CNC Process Monitoring Directly At The Point of Action

The rapid development of digital technologies and the Industrial Internet of Things (IIoT) are the drivers of the industrial revolution currently taking place under the name ‘Industry 4.0’. In the course of this, advanced manufacturing technologies are being combined with IIoT systems with the goal of automated, unmanned production. The necessity for this change results from the constantly growing requirements for accuracy, efficiency as well as flexibility of production processes in combination with the already highly acute shortage of skilled workers. A key technology for meeting these challenges are intelligent monitoring systems that provide quantitative information about the condition of machines and processes. The transparency thus created enables data-based decisions regarding intervention in manufacturing systems.

Particularly in highly value-creating processes with direct influence on the quality of final products, such as milling or drilling, monitoring is of great importance. The background to this statement is, for example, that cutting tools are subjected to high loads and thereby gradually wear out. This aspect has direct impact on the machining accuracy and surface quality of the workpieces produced. In severe cases, this even leads to the production of scrap. Also, a critical degree of wear that is not detected at an early stage can result in breakage of the tool, which in addition to production scrap also comes with cost-intensive machine downtimes. Another cause is frequently occurring unwanted process vibrations, especially in the form of chatter. Reduced surface quality of the manufactured workpieces as well as accelerated wear of tools and critical machine components, such as spindle bearings, are typical consequences. In order to reliably detect and avoid these scenarios and at the same time maximize service life of cutting tools, sensitive and reliable monitoring systems are essential. However, they should not decrease the overall productivity of production facilities by causing machine tool downtimes.

A particularly effective approach is the integration of meaningful sensor technology in direct vicinity of the process zone, because in general, the closer the measurement location is to the point of action, the lower the influence of disturbances on the signals and the higher the achievable sensitivity. A sensor system based on this approach is also capable of acting in parallel with the process. For easy retrofitting in existing machine tools, the sensor integration should not introduce any interfering geometries to the machine working area. Moreover, unchanged regular production flows have to be guaranteed in order to exploit the maximum economic potential of monitoring systems.

The Intelligent Tool Holder Smart Tool

The intelligent tool holder Smart Tool was developed jointly by Fraunhofer IWU and IIS as part of the Fraunhofer Cluster of Excellence Cognitive Internet Technologies CCIT in response to the above challenges. With the Fraunhofer Cluster of Excellence Cognitive Internet Technologies CCIT, the Fraunhofer Society is working across institutes on key technologies for the cognitive, industrial Internet. The goal is to establish a sustainable infrastructure for an agile, flexible and digitized industry. Fraunhofer CCIT is an association of more than 20 Fraunhofer institutes from the fields of microelectronics, information and communication technology, and manufacturing. The joint research and development work focuses on the technology fields of IoT communication, trusted data spaces and machine learning.

Holistic Monitoring System

The Smart Tool system is equipped with extensive sensor technology for measuring process forces, vibrations and temperatures. Thereby, all typically interesting aspects of the process can be monitored. The data is transmitted via wireless link to an analysis unit with graphical user interface, which enables both real-time process monitoring and offline diagnosis. The acquired sensor signals not only create the basis for manual optimization of the machining process. In addition, connection with a machine control system facilitates adaptive process control. For this purpose, meaningful signal features are extracted and converted into decisions for machine tool adaption using modern evaluation algorithms. The effects of these actions are also identified, enabling potentially autonomous adjustments of process parameters. Possible applications comprise an automated tool change at the end of the tool life or an iterative feed rate reduction to eliminate dynamic instabilities.

Self-Sufficient – No Restrictions For Automated Production

The energy required for permanent operation of Smart Tool is generated by an innovative energy harvesting solution. This completely wireless technology is being utilized for the first time in the field of process and tool condition monitoring. A simple component equipped with permanent magnets is mounted on the front of the machine spindle. Through tool rotation electrical voltage is induced in coils integrated on the tool holder. In that way, power is supplied to sensors, signal processing hardware and data transmission. Due to the small size of the magnet holder, neither the automatic tool change nor the available working space of the machine tool is restricted. The 3D-printed, easily adaptable mounting geometry additionally reduces the retrofitting effort to a minimum.

Flexible Use on Any Machine

Almost all electronic components of the tool holder are located on a compact, universal segment between tool chuck and spindle interface. As a result, the latter two geometries are largely freely selectable and permit a large number of application-specific combinations. Since the hardware is completely integrated within the designated segment, Smart Tool has no interfering contours. High static and dynamic rigidity are nevertheless provided thanks to the optimized geometry. Thereby, the system can be used in the same way as a conventional tool holder.

In conclusion, the highly sensitive and easily integratable technology presented here is an important step towards transparent and autonomous manufacturing, as required by the current challenges in the manufacturing industry. Further, increasingly specialized monitoring systems are also likely to be developed in the future and thus digitization of machine tools will continue to proceed. New advances in the field of elementary sensors and integrated circuits will essentially determine this development.

Authors:

Dipl.-Ing. Hendrik Rentzsch – Head of Machine Tool Technology Department Fraunhofer IWU

Alexander Schuster – Research Assistant Fraunhofer IWU

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