Speed control method for a numerical control apparatus

A speed control method for a numerical control apparatus of a machine tool having a tool tip using at least one linear shift axis and at least one rotational axis, comprising the steps of:

• (a) executing a linear interpolation based on a coordinate value on each said linear shift axis of the tool tip and a rotational angle of each said rotational axis at a start point of a shift command, a coordinate value on each said linear shift axis of the tool tip and a rotational angle of each said rotational axis at an end point of the shift command, and a command shift speed in a section defined by said start point and said end point which are given by one block of a machining program, thereby obtaining a distributive shift amount of each said linear shift axis of said tool tip and a distributive rotational amount of each said rotational axis of said machine tool in advance;
• (b) reading a rotational angle of said rotational axis in a present cycle, calculating an amount by which the tool tip is to be displaced in a direction of each said linear shift axis with respect to a rotational center of said rotational axis if said read rotational angle varies in a next cycle by an amount equal to said obtained distributive rotational amount, and obtaining a correction amount for each said linear shift axis required to cancel said amount of displacement;
• (c) in the next cycle, outputting a distribution pulse equivalent to the sum of the obtained distributive shift amount and the obtained correction amount, for each said linear shift axis, while outputting a distribution pulse equivalent to the obtained distributive rotational amount for each said rotational axis,
• thereby controlling the tool tip in such a manner that the speed commanded by the machining program agrees with an actual speed of the tool tip at a machining point.