A Study on Drum Shape Error in WEDM and the Improvement Strategy
|Keywords:||線切割放電加工;鼓形量;加工精度;正常放電比;回授控制;WEDM;drum shape error;straightness accuracy;normal discharge ratio;feedback control||Issue Date:||2010||Abstract:||
WEDM is a machining process using thin wire electrodes. Because of the flexibility of the wire electrodes, there would be vertical alignment geometric error left on the machined surface. In most workshop practice, the error is corrected through multiple surface-finishing process, which is not efficient in time and cost.The previous studies over this issue proposed that the vertical alignment geometric error were out of two mechanisms – either vibration of the wire electrodes, or the ineffective of debris removal – both making the uneven distribution of the electoral discharge alone the wire electrodes.
In this research, with SKD11 alloy of 60mm in thickness as the workpiece, the influences of the wire electrode tension, the servo voltage, and ignition delay time (Td) over the vertical alignment geometric error were observed, and also in conjunction with the ignition delay time of electrical discharged (Td) in order to figure out the suitable control index for feedback automatic control. It showed that, as the wire electrode tension increased, the influence of the wire vibration diminished, which suppressed the removal of the debris making the uneven distribution of electrical discharges. This research also noted that the vertical alignment geometric error could be categorized into two major types: nearly linear type, and irregular type. Different types of the error were attributed to two factors – the flow of debris removal, and the width of discharged gap – which reflected to different normal discharge ratios. Under the adequate normal discharge ratio, the vertical alignment geometric error would be much reduced. Based on the finding stated above, a clean-cut feedback control strategy was proposed in this research. Under the machining setting of servo feeding (G95), the normal discharge ratio was used as the control index to change the discharge off-time for the feedback control. Through on-site experiments, it was proved that, under the machining condition setting of this research, the vertical alignment geometric error could be much decreased with the normal discharge ratio of 40%. For workpiece of 60mm in thickness, the geometric error decreased from 6μm to nearly zero. For workpiece of 80mm in thickness, the geometric error decreased from 11μm to 2μm.
|Appears in Collections:||機械工程學系|
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