Modeling and simulation for the critical bending force of power chucks to guarantee high machining precision

The progressive development of hard turning technology places greater demands upon clamping devices. The chuck is required not only to grip the workpiece securely but also to have high and stable radial chucking stiffness to guarantee high machining accuracy. As a primary property of a chuck, the clamping force, which greatly affects the security and precision of the machining process, must be meticulously determined. However, existing approaches for determining the required clamping force of jaw-chucks have mainly focused on their security. This is especially true for the crucial computation model for determining the critical bending force, which has more influence on the machining accuracy than other components of the cutting force. A new analytic computation model for determining the critical bending force of three-jaw power chucks, taking machining precision into account, is introduced in this study. The finite element method was applied and experiments performed to assist the modeling work. The results show that, when the jaw contacts the workpiece along its full clamping length, the variation in apparent radial chucking stiffness with respect to the direction of the bending force can be eliminated. In addition, an entirely uniformly distributed clamping force is desirable for the maintenance of larger critical bending forces, and a tiny negative initial angle is preferable. Furthermore, the critical bending force and radial chucking stiffness can be improved dramatically by enlarging the clamping length. This approach is helpful in the substitution of hard turning for grinding as the last process of machining.