High-precision Spindle Research Articles

Thermal model of spindles on rolling bearings

By means of the proposed thermal model and corresponding software, the thermal characteristics of high-precision spindles may be quickly assessed in the initial stages of design.

Structure Design of Biaxial Rotary Milling Head with High-Torque, High-Precision and Mechanical Spindle

As a key component of heavy five-axis linkage CNC machine tools, the existing mechanical biaxial rotary milling heads are usually driven by worm gear, timing belt, or multi-stage gears. Worm gear is inefficiency to transmit motion, hard to maintain the accuracy and difficult to eliminate all clearance; Timing belt can be seriously worn and cannot maintain good accuracy; and multi-stage gears transmission has defects such as long chain, low transmission stiffness. This paper aims at designing a biaxial rotary milling head with large output torque of 3000Nm, high precision and high rigidity. So a new design for biaxial rotary milling head with mechanical spindle, gear transmission and swinging fork structure is proposed. An integral supporting axis is applied to increase the rigidity of A-axis; through the closed numerical loop control based on precise encoder and the decoupling technology of feed motion of A/C axis, the contradiction between large reduction ratio and high precision transmission is solved, and then the requirement of high positioning accuracy (10") is met. Theoretical calculation and simulated analysis shown, the new design not only can guarantee transmission accuracy and dynamic characteristics of biaxial rotary milling head, but also can improve its structural stiffness and meet the high demands of large torque, high precision and high rigidity.

Research on the rotational accuracy measurement of an aerostatic spindle in a rolling bearing performance analysis instrument

An ultra-precision instrument is developed to measure the rotational accuracy of a rolling bearing under working conditions. This instrument uses a high-precision aerostatic spindle, which has high-stiffness to support a test bearing under large axial loading. This work demonstrates the capabilities of the double-probe error separation method for the measurement of aerostatic spindle error motion at various axial forces and rotational speeds. The rotational performance of an aerostatic spindle is discussed. The results indicate that the error motion and the form of the error of the measurement surface are reliably separated at various axial forces and rotational speeds. In addition, the error motion of the aerostatic spindle is very stable, increasing slightly and smoothly as the rotational speed increases. The error motion is less than 0.18 um when the speed reaches 1920 rpm. In the future, it will be possible to separate the error motion of an aerostatic spindle from the measurement results of a test bearing.

Rotation error measurement technology and experimentation research of high-precision hydrostatic spindle

The hydrostatic spindle is widely applied in the field of high-precision machine tools, which has some advantages such as high stiffness, high rotary precision, and the high damping shock absorption. The spindle rotation error is an important index to measure the machining accuracy of machine tools. Due to the installing eccentric error of the test bar, conventional method based on the standard test bar to measure the rotation error indirectly is applied to the precision machine tools and common machine tools whose rotation error is greater than 1 μm only. In order to eliminate the installing eccentric error of the standard test bar, it presents a self-reference approach that takes the online finish turning test bar, rather than that of the standard test bar, as the measuring datum. Using the capacitive micro-displacement sensor and the LMS data acquisition equipment as the test platform, it designs a set of spindle rotation error measurement system. Then it studies the frequency domain three-point method and has the rotation error and roundness error of high-precision hydrostatic spindle separated. Experimental study shows that the rotation error and the roundness error of the spindle are 0.9 and 0.3 μm, respectively, under the circumstance of conventional standard test bar as the measuring datum. However, if it takes the online finish turning test bar as the measuring datum, the rotation error and the roundness error of the spindle are only 0.3 and 0.1 μm, respectively. The self-reference method is able to eliminate the installing eccentric error of standard test bar directly, and the measurement system has realized the accurate measurements of the rotation error and roundness error of the high-precision hydrostatic spindle.

An AFM-based methodology for measuring axial and radial error motions of spindles

This paper presents a novel atomic force microscopy (AFM)-based methodology for measurement of axial and radial error motions of a high precision spindle. Based on a modified commercial AFM system, the AFM tip is employed as a cutting tool by which nano-grooves are scratched on a flat surface with the rotation of the spindle. By extracting the radial motion data of the spindle from the scratched nano-grooves, the radial error motion of the spindle can be calculated after subtracting the tilting errors from the original measurement data. Through recording the variation of the PZT displacement in the Z direction in AFM tapping mode during the spindle rotation, the axial error motion of the spindle can be obtained. Moreover the effects of the nano-scratching parameters on the scratched grooves, the tilting error removal method for both conditions and the method of data extraction from the scratched groove depth are studied in detail. The axial error motion of 124 nm and the radial error motion of 279 nm of a commercial high precision air bearing spindle are achieved by this novel method, which are comparable with the values provided by the manufacturer, verifying this method. This approach does not need an expensive standard part as in most conventional measurement approaches. Moreover, the axial and radial error motions of the spindle can both be obtained, indicating that this is a potential means of measuring the error motions of the high precision moving parts of ultra-precision machine tools in the future.

Research on Spindle Bearing Pre-Tightening Based on Spacer Sleeve Adjustment

Machine spindle pre-tightening force is closely related to machining precision. Universal spindle is generally pre-tightened through spacer sleeve adjustment. The calculation method about bearing pre-tightening force is put forward through analyzing and computing the main factors of bearing pre-tightening force, and the machine spindle bearing pre-tightening methods based on pre-tightening force are described in detail. This method is used for installation and maintenance of machine spindle, and it is proved to achieve the accuracy recovery of high-revolution, high-precision machine spindle.

Experimental Investigation of Mechanical Micro Machining for AL6061-T6 Material

This paper is to experimentally investigate the mechanical micro machining properties of AL6061-T6 using tungsten-carbide micro end mills. The cutting simulation based on Finite Element Analysis (FEA) method is also conducted to estimate cutting forces, cutting temperature, and minimum chip thicknesses. The simulation results are used for the determination of experimental machining conditions such as depth of cut, feed rates and cutting speeds. A number of slot micro-milling experiments were performed using 400um diameter micro end mills on a 3 Axis CNC machining center attached with a high precision and high speed spindle unit. The machined surface quality, geometric feature shape, cutting burr generation as well as build-up edges are observed in the cutting experiments. Optimum cutting parameters for a better surface quality and smaller burr sizes are suggested.

공기동압베어링의 성능 해석 및 가공특성 평가

The need is growing for high-speed spindle because various equipment are becoming more precise, miniaturization and high speed with the development of industries. Air-lubricated dynamic bearings are widely used in the optical lithographic manufacturing of wafers to realize nearly zero friction for the motion of the stage. Air-lubricated dynamic bearing can be used in high-speed, high-precision spindle system and hard disk drive(HDD) because of its advantages such as low frictional loss, low heat generation, averaging effect leading better running accuracy. In the paper, numerical analysis is undertaken to calculate the performance of air-lubricated dynamic bearing with herringbone groove. The static performances of herringbone groove bearings which can be used to support the thrust load are calculated. Electrochemical micro machining(ECμ M) which is non-contact ultra precision machining method has been developed to fabricate the air-lubricated dynamic bearing and optimum parameters which are inter electrode gap size, concentration of electrolyte, machining time are simulated using numerical analysis program.

Measurement of Radial and Axial Error Motion in a High Precision Spindle

The performance of a spindle is critical for high precision machining. In this paper, the spindle error motion in a high precision milling center was measured. The spindle is driven by air turbine with rotation speed of 120,000rpm. The radial and axial error motion of the axis of rotation was measured. The capacitive displacement sensors with nanometer resolution were mounted against the master gauge pin through the dedicated setup. Tlusty method was adopted to synchronize angular position of the spindle and data sampling. The measured radial and axial error motion of the spindle were 2.73μm and 2.59μm respectively. Despite of motion errors, the better machining accuracy still can be achieved. It seems that cutting force may improve the rotation performance of a spindle with aerostatic bearing.

Design on Hybrid Ceramic Bearing Based on Water Based Fluid with Controllable Viscosity

This paper presents a high-speed high-precision spindle system with hybrid ceramic bearing, along with its lubricant design and numerical calculation method. A green and viscosity controllable water based fluid as lubricant of the spindle was designed. Viscosities as a function of temperature were tested. Subsequently, numerical calculation was used to deduce a serious of bearing performances. Based on the results rate of the permeation of porous restrictor, some bearing structure parameters and relative optimum lubricant viscosity were determined. The results of thermal elastohydrodynamic simulation and rotor dynamic analysis indicate that the design of the hybrid ceramic bearing meets security and performance specifications. Finally the structure arrangement of the spindle system was finished.

The Dynamic Optimization Analysis of High-Speed Hybrid Thrust Bearing

In this paper, the dynamic characteristics of high-speed hybrid thrust bearing with four-recesses are optimized for different orifice diameter and supply pressure at a given speed condition, the dynamic moment coefficients of hybrid thrust bearing are analyzed, considering the thrust collar angular misalignment case to determine the optimization of orifice diameter and supply pressure under a given speed. The results provide certain reference to the hybrid thrust bearings used in high speed precision spindle.

Intelligent control of microassembly process based on in-process monitoring of pressing force

To realize an automated and intelligent microassembly process, a method has been developed to monitor the position of the shaft in the plate of the high-precision spindle motor to reduce the shaft high and the shaft low problems. The shaft low occurs when the shaft is stuck in the plate lower than the setting tolerance due to the large tolerance of the shaft and the small tolerance of the plate. While the shaft high appears, the shaft is pressed through the plate over the setting tolerance due to the small tolerance of the shaft and the large tolerance of the plate. The plate is expanded to reduce the shaft low before microassembly by preheating technique at the experimentally simulated temperature. The force sensor is utilized to monitor the pressing force. A method proposed to reduce the shaft high introduces the reference voltage as the threshold value, which is obtained by taking the differentiation of the in-process pressing force. A slope detector is developed to calculate the output voltage and the motor driver of machine is controlled when the obtained output voltage is larger than the reference voltage. It is proved that the shaft high and the shaft low are well controlled and reduced regardless of the geometries of the shaft and the plate.

Minimization of cogging torque in permanent magnet motors by teeth pairing and magnet arc design using genetic algorithm

Cogging torque is often a principal source of vibration and acoustic noise in high precision spindle motor applications. In this paper, cogging torque is analytically calculated using energy method with Fourier series expansion. It shows that cogging torque is effectively minimized by controlling airgap permeance function with teeth pairing design, and by controlling flux density function with magnet arc design. For an optimization technique, genetic algorithm is applied to handle trade-off effects of design parameters. Results show that the proposed method can reduce the cogging torque effectively.

Novel two-phase spindle motor for digital video disk applications

Disk drives require high-precision spindle motors for fast and accurate access of data. This paper presents a new two-phase spindle motor for digital video disks (DVDs) and compact disks (CDs). The brushless spindle motor has a pancake-type construction, which has distinct advantages and costs less to manufacture because of the simplicity of the stator structure and the windings. The constraint between the detent torque and the starting torque in the single-phase pancake-type motor design is resolved by using the two-phase scheme. A prototype of the two-phase spindle motor and drive circuit has been made and tested in commercial DVD and CD-ROM drives. Experimental results show that its performance satisfies the requirements of the optical disk drives.

The dynamic performance affected by the axial magnetic force for a DVD spindle motor

The axial magnetic force induced by rotor and stator is an important performance index to affect the dynamic characteristic on a high-precision DVD spindle motor design. In this research, the axial magnetic forces are calculated at varying relative heights between rotor and stator by 3D finite element method (FEM), and compared with the experimental data. Based on the results of simulation and measurement of the axial magnetic forces, the dynamic vibration could be apparently improved and decreased from 76.7 to 3.85 mg at 6000 rpm with disc by a variation of relative height on a DVD spindle motor. The torque constant and running current are also applied as performance indices to evaluate dynamic performance in the paper.

Mechanical and Electrical Run-Out Removal on a Precision Rotor-Vibration Research Spindle

Experimental determination of both steady-state and dynamic performance of a journal bearing requires the use of a high precision spindle with a vanishingly small range of run-out. This was achieved by first eliminating the mechanical run-out of the spindle by grinding the journal specimen while rotating in place. Once the mechanical run-out was removed, the electrical run-out sensed by the displacement proximity-probe-transducers was also removed. Using this procedure the mechanical and electrical run-outs of a research spindle were reduced to less than 0.2 micron (10 μin.), which is better than the resolution of the data acquisition system, 1 micron (50 μin.).

In situ runout identification in active magnetic bearing system by extended influence coefficient method

In this paper, an efficient, yet easy-to-use, in-situ runout identification scheme, based on the extended influence coefficient method, is presented for active magnetic bearing (AMB) systems. It is shown experimentally that the proposed scheme successfully identifies and eliminates the troublesome runout of a well-balanced AMB system in the laboratory, so that a high-precision spindle system can be achieved, while it is in operation.

Disk drive spindle motors and their controls

Disk drives require very high-precision spindle motors for fast and accurate access of data. As storage capacity and access rate increase, more demands are being made of spindle motors with respect to both electrical and mechanical performances. For error-free operations, spindle motors are required to comply with a host of requirements, such as run-outs, both axial and radial, acoustics, mechanical vibrations, electromagnetic interference, contamination, etc. Compliance to these requirements necessitate careful and novel design techniques and highly sophisticated manufacturing processes. This paper discusses various requirements and specific design issues (electromagnetic design, mechanical design, motor control) for the development of spindle motors suitable for different form-factor drives using permanent magnet brushless DC motors.

Analysis of multi-scallop self-adjusting conical hydrodynamic bearings for high precision spindles

This paper deals with the analysis of a new type of special precision conical bearings with multi lobes in the form of scallops, which the author calls ‘crown bearings’. Axial preload and lobe depth are found to be the main controlling parameters that influence the working clearance and oil film stiffness of the bearing. The analysis is valid for any number of lobes and the design graphs for three to six lobed bearings have been given. The working clearance increases with increase in speed, thus the bearing is self-adjusting. The frictional torque is a function of the square root of the surface velocity which indicates a cool running bearing