PURPOSE:To enhance the reliance upon automatic tool replacement by carrying out the clamping and unclamping of a tool with/from a clamp rod in a spindle through the thread coupling system which ensures good followup performance, and furnishing a means to sense that the tool is seated or separated. CONSTITUTION:First the spindle head 1 of a spindle is attached to a tool holder 4 on the mating side, and then a clamp rod 21 is rotated regularly. Now the tool holder 4 is drawn toward the spindle 12 and clamped because of thread advance motion due to thread coupling of a nut part 23 on the clamp rod 21 side with a pull bolt part 22 on the tool holder 4 side. When the tool holder 4 is seated on the seat 24 on the spindle 12 side, an air blow nozzle 36 is blocked, and a pressure rise in the air supply 37 actuates a pressure switch 39, and thus the seating is checked and sensed. Because the clamping involves displacement of a sensing rod 27, a proximity switch 34 with a plunger 32 working as an approaching element allows checking and sensing that the tool holder 4 was clamped certainly.
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