FOR 1845: Ultra-Precision High Performance Cutting (UP-HPC)
The research group FOR1845 "Ultra-Precision High Performance Cutting" (UP-HPC) was a cooperation between the University of Bremen and Leibniz Universität Hannover funded by the German Research Foundation and running from April 2014 to June 2020. In Bremen, the Laboratory for Micro Machining (LFM, Professor Ekkard Brinksmeier) and the Bremen Institute for Mechanical Engineering (bime, Professor Bernd Kuhfuß) were involved, as was the Institute for Manufacturing Technology and Machine Tools (IFW, Professor Berend Denkena) in Hanover. The research group was coordinated by Dr Lars Schönemann (Leibniz IWT, Bremen) throughout its entire duration.
The aim of the research group was to reduce the disproportionately long main and auxiliary times in ultra-precision machining with scientific methods, in order to help this technology to become economically viable in the manufacturing industry. To this end, the following questions were researched in five sub-projects:
Sub-project 1 "Ultra-precise milling with multiple tools"
Sub-project 1 "Ultra-precise milling with multiple tools" (LFM, Bremen) dealt with the research and development of a tool adjustment system for diamond milling. Due to the required precision, single-edged tools are usually used here in order to be able to generate surfaces with a roughness of < 10 nm and a shape deviation of < 100 nm. For the use of tools with multiple cutting edges, a controlled adjustment system is necessary. This was developed and investigated in this sub-project on the basis of a locally limited thermal expansion of the tool holder. For this purpose, a reference design for a tool holder was created and a heat source synchronised with the spindle rotation ("LED ring light") was designed. In the prototypical implementation, it could be shown that up to several thousand revolutions per minute a tool adjustment in the nanometre range is achievable (Figure 1).
Sub-project 2 "Ultra-precise high-speed milling"
Sub-project 2 "Ultra-precise high-speed milling" (LFM, Bremen) was dedicated to the scientific investigation of high cutting speeds in diamond milling of ductile and brittle-hard materials. For metals, the transition to adiabatic shearing at a material-defined speed was demonstrated for the first time for diamond milling, which is associated with decreasing cutting forces and lower tool wear (Figure 2). For brittle-hard materials, an increase in critical chip thickness as a result of higher cutting speeds could not generally be demonstrated. However, it was shown that the achievable surface quality improves in principle, as the proportion of brittle fracture decreases at high speeds.
Sub-project 3 "Electromagnetic ultra-precision linear guidance"
Sub-project 3 "Electromagnetic ultra-precision linear guidance" (IFW, Hanover) developed a magnetically levitated linear axis to increase the feed rate in ultra-precision machining. In addition to a fast and frictionless linear movement, the new axis also enables compensation of path deviations in all six degrees of freedom. The axis developed here achieves an acceleration of up to 14 m/s and a traverse speed above 6000 mm/min with a positioning accuracy better than 300 nm or 1.9 µrad. By using feed-forward control, dynamic path deviations during acceleration and deceleration of the axis could be minimised. Ultimately, the path deviations are less than 1 µm or 2 µrad over the entire travel range of the axis of 90 mm. No critical natural frequencies could be detected below 2300 Hz.
Sub-project 4 "Balancing of spindles for ultra-precise high-speed milling"
Sub-project 4 "Balancing of spindles for ultra-precise high-speed milling" (LFM and bime, Bremen) dealt with the high-precision measurement and automated compensation of unbalances in ultra-precise air bearing spindles at high speeds. It was shown that unbalances can be determined and balanced with higher precision by adaptive adjustment of the spindle connection via a coupling system with tuneable stiffness. In combination with automatic balancing systems based on fluid actuators (fixed-location balancing) as well as ultrasonic motors (spread angle method), a balancing degree of G0.064 could be achieved in a few minutes (Figure 3). The accuracy of these balancing systems is such that unbalance can no longer be detected with conventional systems.
Sub-project 5 "Model-based correction of tool paths in ultra-precision machining"
Sub-project 5 "Model-based correction of tool paths in ultra-precision machining" (bime, Bremen and IFW, Hanover) pursued the goal of optimised control technology for ultra-precision high-speed machining. Different approaches for real-time capable control systems were investigated, in which the control parameters are determined adaptively. It was shown that the following error of a machine axis can be reduced from previously 10 nm to less than 2 nm with the help of dynamic parameter optimisation. These control concepts were applied in particular to the magnetic axis developed in sub-project 3, for which the influences of various control strategies (dynamic feedforward, jerk limitation, input shaping) and their combinations were investigated. The parasitic oscillations, which account for the majority of the dynamic axis errors at high speeds, were successfully minimised. Overall, with the machine control optimised in this way, it is possible to run at such high speeds that production time is reduced by up to 25%.
All of the research group's developments were brought together in a joint test stand (Figure 4) and tested in exemplary machining trials.
The research group FOR1845 "Ultra-Precision High Performance Cutting" was successfully completed in June 2020 after six and a half years. The results were published in a joint final publication, which appeared in Springer Lecture Notes:
Brinksmeier, Ekkard; Schönemann, Lars (Eds.) (2022): Ultra-precision High Performance Cutting. Report of DFG Research Unit FOR 1845. 1st ed. Cham: Springer Nature Switzerland AG (Lecture Notes in Production Engineering). ISBN: 978-3-030-83764-8 (Hardcover), 978-3-030-83765-5 (eBook). DOI: 10.1007/978-3-030-83765-5.