Monitoring & metering
What are the advantages and limitations of piezoelectric
measurement technology for machining processes?
The measuring principle is based on the piezoelectric effect: There is a special quartz crystal at the heart of every piezoelectric sensor, which emits a small electric charge when force is applied. Charge and force are in direct proportional relation to each other. What’s more, the quartz is very stiff and has a high natural frequency. This allows us to measure a wide range of extremely dynamic forces accurately, which is a huge advantage for measuring machining forces. In research and development applications, we mainly use dynamometers, which can precisely measure machining forces in all three dimensions. For monitoring purposes, however, you rarely need such an elaborate sensor. Also, the dynamometers are generally not cost effective for certain industrial applications and do not easily fit into production machines in which there is little space and machine constructional changes are not allowed.
By concentrating on the essentials, Kistler offers
a cost-efficient alternative to dynamometers: We use comparatively simple and compact force or strain sensors, which can be integrated inside tool holders or mounted on machine surfaces such as spindle housings. Our customers have been using this type of sensors in other process monitoring solutions for years. We are now building on this experience for machining processes.
Instrumentation Monthly August 2024
Dynamometers measure the absolute cutting forces. How does the measurement of machining processes with piezo sensors work?
In R&D applications, where dynamometers are being used, absolute force measurements are required, which means measuring and calculating all force vectors and torques by using several sensors. In process monitoring on the other hand, piezoelectric sensors usually allow users to draw conclusions about relevant process properties by monitoring the main force vector. For instance, an increase in tool wear causes higher cutting forces or cutting dynamics. These can either be measured directly in the tool holder or on adjacent machine components via forces that affect the material. Thanks to the integration of force or strain sensors, users can compare process-cycles and detect irregularities. The cooperation between tool manufacturer Paul Horn and the Kistler Group is an example of what such a solution can ultimately look like: Together, we offer ready-to-use sensory tool holders for turning applications. This reduces integration efforts to a minimum and ensures that the sensory tool systems have the same properties in use as the original non-sensory systems. During integration, the force sensor is positioned optimally in the force flow so that the data allows to make relative statements about the force conditions in the process. With such solutions and a connected maXYmos process monitoring unit, processes can be monitored and automated. In the event that the measured forces are outside the intended range or have other deviations, the process monitoring system can provide feedback to the machine or process control system. By doing so, the monitoring solution offers an easy way to obtain data for process optimisation. It also helps to extend tool service life, minimize changeover operations, detect rejects at an early stage and avoid machine crashes as well as unplanned downtimes. In case process forces cannot be measured via the tool holder, you mentioned that a strain sensor on the machine structure might be an alternative. What characterizes the ideal position for the strain sensor to deliver exact measurements and how can users find it?
The best position strongly depends on the machine. You need to consider the total stiffness of the machine and the way the forces move from the cutting point into its structure. Another aspect is the behavior of the cutting forces, which depends on the direction of the cutting and on what you are cutting. Luckily, our international service has many years of experience in installing these sensors. Combined with the expertise from our machining department, we can support each customer’s specific needs.
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