New Path Processing Curved Fibre-Reinforced Plastic Structures
The research and development project “ProCURVE – Process development for the implementation of a novel CNC manufacturing process for the cut-off grinding of spatially curved FRP structures” is funded by the Federal Ministry for Economic Affairs and Climate Action (BMWK) as part of the Central Innovation Program for small and medium-sized enterprises (ZIM) under project no. KK5161701AT0.
CNC manufacturing can satisfy the need of fast and highly individualized production in various industries. However, the production of curved contours and spherical parts for lightweight construction reveals deficiencies of commonly used production processes such as milling, abrasive cutting or sawing, water jet cutting, and laser cutting. In a former project ModuleWorks and its partners have developed a completely new production process, Curved Circular Cutting (CCC), to optimally machine curved contours and spherical shells made of fibre-reinforced plastics with a cut-off grinding tool. One of the challenges of establishing a new production process is planning new toolpaths, a challenge addressed by ModuleWorks.
Years after its introduction, the usage of Computer Numerical Control (CNC) production is still increasing because the technology can serve the need for individual just-in-time manufacturing. This appeal is caused by the cost-effectiveness of small production batches and the ability to produce complex components. With CNC, different production processes can be autonomized such as milling, abrasive cutting or sawing, water jet cutting and laser cutting, which all pave the way for a maximum of flexibility.
This general appeal and the flexibility of CNC based production ranges up to the production of curved contours or spherical components made of fibre-reinforced plastics (FRP) which are important for lightweight construction, for instance in the mobility industry. However, the production of complex shapes made of fibre-reinforced plastics lays bare the limits of milling, abrasive cutting or sawing, water jet cutting and laser cutting. Milling, for example, is a highly flexible process but one with a low feed rate. Furthermore, the processing of composite and hybrid materials, such as fibre-reinforced plastics, is complicated by the fact that layers of these materials may dissolve during the cutting process. Additionally, heavy load on the tool causes abrasion because the tool is in almost constant contact with the surface area of the workpiece. Compared to milling, abrasive cutting or sawing has considerably higher feed rates and better process qualities, but the kinematics of the process only allow for straight cuts. Other processes, such as water jet cutting allow for complex 3D processing yet are high in costs and bad for the environment because water used for cutting is polluted and has to be cleaned afterwards. Equally, laser cutting enables the production of complex forms in a fast way, though material ablation with thermal changes can cause severe damages on the workpiece. Besides that, the process is only applicable for small components. Components made of lightweight materials such as fibre-reinforced plastic composites are usually trimmed through the established processes. Yet, trimming often leads to defects such as thermal damage, delamination, poor dimensional accuracy, and reduced tool life.
To solve these shortcomings, Curved Circular Cutting (CCC), a new cutting technology has been put forward. Curved Circular Cutting is designed to cut spatially curved composite materials made of fibre-reinforced plastics. The core innovation of the process is a novel tool geometry. The tool is a disc-shaped, bend-resistant cutting-off wheel with lateral clearance of the cutting area. The process can cut curved contours and spherical shells composed of fibre-reinforced plastics in a safe, fast, and precise way with minimal wear on the tool. The project requires the development of new toolpaths and the application on an industrial robot.
The success of Curved Circular Cutting has several factors. To name a specific example, the bigger diameter of the tool enables high feed rates, factor 3 to 8 higher than milling. Due to shorter contact times of the tool and the material, the tool has more time to cool down. Apart from being more flexible and faster, the process is cost-efficient and eco-friendly. This is because no costly system technology is required as the process can be applied to 5-axis machining centers and robotic systems that are equipped with a clamping device and a tool holder. For the tool, the application of cut-off wheels is planned, which are cost-efficient compared to other tools and have less tool wear. The only machine prerequisite is a kinematic that has at least five degrees of freedom to cut in a flexible way. For the usage of industrial robots, an innovative end-of-arm-tool will be designed which is able to pick up and guide the cutting tool over the workpiece. Since many, but not all geometric elements can be reached with Curved Circular Cutting, a combined tool holder will be developed. In this way milling can be effortlessly implemented if needed.
More than the machine kinematics, ModuleWorks is specifically involved in new path plans. This involves the realization of process precision of ≤ 100 µm through optimized path planning. Since Curved Circular Cutting just has been invented, freeform cuts and under cuts deserve special attention in path planning. Methods of artificial intelligence are used to optimize the toolpath further. The AI optimized toolpath will consider the initial and final geometry of the workpiece in CAD data, the geometry and features of the tool, and the process type. It also considers the rotation direction of the tool, tool overhangs and the maximum feed rate in relation to the maximum and minimum cutting speed. For collision control and robot kinematics the highest accuracy is intended. Additional flexible components of the machine, such as cables, have to be considered as well. Finally, the robustness of the process has to be factored into the toolpath so that no loops and breakoffs occur, even if geometry problems arise.
Since ModuleWorks is the only company worldwide offering a real-time simulation of production on the CNC management system, the company has been commissioned with the development of a digital twin which is able to simulate upcoming production steps. If collisions are foreseen in the digital twin, manufacturing can be stopped.
The collision simulation for Curved Circular Cutting remains an area for new developments. Due to the high feed rate and the limited computing powers of the control hardware, it is hard to achieve an accurate simulation. The envisaged solution is an intelligent, quasi-real time collision library to check the real-time machine environment. Additionally, a memory buffer is envisioned. The memory buffer temporarily stores data that cannot be processed during peak loads, but that can be made available to the system with a time delay as soon as the load is reduced.
On top of the development of optimized tool paths, the highest level of automatization is aimed for. This eliminates the need for expert knowledge and the process can be employed without additional requirements. This automatization will be reached by measuring the component and utilizing artificial intelligence. More specifically, the difference between the desired geometry of the workpiece and actual cut will be measured so that the quality can be recorded and re-fed to the process chain. For the automatic optimization of the tool path, an individually suited AI will be developed which is able to assess previous experiences, for example fibre-reinforced composites that require a special working direction. In this way, software and hardware are finely tuned to each other.
Curved Circular Cutting (CCC) resolves the limits of existing production processes and is easily accessible for companies. With this approach to production, the project ProCURVE provides all characteristics for an industrial innovation and paves new paths for the industrial processing of curved fibre-reinforced plastic structures.