The Dynamic Stress Study For Micro-Drills

Firstly, the home and overseas researching improvement in micro-hole processing, and the problems and solution methods of micro-drilling are analyzed in this paper. Comparing with Laser Processing, Electron Beam processing, and Electrical Discharge Processing, the high-speed drilling can achieve better micro holes with small dimension error and good surface precision on the wall of the holes, and the processing cost of it is lower for simple equipments, agile operation, and well adaptability, so this processing mode has been applied widely. However, drilling machine system has been hoped deeply demands for it's higher spindle rotating speed and smaller drills'diameter, thus micro drills breaks become a problem to limit the development of high-speed drilling. Now, there are two kinds of solution to this problem with well results for the improvement of processing performances in some degree, the first one is coming from drilling process, then the second tend to consider the drilling machines and processing system. The combination of on-line and real-time monitoring of drilling forces and vibration drilling technique has ameliorated the high-speed drilling properties currently. Although, the stress at the micro-drill danger cross-section from detected drilling thrust and torque could not involve the effects of drills bending deformation and system eccentricity else. Actually, since the small micro-drills stiffness and the avoidless eccentricity of spindle-clamp-drills, the deformation of micro-drills subject to axial force would take place, thus system eccentricity and high-speed rotation should increase this deformation, which give the stress at the micro-drill cross-section, however the stress can not be acquired from the analysis of measured drilling thrust and torque in fact. A dynamic model of micro-drilling spindle system is developed using the element mass, damping and stiffness matrix which are achieved from the Timoshenko beam element, from the rotor dynamics theory with Lagrange theory, and coupling with system eccentricity model, bearing model, and boundary condition at the end of drill in this paper. Eccentricity of spindle-clamp-drills and the drilling thrust and the system rotatory inertia, the gyroscopic moments, shear deformation, and the bearings are incorporated into the model, which will affect the bending deformation of a micro drill during drilling. In order to simplify the rotor dynamic calculation, the part of micro drill with cutting slot has been predigested from equivalent bending deformation principle, and the simplified micro-drill machine system is displayed in this paper. The formula of system forward and backward whirl speeds, critical speeds, critical thrusts, stability determination conditions, and the solution of steady state response are given in this paper, the combined dynamic stress expression is performed from the stress from system bending deformation, and from experimental drilling thrust and torque. The method of the construction of spindle system dynamic model has been checked up by the published references from calculating the critical stable thrust of fixed-fixed and pinned-pinned uniform shaft, the natural frequency parameters of a uniform, non-rotating simply supported shaft, and the whirl speeds of a uniform shaft with isotropic undamped flexible bearing. As a result, the result from the method in this paper is close to the result from the published references in the same condition. In this paper, the critical thrust and whirl speeds of spindle system model are studied. The thrust can not affect anything on whirl speeds of some modes, while whirl speeds of other modes should be decreasing with the increasing thrust; when the thrust approaches to static critical thrust, the whirl speeds of the second mode leads to the destabilization of spindle system. The system critical thrust will decrease sharply after whirl speeds up to a critical value, in other words, a very small thrust will result in system resonance when whirl speeds approach to the first resonant rotating speeds. The effect of improving the first resonant rotating speeds by increasing the bearing stiffness in some range is not obviously until bearing stiffness up to a critical value; In reverse, the bearing damping have an effective influence on the first resonant rotating speeds, thus the big bearing damping contribute to the first resonant rotating speeds weakly. The relationship between the fixed length of the drill, stiffness and damping of the bearings, rotatory speed of the spindle, the system eccentricity, the drilling thrust and the bending deformation amplitude of the drill are determined. As a result, the clamped length of micro drills has a reasonable range; stress of micro drills reduces when the clamped length is in the range. if the bearing damping increase the stress of micro drills decrease, but when the bearing damping is larger than a critical value, the increasing bearing damping does...