Yuhao Gao

Influence of metal chips on drilling quality of carbon fiber reinforced plastic and titanium stacks

The mechanism in drilling of carbon fiber reinforced plastic (CFRP) is different from that of CFRP and titanium stacks. The evacuation of the titanium chips through the hole causes damage to the CFRP. To investigate the quality characteristics of CFRP-layer in drilling of CFRP/Ti stacks, an automated drilling prototype is developed and the drilling experiments of single CFRP and CFRP/Ti stacks are designed respectively for purpose of analyzing the influence of the titanium chips formation with various cutting parameters on the drilling performance of CFRP-layer. The chips morphology are firstly observed, then the mechanism of entrance spalling, high surface roughness (Ra), and hole diameter out of tolerance is studied. The results show that, low feed rate and high cutting speed generates long and folded chips wrapped around the drill, which damage the CFRP strongly. The damage can be controlled by optimization of the cutting parameters. Low feed rate helps to eliminate the entrance spalling, but enlarger the hole diameters in both the CFRP and the titanium layers. To prevent the chips from wrapping on the drill, the cutting speed should be selected in a moderate range. Therefore, the cutting parameters on the titanium layer can be recommended as the feed rate between 0.09 mm/r to 0.13 mm/r, and the cutting speed in a range of 12 m/min to 24 m/min.

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.Copyright

Optimization Design for Normal Direction Measurement in Robotic Drilling

In large assemblies, the perpendicularity of a bolting hole has remarkable effects on the fatigue life and fluid dynamic configuration. While the Computer Aided Design (CAD) model of complexly curved workpieces is hardly satisfied because of manufacturing errors, it is very necessary to measure the normal direction in robotic drilling. One advisable approach is to arrange four laser displacement sensors at the vertices of a rhombus whose center aims at the drilling position. The influencing factors of the measurement precision are firstly discussed in this study, and a novel method to optimize the arrangement size of the displacement sensors for higher precision is introduced. The measurement error for the normal direction consists of the principle error and instrumental error, caused by inconstant curvature of the surfaces and distance measuring errors of instruments, respectively. When the displacement sensors are arranged closer to each other, the principle error will be decreased, whereas the instrumental error will be increased. After the curvature feature of the surface is obtained with the introduced method, the graph of the measurement precision and the arrangement size is plotted. Then, the graph can contribute to developing an optimized design of arrangement size for higher precision. Finally, an example of the curvature obtainment and the arrangement optimization is given. The experimental results show that the optimized design has achieved a higher precision of ± 0.17° for αY and ± 0.26° for αX, whereas the precision of another design is about ± 0.21° for αY and ± 0.29° for αX. The proposed optimization method will bring greater benefit for complexly curved surfaces in practical products and it offers a chance to optimize the arrangement during design phase with little costs.Copyright