Delamination prevention in drilling of composite laminates and force modeling of the milling operation

A model is established which predicts critical thrust force and critical feed rate at which the delamination crack begins to propagate in drilling of composite laminates. Another model is constructed to predict cutting forces in the milling operation. These models have been verified by experiments.;In drilling of unidirectional composites, the delamination zone is modeled as an elliptical plate, with clamped edge and subjected to a concentrated, central load. Based on fracture mechanics, laminate theory and cutting mechanics, expressions are developed for critical thrusts and critical feed rates at which delamination is initiated at different ply locations. A variable feed rate strategy is formulated, which avoids delamination while drilling in a time-optimal fashion.;While it is difficult to model delamination in drilling of multi-directional laminates, experiments have revealed that the critical thrusts and feed rates computed for unidirectional laminates can be conservatively used for multi-directional laminates.;With regard to tool geometry, chisel edge width appears to be the single most important factor contributing to the thrust force and hence delamination. Point angle also plays a significant role. The need to combine small chisel edge and small point angle with newer materials such as polycrystalline diamond is emphasized.;For the milling operation, a plowing effect modification to the existing force models is proposed. Starting with simple, bi-dimensional cutting, the force analysis is successively extended for realistic cutters with geometric features such as lead and rake angles, helix angles and ball-ends. Force equations are developed for peripheral milling, face milling and ball-end milling. The model for ball-end milling is verified experimentally and a good agreement is seen between simulation and experiments.;Based on the analysis of milling and drilling operations, a hollow grinding drill tool was designed and tested. This tool resulted in much smaller thrusts and much better hole quality as compared with the standard HSS drills.;It is hoped that the present study will be useful in avoiding delamination in epoxy-based laminated composites via machining planning, tool design, and adaptive control or active control.