Wireless Strain Measurement in CNC Turning

A 100% Novel Application Demonstration

Monitoring of CNC machining applications allows the production to be analysed and improved, can improve part quality, reduce scrap and help with the overall resource usage associated with a production machine tool. Online process monitoring can allow the machine to become smarter and adapt to its conditions internally. Machine learning and adaptive machining could further help improve manufacturing efficiency. However, in order to allow machines to become “smart”, widespread application of sensing technologies to machine tools must be carried out. Typically state of the art sensing systems for real-time machine monitoring is cost-prohibitive to be applied machine-wide in a plant. Usually, such systems are restricted to R+D and work in a Lab environment. Therefore there is a need for a low cost, flexible sensor system that is easy to apply to currently existing CNC technologies.

Wireless passive strain sensors based on SAW technology were applied to CNC tool holders compatible with the OKUMA CNC turning centre as shown in the images. SAW technology is attractive as the sensors can be deployed cheaply on the substrate and do not need any power supply. The sensing site is passive. The data presented from this project was the first data ever shown where SAW sensors were used to characterise CNC process forces.

Prior to machining operations, the system had to be set up in a simulation of the machining environment. During both the simulation tests and the actual machining, the instrumented tool holders were mounted inside a full 3 axis tool holder force measurement dynamometer. The dynamometer was used to measure forces in all 3 axes in all tests to compare the force signals with the signals generated from the wireless sensors. Shown below is the sensor setup in an Instron loading machine. Predefined load levels were applied and data was taken from all sensors to allow a full ISO calibration to be carried out.

Wireless passive force measurement carried out during a real CNC machining process

Once calibrated the system was installed inside the OKUMA machine tool to test the wireless sensors in a real production environment. CNC turning was carried out and during the tests data was taken from all the sensors in the system and we were able to sample force-related data from the CNC cutting process using the wireless and passive SAW sensing system.

Sample results

Shown below is a sample result set from the system where the signals are plotted on the same graph for comparison. You can clearly see the wireless passive SAW signal following the other industry-standard signal responses. Work is ongoing in this domain as there is an evident demand for a sensing system of this type. This project was the first time SAW technology has been demonstrated it a CNC application.

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