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Machining Science and Technology, 14:224243 Copyright 2010 Mohawk Innovative Technology, Inc. ISSN: 1091-0344 print/1532-2483 online DOI: 10.1080/10910344.2010.489406 DESIGN AND EVALUATION OF AN ULTRAHIGH SPEED MICRO-MACHINING SPINDLE S. Jahanmir, Z. Ren, H. Heshmat, and M. Tomaszewski Mohawk Innovative Technology Inc., Albany, New York, USA ?A new ultrahigh speed micro-spindle has been developed for micro-milling that can be used at rotational speeds approaching 500,000rpm. Since conventional ball bearings or fl uid lubricated journal bearings cannot be used at such speeds, the new micro-spindle uses a set of journal and thrust foil bearings. Prior to fabrication of the micro-spindle, rotordynamic analysis of the rotor with an attached cutting tool confi rmed that the rotor would be stable at the desired speeds. The cutting tool was then attached to the rotor using a shrink-fi t approach. The micro-spindle was integrated with a 3-axis micro-milling machine. Cutting experiments were performed on an aluminum alloy at speeds greater than 300,000rpm using 125 and 300 ?m end-mills. Vibration spectra for free rotation and during cutting confi rmed the dynamic stability of the micro-spindle. The vibration spectrum was dominated by the rotational frequency and was free of deleterious vibrations. The increase in rotational speed to 450,000rpm in micro-milling of aluminum alloy allowed an increase in feed rate to nearly 750mm/min, thus increasing the material removal rate by more than two orders of magnitude. The dimensional accuracy of several straight cuts made at different feed rates was measured. Keywords foil bearings, micromachining, micromanufacturing, ultrahigh speed INTRODUCTION Industriessuchasdefense,medical,electronicpackaging, and transportation require the manufacturing of small-sized precision components with miniaturized features. Micro-manufacturing processes are categorized into 3 main types subtractive, near-net shape, and additive (Rajurkar and Madou, 2006). Some of the processes are downscaled versions of existing macro-manufacturing technologies, and others are innovative methods using various physical and chemical effects. The material subtractive processes include mechanical micromachining, such as turning, drilling, milling, and grinding; electrophysical and chemical processes, such as electro-discharge machining (EDM) and electrochemical Address correspondence to S. Jahanmir, Mohawk Innovative Technology Inc., Albany, NY 12205, USA; E-mail: sjahanmir Ultrahigh Speed Micro-Machining Spindle225 machining (ECM); and energy-beam machining, such as laser, electron, and focused ion beam (Rajurkar and Madou, 2006). Mechanical micromachining processes are downscaled versions of the existing macro-level processes. In these processes, the tools are usually in direct mechanical contact with the workpieces and therefore, a good geometric correlation between the tool path and the machined surface can be obtained. Compared to microelectronic fabrication methods, they have higher material removal rates and the ability to machine complex 2D and 3D micro-shapes in a variety of engineering materials (Cooper, 2005; Ehmann and DeVor, 2006; Rajurkar and Madou, 2006). A few micro-machine tools are commercially available for mechanical micro-machining. One example is the Microlution 363-S, which is a 4-axis machine with an open architecture CNC controller (Anonymous, 2008a; Brown, 2008); This micro-machining center provides 2?m positioning accuracy with 50nm resolution. The automatic quick tool changer can accept cutting tools ranging in diameters from 50?m to 3mm. Although the standard machine is equipped with an electrically driven spindle with a maximum speed of 40,000rpm, electrically driven and air turbine driven high-speed spindles are available for maximum speeds of 80,000 and 160,000rpm. Both high speed spindles use hydrostatic air bearings. Another example is the G4-ULTRA CNC MicroMachining Center from Atometric, which is a 5-axis machine (Anonymous, 2008b). The positioning accuracy is about 0?6?m with a 100nm resolution. The standard spindle operates at a maximum speed of 100,000rpm, but an optional 200,000rpm spindle is also available. The ultrahigh precision micro-machine, ROBOnano Ui, can function as a 5-axis mill, a lathe, a 5-axis grinder, a 5-axis shaping machine, and a high-speed shaper (Anonymous, 2008c). Cutting is nominally accomplished by a single crystal diamond tool. For milling, an air turbine spindle is employed for rotational speeds up to 70,000rpm. ROBOnano is used to machine mirrors, fi lters, gratings, liquid crystal display (LCD) panels, small lenses, micro-moldings, and other small precise parts. Most of the parts require approximately 5 minutes to be machined. However, the machining time and accuracy could be improved if higher rotational speeds could be employed. Another example is the micro-lathe that is 32mm long, 25mm wide, 30.5mm high, and weighs only 100grams. It comprises a friction-driven inchworm system, a main shaft device driven by a micro-motor, and a tool-rest. It has positioning resolution of 25nm. However, a maximum feed of only 24mm/s can be achieved. The main spindle motor can operate at rotational speeds only up to 10,000 rpm (Okazaki, 2000). The same group has also developed a micro-milling machine that consists of an X-Y stage and Z-axis drive unit holding the spindle. The Z-axis drive unit uses 226S. Jahanmir et al. a hollow drive screw, and is driven by a direct drive alternating current (AC) motor. The spindle with diameter of 27mm offers 300,000rpm of maximum rotational speed. The spindle end holds a milling tool with a direct-machined collet chuck of 1mm. High-spindle rotation, high feed rate, and small depth-of-cut contribute to the reduction of machining force, and thus improved precision. Motion control is achieved by a full closed-loop digital servo system, obtaining a feedback signal from a linear scale with a resolution of 50nm. For effi cient material removal and fast production rates, the speed at the cutting point should be comparable to the speeds achieved in macro-machining. While the present micro-machining systems offer spindle speeds less than of 300,000rpm (and most systems are limited to speeds less than 100,000rpm), the small cutting tools (50?m to 1mm diameter) used in micro-machining limit the surface speeds achieved at the cutting point. This causes a severe limitation on production rates, increases tool wear, and limits the degree of dimensional tolerances and precision that can be obtained. The purpose of this effort was, therefore, to develop an ultrahigh speed micro-spindle for machining at rotational speeds near 500,000rpm. EXPERIMENTAL PROCEDURE Auniquehigh-speedmesoscopicsizedturbojetmotor/spindle, previously developed and tested (Salehi et al., 2004), served as the basis for the proposed air-driven motor-spindle system for micro-milling. The entire mesoscopic system, when assembled, weighs only 56g (28mm OD and 32mm length), which includes a 9g rotor (i.e., spindle). Since the desired ultrahigh operating speed exceeds the capability of even hybrid ceramic rolling element bearings with liquid lubrication, two journal and two thrust foil bearings are used (Figure 1). The oil-free bearing foil surfaces are coated with a proprietary, low-friction, wear-resistant Korolon coating (Mohawk Innovative Technology, Inc., Albany, NY) to permit long life and rapid starting (Heshmat et al., 2005). The test setup included air supply lines, a fi ber-optic speed pickup and displacement fi ber-optic probes to monitor the position of the rotor during testing, as shown in Figure 1. An impulse-type air turbine is used to drive the rotor. The mesoscopic turbojet has been successfully tested up to 700,000rpm (Salehi et al., 2004). The clean frequency spectra obtained indicated a well damped rotor bearing system. In these performance tests, over 30 start-stop cycles and more than 1-hour run-time were accumulated. To ensure that the existing spindle could be used for micro-machining and could operate at ultrahigh cutting speeds with an attached tool, rotordynamic analysis was performed to determine the system critical Ultrahigh Speed Micro-Machining Spindle227 FIGURE 1 Cross-section schematic of the mesoscopic turbojet. speeds (i.e., resonances/natural frequencies). Dynamic analysis of the rotor-bearing system was performed using Finite Element Analysis (FEA) software DyRoBeS (Eigen Technologies, Inc., Davidson, NC). Rotor components were modeled based on their geometry and were assigned with appropriate material properties. Because the rotor is axisymmetric, it was divided into beam-type fi nite elements with one station located at each end. Each station has 2 translational and 2 rotational displacements. Each element could be divided into several sub-elements to allow for better accuracy in simulation. Using sub-elements is common in the Finite Element Method and saves computational time with minimal loss of accuracy in the results. The speed-dependent stiffness of the foil journal bearings and the damping coeffi cients estimated for the bearings used in the proposed design were assigned to the spring elements located at the nodes corresponding to the center of each bearing. The stiffness and damping values were calculated based on the geometric parameters and the specifi c design of the foil bearings (Heshmat, 2000; Heshmat et al., 2000). Whirl speed and stability analysis was then conducted to predict natural frequencies of possible vibration modes and evaluate radial stability of the rotor. The analysis was conducted for three confi gurations: (1) rotor with the turbine blades, same as the rotor that had been tested, (2) rotor with an inserted cutting tool, and (3) rotor with an added mass at the thrust disk (i.e., increasing the disk thickness) opposite to the tool end to balance for 228S. Jahanmir et al. FIGURE 2 Three rotor models analyzed: (a) Model No. 1: rotor with turbine buckets and without cutting tool, (b) Model No. 2: cutting tool added to Model No. 1, and (c) Model No. 3: mass added at the opposite end of rotor in Model No. 2. the mass of the cutting tool and maintain the center of gravity at the same location as the baseline rotor without the cutting tool. The cutting tool was simulated as a 1mm OD shaft. The fi nite element models for the three confi gurations are shown in Figure 2. The next issue, prior to fabrication and assembly of the spindle, is the design of tool fi xture. It is required for the tool to be centered as close as possible to the center of the spindle with the smallest possible eccentricity. Also, the mass of the tool holding device must be kept to a minimum and one should be able to balance the tool holder for high speed rotations. The best solution would be a one-piece spindle/tool assembly. However, the overall cost would become too high, since tool replacement would necessitate changing the entire spindle rotor. A common tool holding device is a collet chuck, which is available in small dimensions for the micro-tools selected for this effort. However, the collet chucks have a run out of about 125?m and would add extra mass to the spindle. Furthermore, it is not possible to balance these devices for the ultrahigh speeds required for this effort. After considerable review of the requirements and evaluation of possible tool fi xtures, it was decided to use shrink fi t as a method of tool attachment to the spindle. For this purpose, a round hole was drilled into the center of the spindle to accept the cutting tool. The radial interference was about 10?m and the eccentricity was kept below 2?5?m. While the tool was cooled, and the spindle was heated, the tool was inserted into the spindle. This method also allows for fairly quick tool changes within a few minutes. Ultrahigh Speed Micro-Machining Spindle229 FIGURE 3 Micro-machining setup in edge-milling showing the micro-spindle and stage. Also shown are the two air supply lines. The fully assembled micro-spindle was attached to a 3-axis machine tool for testing and performance evaluation. The machine tool consisted of a 2-axis linear stage (Parker Daedal, Irwin, PA) with a maximum feed rate of 750 mm/min in X and Y directions and a resolution of 20nm. The micro-spindle was attached to an L-shaped holder designed to provide suffi cient stiffness for micro-milling. The machining setup for edge milling is shown in Figure 3. The vertical distance Z was manually adjusted with a micrometer. A personal computer with Labview control software (National Instruments, Austin, TX) was used to control the X-Y motion of the stage for the micro-milling experiments. A regulated 125 psi (862kPa) house air was used to supply high- pressure air to the air turbine. The pressure was regulated to achieve the desired rotational speeds for the cutting experiments. A pair of optical sensors (Philtec, Inc. Annapolis, MD) located in the proximity of the thrust bearing disks (Figure 1) was used to monitor the location of the rotor with respect to the housing. Two sensors could be located near each thrust bearing disk to measure the rotor position, or placed 90apart on one disk to monitor the rotational orbit of the rotor at one thrust bearing disk. The outputs from the sensors were recorded with a multichannel digital tape recorder (Sony, Japan) and analyzed with a high-speed FFT spectrum analyzer (Ono-Sakki, Japan). Starting with a stationary rotor, the supply air pressure was rapidly increased. The rotor speed was monitored through the spectrum analyzer (using Fast Fourier Transform, FFT, of the optical sensor signals). The air pressure was increased until a desired rotational speed was observed in the FFT spectra. Micro-milling tests were conducted at speeds above 230S. Jahanmir et al. 300,000rpm. The vertical position of the micro-spindle was adjusted to obtain a depth of cut of about 10?m. The values for position and speed of the stage in the X and Y directions were inputs to the software and a command was given to initiate the motion of the stage and initiate cutting. A series of parallel cuts in slot-milling confi guration was made in a 6061-T6 aluminum alloy block fi xed to the top surface of the X-Y stage. The feed rate was varied from 25 to 750mm/min. Micro-milling tests were performed with two-fl uted micro-endmills with tip OD of 125?m and 300?m (TS Series, PMT Tools, Janesville, WI). The OD tolerance at the tool tip for these tools is listed as 12?5?m. Most of the cutting experiments were conducted without the use of cutting fl uids. However, a small drop of water-based cutting fl uid emulsion (Rustlik, Danvers, MA) was used in one test. Following the tests, the machined surface was fi rst visually examined, and then viewed under higher magnifi cations with a stereo optical microscope (Leica, Germany). RESULTS Rotordynamic Analysis Three fi nite element rotor models were constructed based on dimensions of the rotor that had been tested up to 700,000rpm. The fi rst rotor model in Figure 2a served as the baseline without the cutting tool. The cutting tool was added to the second model in Figure 2b. The third model in Figure 2c was confi gured to compensate for the added mass of the cutting tool. Using the predicted speed-dependent stiffness values and assumed damping coeffi cients for the two foil journal bearings, the rotordynamic analysis was performed for the three rotor models in a horizontal orientation. The results from the whirl and stability analyses when operating at speeds up to 600,000rpm are presented in Figures 4 and 5. The predicted rigid body natural frequencies for the baseline rotor are also compared in Figure 4 with those measured from the spindle assembly (Salehi et al., 2004), showing a close agreement between the analysis and the experimental results for both the translatory and conical vibration modes. According to the predicted logarithmic decrements in Figure 5 for the two rigid body modes, the second rotor model, shown in Figure 2b, appears to be more stable than the model with an added mass. Logarithmic decrement is a measure of how quickly vibrations decay or vibration energy is dissipated. A negative logarithmic decrement would indicate instability and vibrations would grow without bound. Based on the results of the analysis, the second rotor model was selected for further evaluations. Dynamic analysis of the rotor was performed for the selected model in the vertical orientation to simulate vertical micro-milling conditions. Ultrahigh Speed Micro-Machining Spindle231 FIGURE 4 Whirl speed map for predicted and measured rigid body natural frequencies during horizontal operation. Thepredictednaturalfrequenciesandcorrespondinglogarithmic decrements for the two rigid body modes at rotational speeds up to 600,000rpm are shown in Figures 6 and 7, respectively. The assumed damping coeffi cients were varied from 3.5 to 10.5N-s/m. Both rigid body natural frequencies were slightly reduced as damping increased. As shown in Figure 6, the natural frequencies reside at 26,000rpm for the translatory FIGURE 5 Stability map for predicted rigid body modes during horizontal operation. 232S. Jahanmir et al. FIGURE 6 Whirl speed map for predicted rigid body natural frequencies from Rotor Model No. 2 during vertical operation. mode and at 104,000rpm for the conical vibration mode at the spindle speed of 600,000rpm. Therefore, rotation at 500,000rpm should be attainable, provided that the two vibration modes at lower speeds are passed quickly with suffi cient FIGURE 7 Stability map for predicted rigid body modes from Rotor Model No. 2 during vertical operation. Ultrahigh Speed Micro-Machining Spindle233 inherent damping provided by the foil bearings. As shown in Figure 7, the logarithmic decrement for the two rigid body modes increases with increased damping. These results indicate that a stable performa