Tool changing method and apparatus

A tool changing apparatus to be used in combination with a machining center comprising a column, a tool magazine for storing a plurality of tools and positioning a desired one of the tools at a standby position and for transferring the one tool from the standby position to a tool change position, a headstock slidably supported on the column so as to slide along an axis of a spindle held thereon, a tool changing head mounted on the headstock so as to be turnable about and to be moved toward and away from an axis of the spindle, said tool changing head being provided with two pairs of grippers respectively extended in opposite directions such that one of the two pairs of grippers is positioned to grip a tool held on the spindle when the headstock is located at a position corresponding to said tool change position in response to a tool changing instruction and the other pair of grippers is positioned to grip a next tool positioned at the tool change position, whereby the tool changing head carries out a predetermined tool changing operation, said tool changing apparatus comprising control means including:

• storage means for storing types of tools classified into small type tools and large type tools with reference to a predetermined threshold tool diameter determined on the basis of stroke of the two pairs of grippers;
• type identifying means for identifying the respective types of an on-spindle tool mounted on the spindle and a next tool to be used;
• small type tool feed instruction issuing means for issuing a small type tool feed instruction to move the next tool from the standby position to the tool change position before the headstock is moved to a position corresponding to the tool change position, when the next tool is of the small type;
• large type tool feed instruction issuing means for issuing a large type tool feed instruction to move the next tool from the standby position to the tool change position after the headstock has been moved to the position corresponding to the tool change position, when the next tool is of the large type;
• small type tool return instruction issuing means for issuing a small tool return instruction to return the tool removed from the spindle by the tool changing operation from the tool change position to the standby position after the headstock has been moved toward a machining position, when the on-spindle tool removed from the spindle is of the small type;
• and
• large type tool return instruction issuing means for issuing a large tool return instruction to return the on-spindle tool removed from the spindle by the tool changing operation from the tool change position to the standby position before the headstock is moved toward a machining position, when the on-spindle tool is of the large type.