When a corner shape of a workpiece is machined by a wire electrical discharge machining process, grooving called rough processing is performed firstly while a finishing allowance is left in advance. Then, from the finish machining performed secondly, an electrical condition is switched to one having a low machining energy while an offset amount for the left finishing allowance is gradually decreased. In this way, the corner shape precision is improved.
Here, when the machining process is performed at a constant machining speed, a machining allowance in a corner part is increased or decreased compared with a machining allowance in a straight part. Therefore, when the machining allowance in the corner part is increased a machining remainder occurs compared to a target shape; and when the machining allowance in the corner part is decreased an excessive machining occurs compared to a target shape. As a result, a problem arises in that the corner shape precision is degraded. Here, in order to improve the shape precision of the corner part, there is a need to uniformly control the machining volume for the straight part and the corner part per unit time.
In contrast, in the technique disclosed in Patent Literature 1, the machining volume per unit movement distance of a wire electrode for the straight part and the corner part are accurately calculated based on information stored in a corner control information storage unit. Then, a machining speed ratio between the straight part and the corner part is calculated in advance in response to a machining volume ratio between the straight part and the corner part, the machining speed at the corner part is controlled at the machining speed calculated from the machining speed ratio and the machining speed of the straight part in order to improve the corner shape precision.