A slitting cutter having a disk-shaped cutter body with a plurality of circumferentially spaced insert receiving portions and a plurality of cutting inserts retained therein. Each insert receiving portion has first and second clamping jaws spaced apart by an insert receiving slot, the first clamping jaw resiliently displaceable and having a resilient axis of rotation. The number of cutting inserts resiliently clamped in the slitting cutter is an inner cutting diameter defined by the plurality of resilient axes of rotation multiplied by a spacing factor of between 0.15 and 0.30. A tool key having first and second key prongs is used in combination with the slitting cutter. The second key prong has a thrust surface with a concave profile. In a partially assembled position of the slitting cutter the thrust surface contacts at least one of two spaced apart first and second corner surfaces of the respective cutting insert.