Condition based maintenance of machine tools: Vibration monitoring of spindle units

Machining systems (i.e., machine tools, cutting processes and their interaction) cannot produce accurate parts if performance degradation due to wear in their subsystems (e.g., feed-drive systems and spindle units) is not identified, monitored and controlled. Appropriate maintenance actions delay the possible deterioration and minimize/avoids the machining system stoppage time that leads to lower productivity and higher production cost. Moreover, measuring and monitoring machine tool condition has become increasingly important due to the introduction of agile production, increased accuracy requirements for products and customers' requirements for quality assurance. Condition Based Maintenance (CBM) practices, such as vibration monitoring of machine tool spindle units, are therefore becoming a very attractive, but still challenging, method for companies operating high-value machines and components. CBM is being used to plan for maintenance action based on the condition of the machines and to prevent failures by solving the problems in advance as well as controlling the accuracy of the machining operations. By increasing the knowledge in this area, companies can save money through fewer acute breakdowns, reduction in inventory cost, reduction in repair times, and an increase in the robustness of the manufacturing processes leading to more predictable manufacturing. Hence, the CBM of machine tools ensures the basic conditions to deliver the right ability or capability of the right machine at the right time. One of the most common problems of rotating equipment such as spindles is the bearing condition (due to wear of the bearings). Failure of the bearings can cause major damage in a spindle. Vibration analysis is able to diagnose bearing failures by measuring the overall vibration of a spindle or, more precisely, by frequency analysis. Several factors should be taken into consideration to perform vibration monitoring on a machine tool's spindle. Some of these factors are as follows: the sensor type/sensitivity, number of sensors to be installed on the spindle in different directions, positioning of the vibration accelerometers, frequency range to be measured, resonance frequency, spindle rotational speed during the measurements, measurement condition, including the no-load condition with tool clamped or without a tool, measuring tools and technologies, automatic or manual run of measurement, measurement routine, warning limits, and data handling and analysis, among other factors. The aim of this paper is thus to address CBM and particularly the implementation in the manufacturing industries focusing on the use of vibration monitoring techniques to monitor the condition of the machine tools' spindle units. To conduct this study, a pilot project was followed in real time. The pilot project was performed at a manufacturing company in Sweden. The company's product is gearboxes for the automotive industry, with a production volume of approximately 135,000 units per year. CBM, by online and off-line condition monitoring, using vibration monitoring, has been implemented on different types of machine tools, including horizontal and vertical turning machines, multi-task milling machines and grinding machines.