This work describes a micro-patterning or surface texturing technique on the fixed work surface using a proposed new spindle system with a rotating cutting tool controlled in real time. The proposed spindle system creates micro-sized patterns using rotating tools such as a micro-milling tool and a micro-grinding wheel. The shaft of the spindle is suspended by air bearings, and an electromagnetic actuator controls the radial motion of the spindle housing instead of the shaft. This generates a micro-pattern array on the electro-less Ni-coated workpiece in the micro-milling process. A PID controller is adopted to make the system stable, and adaptive feedforward cancellation is used to effectively compensate for the run-out of the spindle system during machining. The machining results show that this compensation improves the pattern accuracy. It is expected that micro-patterning using the proposed spindle system can be applied over a large surface area.

Micro-patterning technique using a rotating cutting tool controlled by an electromagnetic actuator

This study presents the design and evaluation of a high-speed and precision microspindle used for micromechanical machining. A radial impulse turbine with two rows of nozzles was designed to realize the high-speed performance. A monolithic flexible coupling was used to decouple the turbine shaft and the tool, and then, the tool shank was directly supported by aerostatic bearings, which can minimize the run-out of the tool at high speeds. Additionally, a shape memory alloy (SMA)-based clamp was designed for tool clamping. It can avoid the assembly error caused by stack-up tolerances. The microspindle has simple configuration and compact design with a 30-mm outer diameter and a 95-mm length. The prototype microspindle has a maximum speed of over 240,000 rpm and a tool radial run-out of 2.79 μm. The evaluation results show the feasibility and potential application of the new design concept of the microspindle proposed in this study.

Design and evaluation of a high-speed and precision microspindle

The wettability, friction, adhesion, and optical characteristics of a surface are affected by its micro-topography. In turn, the characteristics of a surface micro-topography can be determined by controlling the trajectory of a cutting tool mounted on a spindle. However, this method is constrained by the tool’s cutting accuracy, which is limited by the run-out error and thermal behavior of the spindle. In this work, an electromagnetic actuator (EMA) is installed at the end of the spindle to compensate for the spindle run-out error and the thermal displacement in real time. The spindle system model includes a cross-coupled stiffness and damping term for the ball bearings. Repetitive controller (RC) reduces the repeatable run-out (RRO) while adaptive feedforward cancellation (AFC) reduces the non-repeatable run-out (NRRO). The control system uses a zero phase error tracking controller (ZPETC) to compensate for its total phase delay. Experimental results show a 70% reduction in run-out error at rotation speeds of 6,000 rpm and 12,000 rpm. The thermal displacement in the radial direction is regulated within a range of 1 µm for more than 3 hours.

Rolling bearing-suspended spindle run-out control using repetitive control and adaptive feedforward cancellation

For more stable operation of radial AC hybrid magnetic bearing (radial AC HMB), it is particularly important to realize optimization design of parameters for its prototype. The paper focuses on solving the several typical defects of traditional methods and puts forward specific countermeasures on improving its disadvantages. Thus, a new parameter design method based on two sets of magnetic field system (the stator core magnetic field system and the air gap field system) is proposed to calculate the structure and electromagnetism parameters of the radial AC HMB. Using the proposed parameter design method, a finite element model of radial AC HMB based on the design results are simulated in finite element analysis software. The simulation results are consistent with the expected parameters calculation results very well, which show accuracy of the parameters and the proposed method. To illustrate the validity of the parameter design approaches further, the verification experiment is conducted. The operating experimental results show that the radial AC HMB has high stability, fast start of suspension and fine anti-disturbance. Moreover, system performance comparison results show the control system based on the new parameter design method has stronger anti-disturbance characteristics than that based on the traditional parameter design method.

Accurate parameter design for radial AC hybrid magnetic bearing

Mechanical micromachining using micro-machine tools is very competitive in manufacturing geometrically complex 3D micro-parts in various engineering materials with advantages of small space, energy-efficient, low equipment production and operation costs. As key components of micro-machine tools, micro-spindles directly determine the performance of micro-machine tools and the application and development of mechanical micromachining technology. This study proposes a novel design concept of the separately spindle and tool and one-piece tool/rotor structure for micro-spindles, whose micro-tool can maintain good rotational accuracy at high rotational speeds. In the principle prototype micro-spindle, an air driven turbine was used as power spindle. The design and optimization of the micro-spindle were conducted. The fabrication and evaluation of the prototype micro-spindle were presented. The results of the principle prototype micro-spindle were analyzed and some efforts for the subsequent micro-spindle were proposed.

Development of a high-speed and precision micro-spindle for micro-cutting

This paper reports the design, analysis, and experimental results of a miniature spindle using magnetic bearings for very high-speed micromachining. To ensure stable high-speed rotation, the shaft was designed such that the first flexible natural frequency was higher than 9 kHz. Four-pole radial magnetic bearings with a permanent magnet bias were located in front of and behind the spindle, and axial air bearings at the spindle covers were used to hold the shaft in the axial direction. Additionally, to ensure small tools were clamped effectively, a clamping mechanism with a shape memory alloy was added at the nose of the shaft. The designed miniature spindle has an outer diameter of 62 mm and a total length of 94 mm, including the housing. The spindle was controlled with a digital signal processor (DSP)-based control system and rotated at up to 200,000 rpm using a built-in air turbine. The stability and ability of the spindle to control the tool orbit to generate micro-features was evaluated.

Woo-Chul Shin

Papers

A magnetically suspended miniature spindle and its application for tool orbit control