TAP FOR CAST EXTRACTING HOLE

PURPOSE:To allow thread cutting to the bottom section of a hole without forming a flat section on the guide section tip and smoothly discharge chips by providing communicating grooves with a U-shaped cross section from the tip edge section of a cutting feed hole to discharge grooves of chips. CONSTITUTION:Communicating grooves 6a-6f with a U-shaped cross section are formed in a taper shape with an upgrade from the tip edge section 5a of a cutting feed hole 5 to discharge grooves 4a-4f. An equation 3S0>=S1 is satisfied, where S0 is the opening area at the tip of the guide section 2 of the cutting feed hole 5, S1 is the overall cross section of the communicating grooves 6a-6f, thus the flowout to the discharge grooves 4a-4f is made smooth, and the discharge of chips is made smooth even if the cutting feed pressure is low. IN addition, the angle alpha formed by the tip edge 3a of a spot facing blade and the communicating grooves 6a-6f is made 90 deg.>=alpha>=45 deg., thus effective thread cutting is allowed even near the bottom section of a blind hole.