The Spindle System And Drills Dynamic Characters Study For High-speed Micro-drills

The micro-drilling is more and more used in the machine, for example in printed circuits boards (PCB) smaller holes down to 0.1 mm diameter need to be drilled through the board layers.Several researches in China and overseas about the improvement of the micro-hole processing and the problems and solution methods of micro-drilling are investigated in this paper. Comparing with Laser Processing, Electron Beam processing, and Electrical Discharge Processing, the high-speed drilling can give good micro holes with small dimension error and good surface precision on the wall of the holes, and the processing cost is lower due to simple equipments, agile operation, and well adaptability, so this processing mode has been widely applied. However, drilling machine system has been more demanded for its higher spindle rotating speed and smaller drills' diameter, the small diameter drills are more susceptible to the drill breakage, thus micro-drill breakage becomes a main factor to limit the development of high-speed drilling. The efforts to remove the broken drill are enormous and can affect the hole's quality and increase the cost of the process. When drilling a small hole, the dynamic characteristics of the drill bits and their relationship to hole's quality are the most critical factors. 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. The stress at the micro-drill danger cross-section from detected drilling thrust and torque could not involve the effects of drills bending deformation andsystem eccentricity else. Because of the smalless of micro-drills stiffness and the avoidless eccentricity of spindle-clamp-drills, the deformation of micro-drills is subjected to axial force. The system eccentricity and high-speed rotation increase this deformation, which gives 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 this paper, 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 conditions at the drill tip. 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 the drilling process. 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 found, the combined dynamic stress expression is performed from the stress, from system bending deformation, and from experimental drilling thrust and torque.Compared with the mean of drilling forces from the data of detecting system, the fluctuation is not too big. The following conclusion can be obtained by the comparison of stress from drilling thrust and torque: the shear stress from torque takes a stronger influence on the drill composite stress than thrust for the too small diameter of micro-drills. In other words, the shear stress from torque is the main part of the drill composite stress ordinarily, thus drills breakage should be attributed to the drilling 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. Further more, the natural frequency of a uniform, non-rotating cantilevershaft has been measured with accelerogragh DH5938 and acceleration sensor YD42 to compare with the result from this paper's method. As a result, the result from the method in this paper is close to the result from the published references and experiments in the same conditions.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 not has any effect to the micro drill stress nearly. Under the condition that there is eccentricity in the drilling system, the stress of micro drills increases rapidly when the spindle speed approaches to resonant speed of the drilling system. The effect of the axial drilling force on the stress of micro drills is related to eccentricity of the drilling system, the micro drill stress increases rapidly when the axial force is larger than a critical value.The system has been simulated numerically to get the transient response process, the track of motion, the Poincare mapping, the system property in Frequency Domain, and the radial and axial displacement. In conclusion, the analytical response of system dynamic model only can show the identity about the system rotating speed, can't reflect the system nature frequency property, and the numerical solution from simulation fluctuate periodicallyaround the analytical solution.The conclusions are valuable to the designing of micro-drill machines and productions.