This paper studies the effect of the clamping conditions on the brittle fracture of the poly-crystalline cubic boron nitride (PCBN) insert during dry high speed machining of gray cast iron. Dry high speed face turning tests are performed on a composite design disc brake rotor to investigate the brittle fracture of the PCBN insert. Scanning electron microscopic analysis is used to examine the fractured surface. A thermo-elasto-plastic finite element analysis is employed to determine the stresses induced in the insert due to thermal and mechanical loading generated during cutting. Different clamping conditions, tool holder materials and clamping torques are considered. The analysis reveals that a steeper temperature drop on the PCBN tool rake face results when a carbide chip breaker is used between the clamp and the insert and also when a Mallory 100 M or Aluminum tool holders are used. As a result high thermal stresses are obtained on the insert rake face. The brittle fracture of the insert is originated at the rake face at a local maximum of tensile stress. The results also indicate that reducing the clamping torque and or removing the chip breaker reduces the local maximum tensile stresses on the rake face and increases the tool life by tenfold.