MODAL ANALYSIS OF CARRIER SYSTEM FOR HEAVY HORIZONTAL MULTIFUNCTION MACHINING CENTER BY FINITE ELEMENT METHOD
The aim of the paper is to reveal and analyze resonance modes of a large-scale milling-drilling-boring machine. The machine has a movable column with vertical slot occupied by a symmetrical carriage with horizontal ram. Static rigidity of the machine is relatively low due to its large dimensions. So it is necessary to assess possible vibration activity. Virtual and operational trials of the machine have been carried out simultaneously. Modeling has been executed with the help of a finite element method (FEM). The FEM-model takes into account not only rigidity of machine structures but also flexibility of bearings, feed drive systems and guides. Modal FEM-analysis has revealed eight resonance modes that embrace the whole machine tool. They form a frequency interval from 12 to 75 Hz which is undesirable for machining. Three closely located resonances (31-37 Hz) are considered as the most dangerous ones. They represent various combinations of three simple motions: vertical oscillations of a carriage, horizontal vibrations of a ram and column torsion. Reliability of FEM-estimations has been proved by in-situ vibration measurements.An effect for stabilization of resonance modes has been detected while making variations in design parameters of the machine tool. For example, a virtual replacement of cast iron for steel in machine structures practically does not have any effect on resonance frequencies. Rigidity increase in some parts (e.g. a ram) has also a small effect on a resonance pattern. On the other hand, resonance stability makes it possible to avoid them while selecting a spindle rotation frequency.It is recommended to set double feed drives for all axes. A pair of vertical screws prevents a “pecking” resonance of the carriage at frequency of 54 Hz. It is necessary to foresee an operation of a main drive of such heavy machine tool in the above resonance interval with the spindle frequency of more than 75 Hz. For this purpose it is necessary to proceed to high-speed machining (HSM).