Quantitative Analysis and Improvement of Countersink Depth in Stack Drilling

While bolt fastening is the most commonly used method for fastening components, hole quality is an important technology standard in modern manufacturing, especially in the aircraft industry. With low stiffness, the machining deformation of aerospace structures has been a striking problem, which makes it difficult to achieve tight tolerance of countersink depth in one-shot drilling of stacked materials. As a result of the manufacturing errors between the workpieces and the digital models, the position of the workpiece surface can hardly be known before drilling. Moreover, the cutting force adds to the deformation of the thin-walled workpiece in the direction of the feed axis. In view of problems mentioned above, a method of position compensation based on the clamp foot displacement is proposed in the paper, which ensures the countersink depth accuracy by compensating for the deformations of clamping and countersinking respectively in different stages of drilling. Some drilling experiments were conducted, in which the forces in the feed direction were real-time monitored and recorded for FEM simulation. The influencing factors of countersink depth error are firstly discussed in this study, which mainly consists of the size of clamping force and the stiffness differences of variable drilling positions. Numerical simulations were carried out to study the deformation characteristics of workpiece under the combining effect of clamping force and cutting force achieved above. Comparing the simulation results and the experiment results, some other influencing factors of countersink depth are discussed.