A cutting insert mounting system adapted for a cutting tool in which, by using a tightening screw, a cutting insert is detachably mounted on an insert mounting seat. A central axis of a screw hole of the insert mounting seat is inclined with respect to a normal direction of a bottom wall surface of the insert mounting seat. When the cutting insert is placed on the insert mounting seat, on a virtual plane defined to expand the bottom wall surface, a central axis of the screw hole is shifted from the central axis of a through hole of the cutting insert to the side of one of two side wall surfaces of the insert mounting seat. On the virtual plane, an axis shift direction of the central axis of the screw hole and an inclination direction of the central axis of the screw hole are different.

A toolholder and method for use with a cutting insert having a clamping opening, comprising a toolholder body and a clamp pivotably mounted to and contacting the toolholder body and pivotable between a first position in which a protrusion of the clamp is disposed in a position in which it is arranged to be received in the clamping opening in the cutting insert to secure the cutting insert to the toolholder body and a second position in which the protrusion is spaced, relative to the toolholder, upwardly from the first position so that the cutting insert is not secured to the toolholder body by the protrusion. A resilient member is arranged relative to the clamp to urge the clamp to the first position and to resist pivoting of the clamp to the second position. An opening in the clamp may receive a portion of a separate lever.