Dynamical Model for the Process of Withdrawal Formation at Deep Drilling of Small Diameter Holes w ith the Increased Relative Length

Abstract

We consider the machining process for drilling deep holes with small diameters (less than 23.0 mm) and increased relative length (length to diameter ratio) over 100. The main problem at drilling such holes is prevention of high-cost products reject due to withdrawal and axis non-straightness. The main operation of the manufacturing process which provides the specified values of the pointed parameters of deep holes is the operation of deep drilling. This operation is performed using a special tool and tooling, with outer lubricant injection, while the workpiece is rotating and the tool is moving forward. The applied tool possesses low twist-and-steer and torsional stiffness, and it works at high rotational frequencies of the workpiece and lubricant pressures. Due to the influence of different factors on the process of deep drilling, this tool is experiencing the kinematic perturbation with the rotational frequency of the workpiece, which is the main reason for further withdrawal variation along the drilling length. Withdrawal formation is a dynamical process, tool lengths may overcome resonance values, thus leading to instability of formation and variation of axis withdrawal. Analysis of results of the available investigations showed that they do not account the features of withdrawal formation at drilling of deep holes of small diameters with the increased relative length. This paper presents results of theoretical investigation on development of the dynamical model for withdrawal process formation, which accounts for the features of drilling deep holes with the increased relative length. Recommendations on decreasing the withdrawal and axis non-straightness, including the withdrawal control, are given.