A novel prediction model for thrust force and torque in drilling interface region of CFRP/Ti stacks

The hybrid structure of CFRP/Ti stacks is becoming widely employed in aerospace structural applications, but drilling the stacks of these two materials at one time is a challenge to manufacturing engineers, especially in the process of drilling interface region, in which the drill edges cut two different materials simultaneously. Excessive cutting forces could result in defects such as delamination, burrs, and interlayer chip, and could also lead to high temperatures and excessive tool wear. In this paper, a mechanistic model is developed to predict the thrust force and torque with time of interface region in drilling of CFRP/Ti stacks. In the model, three-dimensional drilling forces distributed along cutting edges are expressed as the multiplication of transformation matrix, specific energies, and uncut area, which are determined by the drill geometries, cutting conditions, friction, and material properties. A transformation matrix is established to decompose the normal/friction cutting forces on the rake face into tangential/radius/axial forces defined in the drilling coordination. Coefficients of specific energies were calculated from small number of calibration experiments. Experiments were also conducted to validate the model in a wide range of spindle speed and feed rate. The predicted results agree well with experimental results. However, the predictions for torque are lower than the experimental results due to the spring-back of CFRP hole. The model can assist in understanding the drilling process of CFRP/Ti stacks and be used to control cutting forces by selection of drilling conditions and drill geometries.