| The micro-cutting and micro-grind ing technology with micro-machine tools not only is able to manufacture geometrically complex3D miniature components in a wide range of engineering materials, but also has small equipments, low energy consumption and low cost. Therefore, it is one of development directions of green manufacturing. As key components of micro-machine tools, micro-spindles directly determine their performance and the development and application of micro-cutting and micro-grind ing technology. But now available micro-spindles at home and abroad have either high rotational speed but low rotational accuracy, or high rotational accuracy but low rotational speed, and is unable to simply replace tools without reducing rotational accuracy. Aiming at the aforementioned urgent challenge for domestic and foreign researchers, a new design idea and method of the flexible connection structure between power spindle and tool, and one-piece tool/rotor structure for micro-spindles is proposed in this paper. The objective is development of an ultra-high speed and high rotational accuracy micro-spindle, so its related design theory and key technologies are deeply studied. The main contents of this paper are as follows:(1) On basis of the research status analysis of now available micro-spindles, the innovative design concept for micro-spindles is proposed and its feasibility is analyzed. The performance requirements of micro-spindles on rotational speed, rotational accuracy, cutting force and torque are systematically analyzed according to micro-cutting experiments conducted by domestic and foreign researchers and in terms of that, design objectives of the micro-spindle are determined. Based on the correlative theory of gas dynamics, the structure of radial impulse air driven turbine is designed and calculated and its aerodynamic performance, air flow and pressure distribution are studied by simulation. The frictional power loss of the micro-spindle and the total power produced by turbine are analyzed and calculated. The basic design parameters of miniature aerotatic bearings for micro-spindle are estimated. The calculation model of carrying capacity of aerostatic journal bearing and that of aerostatic thrust bearing are established and the load carrying performances of the designed miniature aerostatic bearings used for the micro-spindle are calculated. The changes of load carrying performances of the aerostatic journal bearing with annular orifice restrictors and that of the aerostatic thrust bearing with annular orifice restrictors, with the supply pressure, rotational speed and design parameters are studied by simulation, which provides not only evidence for design optimization of the support bearings for the micro-spindle, but also reference for design of miniature aerostatic bearings of the same type.(2) The structure and performance of now available flexible couplings are compared and analyzed. On this basis, a monolithic micro-flexible coupling including its design concept are innovatively proposed and its error compensation principle is studied. Based on the correlative theory of material mechanics, its design model is established. The structure parameters are designed in detail with the micro-spindle in this paper as application object and the related simulation and optimization are conducted. The fatigue performance of the flexible coupling is theoretically analyzed. The prototype is manufactured and its torsional stiffness is tested. The monolithic micro-flexible coupling has simple and symmetric structure and good dynamic characteristic. It not only avoids stack-up error existing in convetinoal couplings, but also can effectively compensate errors of the micro-spindle including manufacturing error, installing error and motion error and thus increase rotational accuracy of the micro-tool or other driven parts connected with the micro-spindle.(3) A monolithic micro-tool chuck structure scheme using shape memory alloy (SMA) is proposed and its design theory is established. The structure parameters are designed in detail with the micro-spindle as application object and the influence of the centrifugal force on the clamping performance is studied by simulation. The influence of the deformation at low temperature, heat treatment temperature, low training temperature and training times on the the two shape memory effect of SMA ring is experimentally studied. The manufacturing process of the SMA ring using TiNi (Ti49.6%, Ni50.4%) alloy for the maximum two shape memory effect is obtained. The prototype is manufactured and its performance including installing operation and clamping force are tested. The monolithic micro-tool chuck has not only simple configuration and good balance performance, but also no adding installation error, convenient operation, easy size adjustment with application object, so it is an ideal device for the ultra-high speed and ultra-high precision micro-spindle and other high speed precision miniature transmission device.(4) Based on rotor dynamics, the critical speeds of micro-spindle are calculated and compared with the maximum design rotational speed. The stress of turbine shaft and modal shapes and natral frequencies of the whole rotor are studied by simulation. Every component and overall structure of micro-spindle are designed. Its manufacturing process is studied and the prototype is manufactured. Its performance including rotational speed, radial motion error and torque are tested by the test systems build in this paper. The results show that the monolithic flexible connection mechanism compensates the error of power spindle well. Therefore, the design idea and method of the flexible connection structure between power spindle and tool and one-piece tool/rotor structure for micro-spindles is feasible.(5)The main problems existing in the micro-spindle principle prototype are analyzed, which include micro-rotor dynamic balancing, manufacturing of micro-annular orifice restrictor, consistency of gas film clearance and machining deformation of flexible coupling. The micro-spindle is improved according to the existing problems. The improved micro-spindle uses impulse turbine with double-row nozzles, equal diameter turbine shaft, cylindrical outer structure, which bring the benefits of better performance, small size and make it meet the market requirement of equipment miniaturization. The three-point method is studied according to the requirements of high frequency response, small face with large curvature, no contact and error separation of rotational accuracy measurement system for high-speed procision micro-spindles. The corresponding software system for singnal process is developed and simulation study is carried out. The improved prototype is manufactured and its performance including rotational speed, radial motion error and torque are tested. |
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