| 摘要 |
PURPOSE: To provide a pair of recesses for driving a screw difficult to generate deformation, and a screw driving tool suited thereto. CONSTITUTION: A fastener (screw) head 12 having a pair of circumferential recesses 16, 18 formed in the head 12, a ledge 24 in the recesses 16, 18 is located a short distance below the head surface and extending a predetermined angular length from the axial portion of the ledge. The dimension of the ledge is adjustable to accommodate the strength of the fastener material in a manner such that the displacement formation such as a burr of the recess material protruded from the surface of the fastener head 12 is prevented during the normal installation procedures. The drive angle on the recesses 16, 18 is designed to be zero degrees.
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