CNC machining enables very high dimensional accuracy to be achieved without the need for finishing processes such as grinding. In practice, however, the limits of what is possible depend on many factors: the type of operation, the material, the part’s design and the quality of the machinery. Correctly specifying tolerances at the design stage affects not only the functionality of the part, but also production costs and lead times.
In this article, you will learn, amongst other things:
- which ISO tolerance classes are typical for milling, turning and drilling,
- what tolerances can be achieved without grinding in various materials,
- which technological factors affect accuracy,
- when grinding is necessary and how it affects production costs.
Accuracy classes in CNC machining
A tolerance is defined as the permissible range of deviations in the actual dimensions, shape or position of a component from the dimensions specified in the design.
The system of accuracy classes plays a key role in communication between the designer and the production engineer. It enables the unambiguous specification of the precision required for a given component and the manufacturing processes necessary to achieve it. Without clearly defined tolerance classes, the designer risks both overestimating production costs and underestimating the component’s functionality.
Tolerance classes also enable processes to be standardised on an international scale. This facilitates collaboration between companies, allows for the comparison of quotations, and enables production costs and risks to be predicted even before work on an order begins.
ISO tolerance system: IT6, IT7, IT8, IT9
The ISO 286 standard defines an international system of dimensional tolerances. Machining accuracy classes specify permissible dimensional deviations depending on the nominal size of the component. The lower the class number, the greater the accuracy and the higher the technological requirements:
- IT6 – very high accuracy, used in mechanisms and fits requiring manufacturing repeatability,
- IT7 – high accuracy, used in most machine components and precision structures,
- IT8 – this is the industry standard for many mechanical parts with moderate accuracy requirements,
- IT9 – a tolerance used in less demanding structural components.
Tolerances for milling versus turning
Milling and turning differ in the nature of the cutting process, which directly affects the achievable tolerances. Milling is an intermittent process – the tool periodically enters and exits the material, resulting in variable cutting forces and greater susceptibility to vibration.
Turning, on the other hand, is a continuous process in which the tool operates under stable conditions of contact with the material. This makes it possible to achieve tighter tolerances with a lower risk of geometric errors.
Geometric and dimensional tolerances
Dimensional tolerances specify permissible deviations from nominal values, whilst geometric tolerances describe the shape, position and orientation of components. In practice, both types are equally important for the correct functioning of parts.
In many applications, it is precisely the geometric tolerances (e.g. concentricity, flatness or perpendicularity) that determine the quality of the entire mechanism’s operation.
Achievable tolerances for different operations
The capabilities of CNC machining depend largely on the type of operation. Each cutting process has its own specific characteristics, limitations and optimal accuracy ranges. Understanding these differences enables parts to be designed in a way that is both technologically realistic and economically viable.
CNC milling – IT7 class
Modern milling centres enable high-precision CNC milling whilst maintaining productivity. CNC milling to IT7 accuracy class is now the standard in many industrial applications.
Stable clamping, appropriate machining strategies and modern cutting tools ensure process repeatability even in series production.
CNC turning – IT6 class
CNC turning to IT6 tolerance is possible thanks to the high rigidity of the process and cutting stability. In many cases, turning enables better accuracy to be achieved than milling.
This process is particularly well-suited to the production of cylindrical components requiring coaxiality and repeatability.
Drilling and threading
Holes are among the most demanding features in terms of tolerances. Drilling offers good productivity; however, achieving very high accuracy necessitates reaming or boring.
Threading, in turn, requires precise synchronisation of the machine’s movements, as well as adequate cooling and lubrication.
The effect of the material on achievable accuracy
The type of material has a significant impact on the machining process. Properties such as thermal conductivity, hardness and the tendency to build-up determine the achievable accuracy:
- Aluminium and its alloys: Aluminium is a material that is easy to machine. It is possible to achieve a high-quality surface finish and tight tolerances. At the same time, its high thermal expansion requires stable temperature conditions during machining.
- Structural and stainless steels: Steels offer good dimensional stability but generate higher cutting forces. For this reason, a more rigid clamping system is required. Furthermore, stainless steels are also prone to material build-up, which increases the technological requirements.
- Plastics: Plastics are materials that are susceptible to thermal deformation and are elastic. Temperature stability and appropriate cutting parameters are crucial for maintaining accuracy.
Factors affecting machining precision
CNC machining precision does not depend on a single parameter. Rather, it is the result of the interaction of the entire production system. The machine fleet, tools and fixtures, as well as the working environment and the experience of the machinists, are all significant factors. Even minor deviations in any of these areas can significantly affect the final accuracy of the workpiece:
- rigidity of the machine and fixtures – a modern machine tool fleet featuring high-precision machine tools ensures process stability and minimises vibrations. Rigid workpiece clamping minimises errors resulting from deformation,
- tool quality and wear – tool wear directly affects accuracy. It is therefore crucial to regularly inspect and replace worn-out equipment and tools,
- thermal stability of the process – temperature changes affect the expansion of materials and machinery. Thermal stabilisation is therefore vital for accuracy,
Quality control and measurements
Precision machining is inconceivable without equally precise quality control. It is measurements that confirm that the production process meets design requirements and quality standards. Modern quality control using coordinate measuring machines allows even highly complex geometries to be verified.
Quality control is also a key element in ensuring production repeatability. It enables the rapid detection of deviations and the optimisation of the process.
Coordinate measuring machines (CMMs)
CMMs enable accurate measurements of dimensions and geometric tolerances in three axes. This allows for a comprehensive inspection of a workpiece without the need for a multitude of different measuring instruments.
In production, CMMs form the basis of the quality assurance system and measurement documentation.
Roundness gauges for tight tolerances
Roundness gauges allow the measurement of shape deviations in rotary components with very high accuracy. They are particularly important for components that interact within bearing assemblies and gearboxes.
Precise roundness inspection affects the durability and reliability of the entire mechanism.
Optical measuring systems
Thanks to optical measuring systems, we can carry out rapid and non-contact measurements of complex geometries. They are particularly useful for thin-walled and delicate components.
When is grinding necessary?
Despite the extensive capabilities of CNC machining, there are situations where grinding remains an essential finishing process. This applies primarily to very tight tolerances, high surface roughness requirements and the machining of materials after hardening.
Grinding allows a level of precision to be achieved that is unattainable with conventional machining, though it entails additional costs and extends production time. It is essential in the following circumstances:
- IT5 tolerances and higher – these require finishing processes, as conventional machining cannot consistently achieve such tight dimensional deviations,
- very smooth surfaces (Ra < 0.4) – achieving such low surface roughness requires abrasive or honing processes,
- post-hardening machining – the high hardness of the material limits machining capabilities and requires abrasive processing.
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FAQ – frequently asked questions about CNC tolerances
What is the best tolerance achievable without grinding?
Usually IT6–IT7.
Is it possible to achieve IT6 for all dimensions?
No, it depends on the geometry and the material.
How does the size of the part affect the achievable tolerance?
The larger the part, the greater the risk of deformation and geometric errors. Therefore, large components usually require wider tolerances.
How much does machining to IT6 tolerance cost compared to IT8?
Production to IT6 class requires greater precision, time and quality control. As a result, the cost may be noticeably higher than for IT8.
Is an IT7 tolerance sufficient for press-fit connections?
In many applications, an IT7 tolerance is sufficient for standard press-fit connections. However, the final choice depends on the nature of the load and the operating conditions of the component.
