When we cut the metal in machine tools, the vibration on the spindle is generated by unbalance of a high speed spindle or interaction between workpiece and tool in machining. Unbalance and interaction (or chatter vibration) between workpiece and tool in machining are considered as disturbance. High speed and heavy cutting performed for improving the surface quality and productivity are significantly deteriorated by vibration due to these disturbances. The disturbance is an important limiting factor of production rate and surface quality, and reduces the tool life and the dynamic performance of machine tool itself. Therefore we need to compensate the disturbance. Through the compensation of disturbance, we can expect the surface quality and production rate will be improved very much.
In order to compensate the disturbance of the system, we used the electromagnetic actuator (EMA) system. The electromagnetic actuator system consists of 4 electromagnetic poles generating magnetic force. And the spindle system consists of air bearing spindle, EMA, current amplifier and controller. In here, the current amplifier supplies a current to EMA, and the controller control the current for EMA and compensate the disturbance. Therefore we can control the spindle system by adjusting the current of a current amplifier entering into the electromagnetic actuator. The advantage of EMA is non-contact & lubrication, precision drive, changeable stiffness and low maintenance.
Figure 1. Micro-milling using the active air bearing spindle.Figure 2. Schematic of the active air bearing spindleFigure 3. Mechanical model of the active air bearing spindleFigure 4. Bondgraph model of the active air bearing spindle
2. Hammering test
Hammer testing is a straight forward method which yields good results under most conditions. This testing technique makes use of the fact that when a (mechanical) structure is excited by means of a Dirac pulse, the structure responds with all its eigenvalues (i. e. natural frequencies and damping). In practice, a true Dirac pulse does not exist since its theoretical duration is zero. In general, as the impact duration increases the range of excited frequencies decreases. Impact tips mounted to a force impulse hammer consist of different materials (steel, plastic, and various rubbers), each yielding different excitation durations and different excitation frequency ranges. Depending upon the frequencies of interest of the structure under test, the appropriate impact tip is mounted to the hammer.
Figure 5. Experimental setup for the hammering testFigure 6. Experimental result of the hammering test (Frequency response of the plant)