Cutting processes can now be monitored reliably – thanks to piezoelectric measurement technology

Cutting processes generally have extremely diverse characteristics that make it very difficult to monitor them reliably. Micromachining processes in particular demand increased vigilance – in fact, they are virtually impossible to monitor. The remedy: piezoelectric force sensors from Kistler. Thanks to their very wide measuring range, these sensors reliably capture even the smallest forces to supply information about the condition of the cutting edge.

During a machining process, the forces on workpieces and cutting tools are high in relation to the area on which they act. This limits cutting insert lifetimes, which may differ depending on the material and the application. Lifetimes can vary enormously, especially if micromachining processes are involved and the materials are difficult to machine. Another factor is that we cannot use our own eyes or ears to determine the current condition of the tool edge, so we are left with no reliable information for implementing integration into automated processes.

Gunnar Keitzel, Head of SBF Cutting Force
Gunnar Keitzel, Head of SBF Cutting Force

Ensuring reliable micromachining processes: a challenge

This was also the challenge that faced the production team at Kistler, the measurement technology expert based in Winterthur, Switzerland. Kistler's production range includes cartridges and membranes for exceptionally sensitive pressure sensors. A nickel-iron alloy is used for these parts due to their extremely compact design and wall thicknesses of as little as 0.06 mm. This alloy has a lower thermal expansion coefficient and a higher temperature range than materials used for more common cartridge types. However, it is very difficult to machine this material. It reacts with extreme sensitivity and any forceful contact causes immediate deformations, leading to frequent tool breakages that result in scrap during production. Tool lifetimes are also very short and extremely variable.

Information for automated machining processes

Severin Hosmann, Team Head at Kistler, explains: 'The very wide variations in tool lifetimes caused a lot of trouble for our production staff. They had to keep their eyes constantly on each individual machine. Sometimes they could only machine 20 cartridges with one cutting edge, but on other occasions they could manage 50. This was very inefficient, so we needed a system that could permanently optimize the process.'

The team began with an attempt to assess the cutting edge's condition by monitoring the drive power of the main spindle motor. However, this method is only suitable for workpieces with bore diameters of 3 mm or more. Little can be gained from applying this approach to micromachining, because the mass of the machine spindle is too great and the friction losses in the spindle are too high: in some cases, they act as disturbance variables that generate larger signal components than the cutting process itself. This means that a solution of this sort is not sensitive enough to be used for automation.

The solution: piezoelectric force measurement

An innovative sensor concept based on piezoelectric force measurement was eventually developed as the solution to these problems. This involved adding Kistler's own high-resolution SlimLine force sensors to the turning tools. Thanks to their excellent rigidity, SlimLine sensors are especially suitable for measuring rapidly changing tensile and compression forces. The measuring chain, the connection to the machine control and software specifically programmed for this application were also installed. Kistler's metalworking specialists conducted an extensive series of tests to prove that the sensor concept was suitable for the purpose. During each turning test, all the test dimensions (such as workpiece thickness) were recorded in a measurement log. Within a very short period, the team completed about 1 500 cutting tests that yielded important findings as the basis for further action.

Force signals provide information about wear

Gunnar Keitzel, Head of Kistler's Strategic Business Field Cutting Machining, was highly satisfied with the results: 'This series of tests was a complete success. They showed us that our sensor concept is ideal for measuring forces in micromachining. Even the smallest changes in the process can be visualized.' Piezoelectric force measurements made it possible to demonstrate that tool condition and geometry exert a significant influence on the force level in the machining process. As wear increased, there was a disproportionately high increase in tool loads. This led to significantly shorter edge lifetimes accompanied by sudden tool breakages. In turn, these problems caused numerous production outages and increased the scrap rate.

From process optimization to condition monitoring

These findings were used to adapt the machining processes. Measures taken included optimizing the cutting volume so as to minimize the acting forces. At the same time, the direct connection to the machine control allows dynamic adaptation of the tool according to the condition of the cutting edge. The result: significantly longer tool lifetimes that lead to improved process reliability. Continuous tool monitoring is possible thanks to SlimLine force sensors from Kistler. They measure even the smallest forces that act during the production process. By referring to this information, production staff know when a cutting edge needs to be replaced – so scrap is prevented before it can even occur.

Reliable data to calculate physical limits

Severin Hosmann points out: 'To measure is to know. To know is to understand. We need to understand the processes that take place in the machine before we can analyze the parameters and draw conclusions about tool performance and quality assurance. Until now, we had to work with information from the manufacturer on feed, speed, cutting depth and so on. But that information leaves a lot of leeway for action, so what we did was largely based on guesswork. Now, thanks to the new system, we can work with numbers, data and facts. From now on, tool parameters are expressed in terms of cutting force. In our case, this made the entire process transparent. And that allows us to calculate the physical limits in advance.'

A concept proven in practice

The system has since proven its viability in practice and is now being launched on the market as the Piezo Tool System. This system – the PTS – is equally suitable for process optimization and condition monitoring, in large-scale series manufacturing as well as production of single items. Gunnar Keitzel: 'Our production colleagues were looking for a suitable way of monitoring their machining processes. And at the same time, we needed an application that we could use for practical tests on our sensors. This created the possibility of testing the system directly on site, here in our own production facility. Good collaboration among everyone involved was a critical factor in bringing this project to a successful conclusion.'