As illustrated in FIG. 4, the prior art oil basin for electric discharge machine generally has two movable walls and L-shaped fixed walls to form an oil basin which has a magnetic worktable placed inside. The two movable walls are initially open to place a workpiece on the worktable. The two walls are then closed and oil is injected through the bottom of the oil basin from the inlet into the basin so as to cover the top of the workpiece. In processing, oil is drained out from the outlet which is located in the same side as the inlet. Residue is filtered out and the oil is reinjected into the basin.
However, there are some apparent defects which exist in the above mentioned oil basin. For instance:
1. The oil, after processing, will from the inner sides of the two movable walls onto the floor when the doors are open.
2. The oil, in processing, is drained out of the basin from the same side as it enters rendering it difficult to filter out all the residue which may cause swirl, produce short circuits, or produce a rough surface on the workpiece.