CNC (Computer Numerical Control) machining is a method of manufacturing components in which the movements of the machine tool – axes, feeds, speeds and machining cycles – are determined by a control programme rather than by manually guiding the tool. In industrial practice, CNC mainly refers to machining, i.e. milling, turning, drilling, threading and integrated processes such as mill-turn.
The key feature of CNC is not automation itself, but the repeatability of the process and the translation of technical knowledge into data. Once a process has been correctly designed and verified, it can be reproduced in series production, controlled through tool adjustments, and documented in accordance with quality requirements. It is precisely this feature that makes CNC the dominant technology in precision, small- and medium-volume production.
From technical drawing to finished component
Properly organised CNC machining always starts with the input data. A technical drawing or 3D model must include information on the material, tolerances, surface roughness, threads and functional requirements. Without this data, even the best CNC machine cannot guarantee consistent quality.
During the technology stage, the base plates, number of fixings, sequence of operations and cutting strategies are selected. The CAM system then generates toolpaths, which are translated by the post-processor into a programme that can be understood by the specific CNC controller. This is a critical stage, as different controllers interpret the code in different ways. Research into CAD/CAM/CNC integration shows that the lack of standardisation at this level is one of the main barriers to programme portability (VTT, Finland).
Only once the workpiece has been set up on the machine, the tools have been measured and the first piece has been checked can the process be considered ready for mass production.
CNC programming and ISO 6983
The most common programming language for CNC machine tools is G-code, as described in the ISO 6983 standard. This standard defines how commands controlling tool movement and auxiliary functions, such as tool changes or coolant activation, are written.
ISO 6983 describes control at the level of the tool path relative to the machine axes. This approach is effective and well-established, but it has a significant limitation: the code does not convey the full information regarding the machining intent. The programme contains information on ‘how the tool moves’, but not always ‘why’ or ‘which feature is being machined’. For this reason, post-processors and the technologist’s expertise play a key role in industrial practice. The ISO 6983 standard remains the foundation of CNC, but it is not the only direction of development for numerical control.
STEP-NC and ISO 14649 – a move towards greater integration
The solution to the limitations of conventional G-code is the STEP-NC concept, as described in ISO 14649 and the related ISO 10303 (STEP) standards. Rather than merely describing toolpaths, STEP-NC conveys information on machining features, operations, tools and process parameters.
ISO 14649-10 defines process data and the interface between programming systems and CNC controls. In theory, this enables more intelligent control, easier process adaptation and better integration with CAD/CAM systems. In practice, STEP-NC has not yet replaced G-code in mass production, but it represents an important direction of development and a benchmark for modern control systems.
CNC machining and tolerances – the role of ISO 2768
A common misconception is the question: ‘What level of accuracy does CNC provide?’ Accuracy does not depend solely on the machine, but on the entire system: thermal stability, clamping, tools, machining strategies and quality control.
Technical documentation very often uses the general tolerances specified in ISO 2768, which apply when the drawing does not specify individual tolerances. The ISO 2768-1 standard defines tolerance classes for linear and angular dimensions, thereby simplifying drawings and standardising the interpretation of requirements.
The use of ISO 2768 helps to avoid disputes at the acceptance stage and provides the process engineer with a clear basis for process design. At the same time, this standard does not replace functional tolerances – critical components should always have individually specified tolerances.
Number of axes and CNC machining capabilities
CNC machines are available in 3-, 4- and 5-axis configurations. The difference lies not only in ‘precision’, but primarily in the number of set-ups and the availability of surface area. Multi-axis machining allows more operations to be carried out in a single set-up, which reduces positioning errors and shortens production time.
In industrial practice, 5-axis machining is not required for every workpiece. Its real advantage lies in reducing changeover times and the ability to machine complex geometries, rather than simply delivering ‘better quality’ in every application.
When does CNC machining make business sense?
CNC machining is particularly well-suited to manufacturing environments where repeatability, quality control and the ability to iterate quickly on a design are key. It is especially effective for prototyping, small- and medium-volume production, and components with complex geometries.
It becomes less cost-effective when the part is very simple and produced in small batches, and the set-up costs outweigh the production costs. In such cases, CNC machining is still technically feasible, but it is not always economically viable.
The most common mistakes when working with CNC
The most common problem is a lack of complete input data: missing general tolerances, missing reference planes, or ambiguous surface requirements. The second mistake is the belief that a CNC programme is ‘universal’ and can be transferred between machines without a second thought. The third is imposing excessively tight tolerances without any real functional need.
In practice, a properly designed CNC process starts with the documentation and ends with stable production, rather than with the machine itself.
Summary
CNC machining is not a single technology, but a complete manufacturing system based on data, standards and process control. Standards such as ISO 6983, ISO 14649 and ISO 2768 are not an add-on to CNC, but an integral part of it. Understanding their role enables the design of stable, predictable and scalable processes. This is precisely why CNC remains one of the cornerstones of modern industrial manufacturing.
FAQ – CNC Machining (Technical)
In CNC machining, the movements of the tool and workpiece are controlled by a programme rather than by the operator manually guiding them. This ensures greater repeatability, the ability to work in batches, and easier quality control.
The most common standards are ISO 6983 (G-code programming), ISO 14649 (STEP-NC – process data) and ISO 2768 (general tolerances). These standards streamline communication between design, technology and production.
No. Although G-code forms the basis, individual control systems use their own dialects and cycles. That is why programmes are generated using post-processors tailored to a specific machine.
If the drawing does not specify individual tolerances, the general tolerances of ISO 2768 are often used. However, accuracy depends on the entire process: the machine, the tools, the clamping and the inspection.
Not always. 5-axis machining primarily reduces the number of set-ups and facilitates the machining of complex geometries. For simple parts, 3 axes may be entirely sufficient.
CNC loses its economic viability for very simple, one-off parts, where the cost of setting up the process exceeds the cost of production. In such cases, conventional methods are often preferable.
