Machine tools need to be particularly rigid and dynamically stable to ensure high machining quality. Quality losses, among other things, are caused when the process frequency excites the resonance frequencies of the machine and causes it to vibrate. At Karlsruhe Institute of Technology (KIT), researchers have developed an innovative lightweight sliding carriage whose eigenfrequencies can be adjusted flexibly by filling chambers with a fluid. Such novel design allows to considerably increase the production quality and productivity.

Machine tools are vibratory systems. The machine resonance frequencies excited during the machining process may not only cause losses in the quality of the machined parts due to vibrations, e.g., due to chatter, but may also increase wear on the machine components. Machining with interrupted cuts, for instance milling, is particularly critical. A reduced cutting efficiency could avoid such drawbacks but would increase the machining time and be to the disadvantage of efficiency, especially if one takes into account that adjustments do not only affect single operating points but the entire operating range. Since, in addition, processes in industrial practice are often customised, machine tool manufacturers are faced with the challenge of designing machines in such a way that eigenfrequencies can be flexibly adapted to the process anytime.

Researchers at KIT's wbk Institute of Production Science have developed an innovative solution based on the idea that an active change in the mass of the machine tool, in particular in the mass of its moving parts, causes a shift in eigenfrequencies if the mass change is significant relative to the initial mass. The KIT researchers thus designed the machine tool components such as spindle take-up, sliding carriage, and machine stand as lightweight parts using carbon-fiber reinforced plastics (CFRP) which, compared to conventional steel and cast materials, are much lighter while having a high stiffness.

The researchers around Professor J?rgen Fleischer, Head of wbk Institute of Production Science, developed a CFRP sliding carriage with chambers allowing adjustment of the dynamic behaviour through having been filled with a fluid independently from one another by use of a pump. The total mass of the machine thus can be controlled variably. The dynamics of the moving parts is not affected in that way. Quite on the contrary, due to the fact that critical vibrations are not being reduced anymore through process parameter adjustments but through a shift in eigenfrequencies caused by the different fillings, the CFRP sliding carriage even contributes to an increased cutting efficiency. The more chambers are filled, the greater the mass of the sliding carriage, the lower the sliding carriage's eigenfrequency.

To investigate the potential of their approach, the Karlsruhe researchers set up a basic test stand consisting of a CFRP sliding carriage with chambers. The sliding carriage is connected to the frame-mounted motor and drive motor via guiding carriage, guide rails, and a ball screw. Based on a so-called experimental modal analysis, the researchers determined the shift of frequencies by exciting the sliding carriage by means of an impact hammer and assessing the eigenfrequencies.

The Machine Tool with Variable Resonance Frequencies will be presented at the KIT stand (C18), hall 2, Hannover Messe 2014 (April 07 to 11, 2014).

Contact: Dipl.-Ing. Simon Frederik Koch, wbk Institute of Production Science, Karlsruhe Institute of Technology, Germany. Email: simon-frederik.koch@kit.edu