to the number of tool changes required.
hardness : Compared to softer materials, harder materials such as aluminum, stainless steel, brass and copper can be machined more easily and accurately. The reason for this is that the machine tool can change dimensions more easily when in contact with softer materials. Therefore, milling more delicate materials requires patience.
Complexity and design in pieces
CNC part design is the most important element in maintaining tight tolerances. Adjustments during the design phase not only ensure consistently tight tolerances, but also improve quality, increase customer satisfaction and reduce costs.
Tolerance control can be influenced by part geometry, overall dimensions, and wall thickness specifications. The different contraction rates that thick walls can exhibit within their thick sections make it difficult to maintain accurate tolerances due to the different contractions within different sections.
Tighter tolerances – testing impact parts
It is difficult and takes longer to verify tighter tolerances. This is because better measuring equipment and testing techniques are needed. This also increases parts production costs.
For example, vibrations during milling can cause chatter marks. Although these marks can be inspected visually, obtaining accurate data such as mark depth and length requires sophisticated inspection tools. These marks can get in the way and cause noise and vibration during assembly (especially with tighter tolerances).
Choose only if necessary
Many people tend to be very precise when choosing a tolerance. Choosing a tolerance of 0.001mm instead of 0.01mm can result in a two to three times increase in price. Tighter tolerances require more careful manufacturing and additional testing to ensure the accuracy of all parts.
In some situations, tighter tolerances are required. However, part tolerances should depend primarily on how they work together, what they are used for, how they are manufactured, and how sensitive the characteristics are to variation.
