Abstract: A cutting tool assembly wherein a replaceable cutting insert is clampingly held between a pair of clamping jaws of an insert holder which are respectively provided with pairs of clamping surfaces one pair of clamping surfaces being of uniformly curved cross-sectional shape, while the other pair of clamping surfaces being of non-uniformly curved cross-sectional shape; the arrangement being such that when the insert is clamped between the jaws the respective clamping surfaces contact each other along discrete, axially directed locations which are symmetrically disposed with respect to and axially spaced from a median plane of the insert.