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Online process monitoring in machine tools from 3plusplus

Our cluster partner 3plusplus, together with the IWF of the TU Berlin, used measurement data and real-time data for the process monitoring of a machine tool to investigate the progressive wear at the cutting edge of a tool as well as the surface wear.


Fig. 1: The 3plusplus software enables the measurement of the roughness (1), the sensor signal (2) as well as the roughness depth (Copyright 3plusplus GmbH, 2022)


By integrating a laser-based measuring system into the turning process, component characteristics are extracted parallel to the process: The roughness characteristics of the surface of the currently manufactured component and geometric features such as shaft diameter, radii and chamfers. Based on this online profile laser measurement, a process control of the component roughness was developed that enables higher precision and efficiency of the machine tools, which significantly reduces manufacturing costs and deviations. The process-reliable guarantee of quality characteristics through compensation of disturbance variables, even with worn tools, eliminates the need for premature tool changes. In addition, online monitoring is possible even without prior knowledge and eliminates the need for subsequent testing of work pieces for a wide range of quality characteristics.



Profil sensor integration

In addition to guaranteeing a process-safe measurement, the requirement for the installation of the profile sensor was the realization of a plug&play solution. The sensor must be aligned in such a way that both the mold geometry based on the molded-off mold cutting edge and the resulting component roughness can be measured without influencing or even bringing the production process to a standstill, for example due to collision. The device for mounting the sensor was attached to the tool turret. A CAD 3D model was used to determine the collision-free travel paths and to ensure that the measuring range of the profile sensor was not left during production. One disadvantage of attachment to the tool turret is the lack of a y-axis. Without actuator-based kinematics, precise, repeatable positioning of the sensor is not possible. Therefore, a software-based calibration of the measuring system was developed, which also compensates for the variation of the radial incident angle of the laser beam due to incremental decrease of the shaft diameter during manufacturing.


Communication via OPC UA

The interface meets the requirements for real-time capability of the laser's machine communication, measurement data acquisition and processing, and machine control. The laser sensor and raw data measuring device are connected via Ethernet to a measuring computer on which the 3plusplus software is installed. The external communication unit consists of a PLC and an OPC UA server. Through the PLC, the control algorithm can be implemented to actively intervene in the manufacturing process. The bidirectional communication between external software and the machine control is ensured by the OPC UA server. It provides machine-internal measurement data for the control loop and transmits commands to the external system.


Parameters from recorded measurement data

As a basis for the laser-based detection of complex geometric structures and topographic surface parameters, raw data from differently parameterized machining processes were recorded. The machine-internal and the sensor-based external measurement data provide extensive information about the manufacturing process. By means of the tool coordinates provided by the OPC UA server, it is possible to assign process-parallel measurement data, such as cutting forces, spindle power and current roughness parameters to a fixed position on the workpiece. 3plusplus developed a software plug-in to extract characteristic values from the acquired measurement data. By using the plug-in, process-dependent parameters such as feed rate, current depth of cut and manufactured diameter of the shaft can be determined from the profile sensor data. The measuring system is suitable for detecting wear-related disturbance variables in the process and precisely determining the time of the tool change.


Prospects

The project resulted in a teach-in-free system for online process monitoring during turning operations. The plug-in for roughness measurement developed by 3plusplus can be used as a stand-alone solution or in combination with other modules of the modular software system for extensive automation and measurement tasks. The software is always tailored to individual requirements. Further work on online laser profile measurement is planned: the implementation of process-parallel manipulation of the infeed axis, the combination with AI, and the coupling of additional sensor technology.

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