Method of machining work in numerically controlled lathe

A method of machining a work in a numerically controlled lathe having a rotatable spindle, a first tool rest configured to move back and forth relative to the spindle in a spindle axis line direction and in a direction crossing a spindle axis line, and a second tool rest configured to move back and forth relative to the spindle in the spindle axis line direction, the method being characterized by comprising the steps of:

• judging which of the first tool rest and the second tool rest tools used in current machining are installed on;
• judging whether tools used in next machining are the tools installed on the first tool rest or the tools installed on the second tool rest;
• judging whether interference is caused between the first tool rest and the second tool rest during movement when, as to the first tool rest and the second tool rest, the tool rest on which the tools used in the current machining are installed is different from the tool rest on which the tools used in the next machining are installed;
• obtaining, for both the first tool rest and the second tool rest, interference boundary positions at which the first tool rest and the second tool rest are in proximity but do not interfere with each other, on the movement paths of the respective tool rests, when interference is caused between the first tool rest and the second tool rest;
• moving one tool rest toward the standby position at a fast feed speed when the tool rest on which the tools used in the current machining are installed is different from the tool rest on which the tools used in the next machining are installed;
• obtaining a feed speed for the other tool rest so that the other tool rest on which the tools used in the next machining are installed reaches the interference boundary position within the same time as the time in which the one tool rest reaches the interference boundary position, and moving the other tool rest toward the interference boundary position at the obtained feed speed;
• and
• increasing the speed of the other tool rest to a fast feed speed when the other tool rest passes the interference boundary position so as to move the tools used in the next machining to a machining start position in the case where the feed speed obtained for the other tool rest is less than the fast feed speed.