Design and Evaluation of an Ultra Precision Rotary Table for Freeform Machine Tools
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This paper describes the design and evaluation procedure of an ultra-precision rotary table for freeform generating machined tools. Design of the thrust and journal hydrostatic bearings and experimental evaluation of the table were performed. To get the compact size and less lost motion direct drive servomotor with ultra precision encoder. From the considered design, following performance were confirmed by experiment. The total stiffness of the prototype rotary table was 483.6 and 97.6 for axial and radial direction, respectively. Rotational accuracy of the table was investigated by capacitive sensor and reversal measurement technique, and 0.10 radial direction and 0.05 axial direction of the rotational accuracy were confirmed. The micro resolution of the table was also investigated with displacement of capacitive sensor, and of micro resolution was confirmed. Index accuracy of the table was evaluated by the autocollimator and polygon mirror, and the arcsec accuracy and arcsec repeatability of the table were confirmed. Those are under the general requirements of ultra precision rotary tables for freeform generating machined tools.Keywords:
Table (database)
Machine tool
Rotary encoder
Accuracy and precision
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Profile measurement is a key technical enabler in the manufacturing of highly curved freeform surfaces due to their complex geometrical shape. A current optical probe was used to measure nearly rotary freeform surfaces with the help of one rotation axis, because the probe needs to measure along the normal vector of the surface under the limitation of the numerical aperture (NA). This kind of measuring system generally has a high cost due to the high-precision, multi-axis platform. In this paper, we propose a low-cost, dual-axis rotation scanning method for a highly curved freeform surface with an arbitrary shape. The optical probe can scan the surface profile while always keeping consistent with the normal vector of the measuring points with the help of the double rotation axis. This method can adapt to the changes in curvature in any direction for a highly curved freeform surface. In addition, the proposed method provides a system error calibration technique for the rotation axis errors. This technique can be used to avoid the dependence of the measuring system on the high-precision platform. The three key system errors that affect the measurement accuracy such as installation error of the B-axis, A-axis, and XZ perpendicularity error are first analyzed through establishing an error model. Then, the real error values are obtained by the optimal calculation in the calibration process. Finally, the feasibility of the measurement method is verified by measuring one cone mirror and an F-theta mirror and comparing the results to those obtained using commercial equipment. The maximum measurable angle of the system is ±90°, the maximum measurable diameter is 100 mm, and the measurement accuracy of the system reaches the micron level in this paper.
Aperture (computer memory)
Normal
Coordinate-measuring machine
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Citations (11)
The performance of high-precision optical systems using spherical optics is limited by aberrations. By applying aspherical and freeform optics, the geometrical aberrations can be reduced or eliminated while at the same time also reducing the required number of components, the size and the weight of the system. New manufacturing techniques enable creation of high-precision freeform surfaces. Suitable metrology (high accuracy, universal, non-contact, large measurement volume and short measurement time) is key in the manufacturing and application of these surfaces, but not yet available. In this thesis, the design, realization and testing of a new metrology instrument is described. This measurement machine is capable of universal, noncontact and fast measurement of freeform optics up to O500 mm, with an uncertainty of 30 nm (2s). A cylindrical scanning setup with an optical distance probe has been designed. This concept is non-contact, universal and fast. With a probe with 5 mm range, circular tracks on freeform surfaces can be measured rapidly with minimal dynamics. By applying a metrology frame relative to which the position of the probe and the product are measured, most stage errors are eliminated from the metrology loop. Because the probe is oriented perpendicular to the aspherical best-fit of the surface, the sensitivity to tangential errors is reduced. This allows for the metrology system to be 2D. The machine design can be split into three parts: the motion system, the metrology system and: the non-contact probe. The motion system positions the probe relative to the product in 4 degrees of freedom. The product is mounted on an air bearing spindle (??), and the probe is positioned over it in radial (r), vertical (z) and inclination (?) direction by the R-stage, Z-stage and ?- axis, respectively. The motion system provides a sub-micrometer repeatable plane of motion to the probe. The Z-stage is hereto aligned to a vertical plane of the granite base using three air bearings, to obtain a parallel bearing stage configuration. To minimize distortions and hysteresis, the stages have separate position and preload frames. Direct drive motors and high resolution optical scales and encoders are used for positioning. Mechanical brakes are applied while measuring a track, to minimize power dissipation and to exclude encoder, amplifier and EMC noise. The motors, brakes and weight compensation are aligned to the centres of gravity of the R and Zstage. Stabilizing controllers have been designed based on frequency response measurements. The metrology system measures the position of the probe relative to the product in the six critical directions in the plane of motion of the probe (the measurement plane). By focussing a vertical and horizontal interferometer onto the ?-axis rotor, the displacement of the probe is measured relative to the reference mirrors on the upper metrology frame. Due to the reduced sensitivity in tangential direction at the probe tip, the Abbe criterion is still satisfied. Silicon Carbide is the material of choice for the upper metrology frame, due to its excellent thermal and mechanical properties. Mechanical and thermal analysis of this frame shows nanometer-level stabilities under the expected thermal loads. Simulations of the multi-probe method show capabilities of in process separation of the spindle reference edge profile and the spindle error motion with sub-nanometer uncertainty. The non-contact probe measures the distance between the ?-axis rotor and the surface under test. A dual stage design is applied, which has 5 mm range, nanometer resolution and 5° unidirectional acceptance angle. This enables the R and Z-stage and ?-axis to be stationary during the measurement of a circular track on a freeform surface. The design consists of a compact integration of the differential confocal method with an interferometer. The focussing objective is positioned by a flexure guidance with a voice coil actuator. A motion controller finds the surface and keeps the objective focused onto it with some tens of nanometers servo error. The electronics and software are designed to safely operate the 5 axes of the machine and to acquire the signals of all measurement channels. The electronics cabinet contains a real-time processor with many in and outputs, control units for all 5 axes, a safety control unit, a probe laser unit and an interferometry interface. The software consists of three main elements: the trajectory planning, the machine control and the data processing. Emphasis has been on the machine control, in order to safely validate the machine performance and perform basic data-processing. The performance of the machine assembly has been tested by stability, single track and full surface measurements. The measurements focus on repeatability, since this is a key condition before achieving low measurement uncertainty by calibration. The measurements are performed on a O100 mm optical flat, which was calibrated by NMi VSL to be flat within 7 nm rms. At standstill, the noise level of the metrology loop is 0.9 nm rms over 0.1 s. When measuring a single track at 1 rev/s, 10 revolutions overlap within 10 nm PV. The repeatability of three measurements of the flat, tilted by 13 µm, is 2 nm rms. The flatness measured by the uncalibrated machine matches the NMi data well. Ten measurements of the flat tilted by 1.6 mm repeat to 3.4 nm rms. A new non-contact measurement machine prototype for freeform optics has been developed. The characteristics desired for a high-end, single piece, freeform optics production environment (high accuracy, universal, non-contact, large measurement volume and short measurement time) have been incorporated into one instrument. The validation measurement results exceed the expectations, especially since they are basically raw data. Future calibrations and development of control and dataprocessing software will certainly further improve these results.
Dimensional Metrology
Surface Metrology
Coordinate-measuring machine
Position (finance)
Realization (probability)
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Aiming at the characteristics of flat and small parts assembly,a set of micro-assembly systems platform based on vacuum suction is designed. The vacuum adsorption micro-gripper parts realize clamping and placing. The double CCD cameras achieve the pose and position parameters recognition. Positioning accuracy of displacement instrument in the of(4~ 6)μm is used to part orientation. Error source of displacement table is introduced,and it emphatically calculates the accumulated error of stepper motor and leads screw in the displacement table. The calculated results is(±1.405)μm within a reasonable range. The re-orientation accuracy plays important role in accuracy of displacement instrument basing on the close load control of grating,and the re-orientation accuracy of less than 3μm is verified by lots of experiments.
Stepper
Stepper motor
Position (finance)
Table (database)
Positive displacement meter
Accuracy and precision
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The accuracy of workpieces machined on the tilting rotary tables of five-axis machining centers depends on the accuracy of the axis of rotation. A 3D probe was developed to measure the accuracy of the axis of rotation under simultaneous three-axis motion to avoid the inconvenience of existing measurement instruments. This paper proposes a measurement method for the parallelism error between the sensitive directions and the movement directions of the linear axes. In addition, this paper proposes a correction method using a rotational matrix consisting of the measured parallelism errors and a display method to show the motion errors of the axis of rotation as an application of the 3D probe.
Rotation matrix
Machine tool
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This paper describes the measuring methods for positioning accuracy of tilting axes in five-axis controlled machining centers with a tilting rotary table. The top surface of the tilting rotary table is generally in higher position than the tilting axis. In this case, there is no measuring method except for a manually operated clinometer. Thus, a clinometer consisting of a precision rotary encoder and a bubble level was firstly developed. In the method, operators have to read visually the scale of the level and the reading accuracy is dependent on their ability. Instead of the bubble level, an electronic level with high resolution was employed for comparison. A ball bar equipment was also applied to measure the angular positioning accuracy of the tilting rotary table, as the ball bar equipment can measure the angular displacement continuously.
Ball (mathematics)
Rotary encoder
Table (database)
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A model machine of multifunctional form and position measurement instrument controlled by a personal computer has been successfully developed. The instrument is designed in rotary table type with a high precision air bearing and the radial rotation error of the rotary table is 0.08 μm. Since a high precision vertical sliding carriage supported by an air bearing is used for the instrument, the straightaway motion error of the carriage is 0.3 μm/200 mm and the parallelism error of the motion of the carriage relative to the rotation axis of the rotary table is 0.4 μm/200 mm. The mathematical models have been established for assessing planar and spatial straightness, flatness, roundness, cylindricity, and coaxality errors. By radial deviation measurement, the instrument can accurately measure form and position errors of such workpieces as shafts, round plates and sleeves of medium or small dimensions with the tolerance grades mostly used in industry.
Flatness (cosmology)
Roundness (object)
Measuring instrument
Air bearing
Machine tool
Position (finance)
Coordinate-measuring machine
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For the importance of measuring accurately the angular rate in the performance evaluation of rotary table,a high-speed video method for measuring the angular rate of rotary table was proposed.A collimator tube was fixed to the rotary table as a measureable objective,and the space position of the objective was corresponding to the rotating angular position of the table.In the rotating process of rotary table,the objective space position was recorded with a high-speed camera and thus the measurement was implemented.The orientation method with the pixel subdivision technology was adopted to improve the measurement precision,where the subdivision precision could reach 0.03 pixels.This method was used to collect and analyze the test data for a certain type of low-speed rotary table.The error analysis result shows that the precision of rate measurement can reach 3.405×10-4 when the angular rate of rotary table is 0.2°/s.It means that the present method can be used for measuring the angular rate of high-precision rotary table.The instantaneous angular rate of rotary table can be measured due to the high sampling frequency of high-speed camera.The objective evaluation for the angular rate of rotary table is realized with the non-contact measurement way in the proposed method.
Table (database)
Position (finance)
Collimator
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Measurement of five DOF motion errors in a ultra precision feed table was attempted in this study. Yaw and pitch error were measured by using a laser interferometer and roll error was measured by using the reversal method. Linear motion errors in the vertical and horizontal directions were measured by using the sequential two point method. In this case, influence of angular motion errors was compensated by using the previously measured ones by the laser interferometer and the reversal method. The capacitive type sensors and an optical straight edge were used in the reversal method and the sequential two point method. Influence of thermal deformation on sensor jig was investgated and minimized by the periodic measurement according to the variation of room temperature. Deviation of gain between sensors was also compensated using the step response data. 5 DOF motion errors of a hydrostatic table driven by the linear motor werer tested using the measurement method. In the horizontal direction, measuring accuracies for the linear and angular motion were within and arcsec, respectively. In the vertical direction, they were within and arcsec. From these results, it was found that the introduced measurement method was very effective to measure 5 DOF motion errors of the ultra precision feed tables.
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An automated setup for non-tactile high-precision measurements of roundness and cylindricity of ring gauges is presented in this paper.The aim is to overcome classical problems of tactile and radial roundness measurements such as the error influences of the used rotary table and the work piece alignment and thus to increase the accuracy and reduce the measurement time.To achieve those aims a double interferometer concept was chosen and combined with a measurement system for the work piece alignment, a high precision rotary table and an automated four-axes adjustment unit.The main alignment errors of the work pieces (e.g.ring gauges) such as eccentricity and tilting are either suppressed or directly detected and consequently reduced by the automated four-axes adjustment unit.Due to the non-tactile measurement concept and the contactless energy supply of the fouraxes adjustment unit, the radial run of the rotary table is not affected.
Roundness (object)
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Citations (27)
A method of flatness measurement of noncontinuous flat part using precision rotary table,micrometer,flat ruler,autocollimation is introduced.A noncontinuous flat part with Ф2310mm diameter was measured; the data processing method and measuring results were given.The measurement uncertainty of the method is 5 μm.
Flatness (cosmology)
Table (database)
Micrometer
Ruler
Accuracy and precision
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Citations (1)