Journal of Advanced Mechanical Design, Systems, and Manufacturing Vol. 10(2016) No. 5

Unexpected glitches typically occur on the finished surface machined by the 5-axis machining centers, because of geometric and dynamic synchronous errors of the machine. In this study, actual ball-nosed end milling tests of hemispheres and its finished surface simulations with different geometric errors and different position loop gain of feed drive systems were carried out, in order to clarify the influence of the geometric and dynamic synchronous errors onto machined surface. As the results, it is clarified that the influence of geometric errors onto the machined surface is depending on the relationships between the movement of the axes and the surface geometry. In addition, the dynamic synchronous error also influences the machined surface when the velocity of translational and rotary axes changed rapidly.

This paper proposes a new optical angle sensor, in which a mode-locked femtosecond laser referred to as the optical frequency comb is employed as the light source for the sensor. By using the optical frequency comb, whose carrier frequency is well stabilized by using an external frequency standard with the uncertainty of 10^-11, both the sensor stability and the sensor sensitivity are expected to be improved. In this paper, the angle error caused by the frequency fluctuation of the laser beam is considered in the case of both the mode-locked femtosecond laser and a laser diode, which is a conventional light source for the optical angle sensor. A prototype optical sensor head with the mode-locked femtosecond laser is then fabricated for the angle sensor. Since the light wavelength of the mode-locked femtosecond laser used in this paper is out of the visible range, an alignment method based on laser autocollimation with a retroreflector is introduced to determine the zero-position for the angle sensor. This method is also effective in suppressing the influence of the alignment error such as a cosine error. Some basic experiments are carried out to verify the feasibility of the developed angle sensor with the optical frequency comb.

A demand for a nanoscale patterning method with high throughput exists for the fabrication of next-generation devices. Nanoimprint lithography (NIL) is a major breakthrough for obtaining the nanoscale pattern because of its high resolution and simple process. Because the resolution of NIL depends on the features of the master mold, it is very important to prepare a fine mold. Electron beam lithography (EBL) is typically used for the fabrication of NIL molds because EBL has a resolution of sub-10 nm. However, EBL is low throughput process and performed in a vacuum; it is difficult to obtain a large mold via EBL. On the other hand, direct laser writing (DLW) performed in the atomosphere is also promising method for the fabrication of a NIL mold because DLW can fabricate an arbitrary pattern in a large area. However, it is difficult to achieve the resolution of sub-100 nm with DLW because of a diffraction limit. To fabricate a fine mold in a large area, we have developed a miniaturization technique of a replica mold made of an elastic UV-curable resin using mechanical deformation. Although, the miniaturization of a line and space (LS) pattern of several hundreds nm width was already demonstrated, the miniaturization characteristics of a several micron and sub-100 nm width pattern are not examined. In this study, the deformation characteristics of the LS pattern with the micron and sub-100 nm width are examined. As a result, it became clear that our miniaturization technique can apply to the micron pattern. In addition, we succeeded in reducing the size of an LS pattern with a line width of less than 50 nm.

Roll-to-roll nanoimprint lithography (RTR-NIL) has received considerable attention because it permits large-area nanopatterning with both high resolution and high throughput. However, the application of RTR-NIL is restricted by difficulties in fabricating nanoscale roll molds. Seamless roll molds are especially desirable, because the presence of seams reduces the yield of the imprinted product. We have previously developed a technique producing seamless molds by direct writing with an electron beam onto a rotating cylindrical substrate. We have now developed a method for fabricating fine patterns on roll molds for RTR-NIL by using electron-beam lithography (EBL) with a positive-type electron-beam resist and an aluminum roll as a substrate, which is rotated in a scanning electron microscope. The electron beam is focused at a single point on the surface of the roll mold and the dot pattern is produced by switching the beam on and off. Dot and line-and-space patterns are obtained by developing the exposed substrate. In this study, we investigate the fabrication characteristics of a line-and-space pattern and a dot pattern by using EBL with a rotating stage. As a result, we produced fine dot patterns with a diameter of less than 70 nm and a fine line-and-space pattern with a width of less than 70 nm.

Several methods of evaluating the motion accuracy of the rotary axes of five-axis machining centers have been proposed in past studies. Because it is known that particular motion errors exist near motion direction changing points, it is important to evaluate the behavior of the rotary axes near these points. However, the influence of the motion error of the translational axes is included in conventional evaluation results because the translational axes reverse at the motion direction changing points about the rotary axes. In this study, a measurement system for evaluating the dynamic characteristics of the rotary axes near the motion direction changing points about these axes, excluding the influence of the translational axes, was developed. The measurement tests were also conducted using this measurement system for evaluating the dynamic characteristics of the tilt axis near the motion direction changing points about this axis. In addition, an actual machining test was conducted to evaluate the influence of the motion errors of the tilt axis near the motion direction changing points. As a result, it was confirmed that the behavior of the tilt axis near the motion direction changing points can be evaluated by using the proposed measurement method. It was also confirmed that the influence of the motion error on the machined surface is related to the size and shape of the tool and the geometric relationship between the motion error and the machined surface. It was also confirmed that the machined shape does not always contain defects when motion errors exist depending on the relationship between the motion error and the machined surface.

The use of machine tools for on-machine coordinate measurement on the workpiece is becoming commonplace. However, numerous errors can adversely affect the measurement accuracy. For instance, the inter-axis parameters are major contributors to the overall machine tool's inaccuracy hence estimation and compensation of such errors are prerequisite to fully exploit the machine's measurement capability. This paper presents a scheme to assess the accuracy of coordinate measurement by probing a precision sphere mounted on the machine table for different rotary axes indexations. Only the sphericity of the reference sphere is assumed and neither its size nor position. All individual probing data from all indexations are used for the assessment of the apparent out of sphericity of the reference sphere. Machine readings are processed either directly, using the machine nominal model, or using a compensated model. This provides a fast method to validate on-machine measurement before and after compensation with the exception of isotropic scale effects.

Exchangeability of machine tools requires identical results of identical machining operation. For estimating the behaviour of machine tools, which are highly integrated mechatronic systems, the interaction of electromechanic system and controller must be observed together. Furthermore, it is possible to measure friction at start-up in serial production of machine tools and at the production plants of machine tool consumers without any additional measuring equipment. By these measurements of friction severe variances of behaviour of feed axes have been detected. These are mainly caused by assembly variances. Especially the alignments and preloads of bearings and ball screw have a strong effect because these components have a high stiffness. This means even smallest deviations result in high constraining forces, which directly affect the friction behaviour. Since it is impossible to determine the reasons of these variances in completely assembled machine tools, a test stand of a ball screw driven axis is examined. This test stand allows both detailed identification of components’ friction behaviour and definite variations of assembly conditions, namely alignment and preload, of bearings and ball screw nut. Hence a detailed investigation of effects of assembly variations becomes possible and has been conducted for alignment of ball screw nut and loose bearing. However, by this examination only the effects on the complete system can be examined but not the situation of constraining forces within the components. The knowledge of these internal forces is important because they directly affect the life expectancy of components and therewith feed drives. In order to improve the significance and to estimate the effect of assembly variances in detail, a modelling method based on multibody simulation has been developed and verified by comparison of measuring results of the test bed and simulation.

In vertical positioning systems, the friction force and the gravity load cause significant error. In order to minimize the effect of friction and gravity load, this paper presents the noncontact gravity compensator applying magnetic fluid seals. The structure of the magnetic fluid seals was proposed for the linear motion system, and the dimensions of the magnetic fluid seal components were determined based on the result of the magnetic field analysis using FEM. Magnetic field analysis was used in order to investigate the relationship between the seal capacity and varied dimensions. Based on the results of the analysis, several types of magnetic fluid seal units were manufactured, and the analytical results were compared with the results of the seal capacity evaluation experiment in order to confirm the validity of the magnetic field analysis. In addition, the gravity compensating system using the magnetic fluid seals was constructed and applied to a vertical positioning system. Positioning resolution was evaluated with stepwise positioning experiments. Tracking performance and the response of the pressure were evaluated by the continuous path control experiment. According to these experimental results, the effectiveness of the gravity compensator with magnetic fluid seal was confirmed.

To improve the wear resistance of small machine parts such as microscale gears, we developed a novel surface treatment process, laser-induced local surface treatment. Disk-shaped austenitic stainless steel (SUS316L) specimens were soaked in Al(NO₃)₃ solution and subsequently irradiated by a nano-pulse fiber laser beam. The treated surfaces were then observed under a scanning electron microscope and analyzed using an energy dispersive X-ray spectrometer. The effects of defocus amount, tribology behavior, and wear resistance on the characteristics of the treated surface were examined in detail. The surface roughness of the treated specimens with the defocus range of -1 to -2 mm was lower than that of the treated specimen without defocus. The thickness of the treated layer decreased when the defocus amount was increased within the abovementioned range of values. However, when treatment was performed with a defocus amount of -3 mm, no noticeable change was observed on the treated surface; this was attributed to the difference in laser-energy density. The friction coefficient of the laser-treated specimen with the defocus level of -1 mm was much lower than that of the untreated specimen. This implies that the wear resistance of austenitic stainless steel can be improved with the proposed surface treatment method.

This paper presents a design study of an optical configuration for the fabrication of a two-dimensional grating, which will be used as a scale in a planar encoder system. For the modified two-axis Lloyd's mirror interferometer, in which major modifications have been made to the conventional one-axis Lloyd's mirror interferometer, computer simulation is carried out based on wave optics. Three-dimensional light intensity distributions of the fringe patterns are calculated to investigate the effect of the polarization modulation. In addition, a relationship between the asymmetry of the cross-sectional profiles of the grating pattern structures and the designed grating pattern period is also investigated. Furthermore, pattern exposure tests have been carried out by using a prototype optical setup for the modified two-axis Lloyd's mirror interferometer.

A noncontact scanning electrostatic force microscope (SEFM) for surface profile measurement is introduced, and applied to the profile measurement of insulating samples. The SEFM calculates the tip-sample distance with an algorithm named ‘dual-height method’, and compensates for the fluctuation of the tip-sample distance to obtain accurate surface profile image. Glass and acrylic, which are commonly applied to optical components, are prepared as the insulating samples, and bias voltage is applied to the samples with an electrode clamp. On the glass sample, electrostatic force is observed in the case that the bias voltage of 50 V is on. On the acrylic sample, however, electrostatic force is not observed regardless of the bias voltage. Surface profile measurement of the glass sample is demonstrated. The tip-sample distance image is created with the dual-height method from the data of the tip trajectories and the frequency shift signals obtained through the constant tip-sample interaction scan. The results show that the tip-sample distance is kept larger than 100 nm, which is more than ten times larger than that of conventional noncontact SPMs. The large tip-sample distance brings the SEFM a high robustness against the disturbances from environment, which would cause tip-sample collision in the conventional noncontact SPMs. The same area on the same sample is measured also in an atomic force microscope (AFM) mode, which is realized by setting the bias voltage 0 V. The SEFM image agrees with the AFM image quantitatively, showing that the SEFM with the dual-height method is applicable to noncontact profile measurement of the glass. On the other hand, it is difficult to utilize the SEFM in the profile measurement of the acrylic sample because the electrostatic force on the acrylic is not controllable with the voltage of the electrode clamp.

Hypoid gear machining is a critical process in automobile industries. Many studies have discussed the gear machining in the geometrical model for manufacturing and design of the gears. In terms of the cutting process, the accuracies of the gears largely depend on the cutting forces. Therefore, the cutting force should be estimated to perform the high accurate gear cutting. However, few studies have been done on prediction of the cutting force. Analytical predictions of the cutting forces are required for manufacturing of gears with high qualities. This study presents an analytical model to predict cutting force in the hypoid gear cutting, in which the gear grooves are machined by sets of inner and outer cutting blades. Both of the blades finish the side and the end faces in the gear grooves. Different chip flows occurs on the end and the side edges of the blades. The chip flows are modeled by piling up orthogonal cuttings containing the cutting and the chip flow directions, where the chip flow direction is determined to minimize the cutting energy. The cutting forces loaded on the inner and the outer blades are predicted in the chip flow models. The presented model is validated in the cutting tests with measuring the cutting forces.