Fabrication of free-form surfaces using a long-stroke fast tool servo and corrective figuring with on-machine measurement

Fabrication of free-form surfaces that are frequently demanded for the construction of optical imaging systems is described. To obtain a tool motion with large amplitude and high bandwidth, a novel long-stroke fast tool servo is proposed and installed on the Z-axis of a diamond turning machine as an additional synchronized axis. In addition, a special on-machine measurement device is used to measure the optical parameters of the machined surface and to compensate for the residual form of errors that are commonly produced in the diamond turning process. Actual machining test results show that the proposed procedures are capable of generating the copper free-form mirrors of 50 mm diameter to a form accuracy of 0.15 μm in peak-to-valley value error.     

Estimation and correction method for the two-dimensional positionerrors of a planar XY stage based on motion error measurements

This paper describes an estimation and correction method for the two-dimensional (2D) position errors of a planar XY stage that is driven along the Y-axis by two linear motors. The 2D position errors of the stage were estimated and corrected based on measured motion errors from a conventional laser interferometer system. To compensate for the planar XY stage 2D position errors, corrections were introduced for the yaw, perpendicular, straightness and 1D position errors along each axis, which are predominantly caused by linear scale, yaw, and pitching motion errors. The effect of the motion error corrections was evaluated by diagonal measurements based on the ISO230-6 standard and six different 1D position error measurements along the X and Y-axes. By applying error motion corrections, the diagonal systematic deviation at the center point was improved from 499.7 μm to 1.16 μm, and the estimated maximum 2D position errors were improved to −0.263 and 0.530 μm in the X and Y directions, respectively. The diagonal systematic deviation at a corner point was 1.23 μm and the estimated maximum difference between the corner and center points improved from −2.603 and 2.603 μm to −0.05 and 0.12 μm in the X and Y directions respectively.     

Development of a multi-step measuring method for motion accuracy of NC machine tools based on cross grid encoder

Machining accuracy is directly influenced by the quasi-static errors of a machine tool. Since machine errors have a direct effect on both the surface finish and geometric shape of the finished work piece, it is imperative to measure the machine errors and to compensate for them. A revised geometric synthetic error modeling, measurement and identification method of 3-axis machine tool by using a cross grid encoder is proposed in this paper. Firstly a revised synthetic error model of 21 geometric error components of the 3-axis NC machine tools is developed. Also the mapping relationship between the error component and radial motion error of round work piece manufactured on the NC machine tools are deduced. Aiming to overcome the solution singularity shortcoming of traditional error component identification method, a new multi-step identification method of error component by using the cross grid encoder measurement technology is proposed based on the kinematic error model of NC machine tool. Finally the experimental validation of the above modeling and identification method is carried out in the 3-axis CNC vertical machining center Cincinnati 750 Arrow. The entire 21 error components have been successfully measured by the above method. The whole measuring time of 21 error components is cut down to 1–2 h because of easy installation, adjustment, operation and the characteristics of non-contact measurement. It usually takes days of machine down time and needs an experienced operator when using other measuring methods. Result shows that the modeling and the multi-step identification methods are very suitable for ‘on machine’ measurement.     

Dynamic variable depth of cut machining using piezoelectric actuators

This paper describes the design of a piezoelectric actuated cutting tool and implementation of digital servo controls for machining surfaces with dynamically varying depth of cut. Through a flexure hinge, the tool holder could generate 50 μm travel at the tip of the cutting insert. Tool motion errors of less than 0.5 μm were achieved in tracking cyclic waveforms by employing a digital repetitive servo control. When applied to turning aluminum and steel workpieces with variable depth of cut using carbide tools, less than 5 μm machined surface errors were measured.     

Accuracy enhancement of five-axis machine tool based on differential motion matrix: Geometric error modeling,identification and compensation

This paper presents the precision enhancement of five-axis machine tools according to differential motion matrix, including geometric error modeling, identification and compensation. Differential motion matrix describes the relationship between transforming differential changes of coordinate frames. Firstly, differential motion matrix of each axis relative to tool is established based on homogenous transformation matrix of tool relative to each axis. Secondly, the influences of errors of each axis on accuracy of tool are calculated with error vector of each axis. The sum of these influences is integration of error components of machine tool in coordinate system of tool. It endows the error modeling clear physical meaning. Moreover, integrated error components are transformed to coordinate frame of working table for integrated error transformation matrix of machine tools. Thirdly, constructed Jacobian is established using differential motion matrix of each axis without extra calculation to compensate the integrated error components of tool. It makes compensation easy and convenient with reuse of intermediate. Fourthly, six-circle method of ballbar is developed based on differential motion matrix to identify all ten error components of each rotary axis. Finally, the experiments are carried out on SmartCNC500 five-axis machine tool to testify the effectiveness of proposed accuracy enhancement with differential motion matrix.     

A six-degree-of-freedom measurement system for the motion accuracy of linear stages

This paper presents a new six-degree-of-freedom measurement system (6DMS) which has the capability to measure simultaneously all six motion errors of a linear stage. This system employs three parallel laser beams to detect three relative linear distances of a moving body by Doppler effect so that the positioning, yaw and pitch errors can be separated. With an additional beam splitter and two quadrant photodetectors to detect the lateral shifts of the returned beams, the remaining two straightness errors and roll error can be obtained at the same time. In comparison with the HP5528A system, the accuracy of the positioning error is about 0.01 μm to the range of 10 m, the straightness error is about 1 μm within the measuring range of ± 0.1 mm, and angular errors are all about 1 arcsec within the range of ± 50 arcsecs. This system is simple in principle and can be easily equipped to any moving stages, such as linear stages, X-Y tables, CMMs, and machine tools.     

Enhanced virtual machining for sculptured surfaces by integrating machine tool error models into NC machining simulation

Sculptured surface machining is a time-consuming and costly process. It requires simultaneously controlled motion of the machine axes. However, positioning inaccuracies or errors exist in machine tools. The combination of error motions of the machine axes will result in a complicated pattern of part geometry errors. In order to quantitatively predict these part geometry errors, a new application framework ‘enhanced virtual machining’ is developed. It integrates machine tool error models into NC machining simulation. The ideal cutter path in the NC program for surface machining is discretized into sub-paths. For each interpolated cutter location, the machine geometric errors are predicted from the machine tool error model. Both the solid modeling approach and the surface modeling approach are used to translate machine geometric errors into part geometry errors for sculptured surface machining. The solid modeling approach obtains the final part geometry by subtracting the tool swept volume from the stock geometric model. The surface modeling approach approximates the actual cutter contact points by calculating the cutting tool motion and geometry. The simulation results show that the machine tool error model can be effectively integrated into sculptured surface machining to predict part geometry errors before the real cutting begins.     

A new compensation method for geometry errors of five-axis machine tools

The present study aims to establish a new compensation method for geometry errors of five-axis machine tools. In the kinematic coordinate translation of five-axis machine tools, the tool orientation is determined by the motion position of machine rotation axes, whereas the tool tip position is determined by both machine linear axes and rotation axes together. Furthermore, as a nonlinear relationship exists between the workpiece coordinates and the machine axes coordinates, errors in the workpiece coordinate system are not directly related to those of the machine axes coordinate system. Consequently, the present study develops a new compensation method, the decouple method, for geometry errors of five-axis machine tools. The method proposed is based on a model that considers the tool orientation error only related to motion of machine rotation axes, and it further calculates the error compensations for rotation axes and linear axes separately, in contrast to the conventional method of calculating them simultaneously, i.e. determines the compensation of machine rotation axes first, and then calculates the compensation associated with the machine linear axes. Finally, the compensation mechanism is applied in the postprocessor of a CAM system and the effectiveness of error compensation is evaluated in real machine cutting using compensated NC code. In comparison with previous methods, the present compensation method has attributes of being simple, straightforward and without any singularity point in the model. The results indicate that the accuracy of positioning was improved by a factor of 8–10. Hence, the new compensation mechanism proposed in this study can effectively compensate geometry errors of five-axis machine tools.     

Theory and application of a combined feedback–feedforward control and disturbance observer in linear motor drive wire-EDM machines

This paper presents a combined two-degree-of-freedom controller and disturbance observer design for a direct drive motion control system actuated by permanent-magnet linear synchronous motors (PMLSM). A feedback controller based on pole-placement design method is proposed to achieve desired tracking performance as well as stabilize the closed-loop system. A newly designed feedforward controller is proposed to reduce tracking errors based on an inverse model of the direct drive system. A digital disturbance observer is implemented to be included in the proposed feedback–feedforward control structure to compensate for nonlinear friction, cogging effects, and external load disturbance. Furthermore, the proposed control scheme has been verified as being internally stable. Experimental results indicate that the proposed controller can achieve a high contouring accuracy of ±0.3 μm as well as provide disturbance rejection and robustness. The maximum contour error of circular trajectory was reduced from 8.5 to 3.2 μm in comparison with proportional-integral-derivative (PID) controller.     

Effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool

The effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool are studied experimentally with a theoretical analysis. Problems in selecting the optimum burnishing parameters and some burnishing mechanisms are discussed. With suitable parameters employed, the new no-chip finishing process developed can eliminate or reduce the cutting marks left on the workpiece surface by diamond cutting tools, with its surface roughness reduced to Ra=0.026 μm from the original 0.5 μm.     

Measuring and compensating for 5-DOF parasitic motion errors in translation stages using Twyman–Green interferometry

A new metrological method is presented that performs simultaneous real-time measurements and compensates for 5-DOF parasitic motion errors in translation stages for precision profiling of optical surfaces. Two plane mirrors are used to obtain motion-dependent interferometric fringes generated by the optical principles of Twyman–Green interferometry. The fringes generated are monitored using high-speed 2D photodiode arrays and analyzed to determine the five separate motion error components in real time. Simultaneously, null control is performed to suppress the measured motion errors independently using piezoelectric actuators through real-time feedback control while the machine axis is moving. The experimental results demonstrate that 5-DOF parasitic motion errors are effectively measured and compensated to within 10 nm for translational motions and 0.15 arcsec for angular motions.     

Ultra-precision surface finish of the hardened stainless mold steel using vibration-assisted ball polishing process

A vibration-assisted spherical polishing system driven by a piezoelectric actuator has been newly developed on a machining center to improve the burnished surface roughness of hardened STAVAX plastic mold stainless steel and to reduce the volumetric wear of the polishing ball. The optimal plane surface ball burnishing and vibration-assisted spherical polishing parameters of the specimens have been determined after conducting the Taguchi's L₉ and L₁₈ matrix experiments, respectively. The surface roughness R_a=0.10 μm, on average, of the burnished specimens can be improved to R_a=0.036 μm (R_max=0.380 μm) using the optimal plane surface vibration-assisted spherical polishing process. The improvement of volumetric wear of the polishing ball was about 72% using the vibration-assisted polishing process compared with the non-vibrated polishing process. A simplified kinetic model of the vibration-assisted spherical polishing system for the burnished surface profile was also derived in this study. Applying the optimal plane surface ball burnishing and vibrated spherical polishing parameters sequentially to a fine-milled freeform surface carrier of an F-theta scan lens, the surface roughness of R_a=0.045 μm (R_y=0.65 μm), on average, within the measuring range of 149 μm×112 μm on the freeform surface, was obtainable.     

Swept volumes of toroidal cutters using generating curves

Modelling the volume swept by a tool as it moves from one programmed position to another in three, four and five-axis milling is a challenging task. In this paper, a technique for generating the volume swept by a toroidal cutter in its motion along a given tool path, is presented. This technique is based on identifying “generating curves” along the path and connecting them into a solid model of the swept volume. The swept volume simulations are verified experimentally for three test pieces. These are also compared with the simulation results from the Z-map technique. The results of the experimental verification show the method to be accurate to within 10 μm for the three test pieces. Furthermore, the computation time for the new technique is significantly less than the Z-map method. It is concluded that the proposed method has the potential of allowing fast simulation and verification of multi-axis tool paths.     

Servo scanning 3D micro-EDM based on macro/micro-dual-feed spindle

Using the end discharge of micro-rod-shaped electrode to scan layer by layer, micro-electrical discharge machining (EDM) can fabricate complex 3D micro-structures. During the machining process, the discharge state is broken frequently due to the wear of the tool electrode and the relative scanning motion. To keep a favorable discharge gap, the feed spindle of the tool electrode needs the characteristics of high-frequency response and high resolution. In this study, an experimental system with a macro/micro-dual-feed spindle was designed to improve the machining performance of servo scanning 3D micro-EDM (3D SSMEDM), which integrates an ultrasonic linear motor as the macro-drive and a piezoelectric (PZT) actuator as micro-feeding mechanism. Based on LabVIEW and Visual C++ software platform, a real-time control system was developed to control coordinately the dual-feed spindle to drive the tool electrode. The micro-feed motor controls the tool electrode to keep the favorable discharge gap, and the macro-drive motor realizes long working range by a macro/micro-feed conversion. The emphasis is paid on the process control of the 3D SSMEDM based on macro/micro-dual-feed spindle for higher machining accuracy and efficiency. A number of experiments were carried out to study the machining performance. According to the numerical control (NC) code, several typical 3D micro-structures have been machined on the P-doped silicon chips. Our study results show that the machining process is stable and the regular discharge ratio is higher. Based on our fundamental machining experiments, some better-machined effects have been gained as follows. By machining a micro-rectangle cavity (960 μm×660 μm), the machined depth error can be controlled within 2%, the XY dimensional error is within 1%, the surface roughness R_a reaches 0.37 μm, and the material removal rate is about 1.58×10⁴ μm³/s by using a tool electrode of Φ=100 μm in diameter. By machining multi-micro-triangle cavities (side length 700 μm), it is known that the machining repeatability error is <0.7%.     

An application of real-time error compensation on a turning center

A real-time error compensation (RTEC) system is developed to correct thermally-induced and geometric errors on a four-axis dual-spindle turning center. These errors vary with different cutting tool positions as well as different thermal conditions, thus real-time correction is required. Two problems in the current RTEC approach are addressed: (1) the difficulty in the actual measurement of error components according to the defined coordinates and (2) the selection of a small set of appropriate temperature variables from numerous candidate thermal sensors on a machine structure. A flexible error measurement method and an optimal temperature variable selection process are proposed to overcome these difficulties. After machine errors were characterized and modeled, the effectiveness of the developed RTEC system was evaluated by using laser inspection and an actual cutting test. The maximal diagonal displacement error is corrected from 75.0 to 7.5 μm. In the cutting test, the part diameter error of a car steering joint is reduced from 60 to 10 μm.     

Enhancement of dynamic stability of cantilever tooling structures

The least rigid components of machining systems are cantilever tools and cantilever structural units of machine tools (rams, spindle sleeves, etc.). These components limit machining regimes due to the development of chatter vibrations, limit tool life due to extensive wear of cutting inserts, and limit geometric accuracy due to large deflections under cutting forces. Use of high Young's modulus materials (such as sintered carbides) to enhance the dynamic quality of cantilever components has only a limited effect and is very expensive. This paper describes a systems approach to the development of cantilever tooling structures (using the example of boring bars) which combine exceptionally high dynamic stability and performance characteristics with cost effectiveness. Resultant success was due to: (1) a thorough survey of the state of the art; (2) creating a “combination structure” concept with rigid (e.g. sintered carbide) root segments combined with light (e.g. aluminum) overhang segments, thus retaining high stiffness and at the same time achieving low effective mass (thus, high mass ratios for dynamic vibration absorbers, or DVAs) and high natural frequencies; (3) using the concept of “saturation of contact deformations” for efficient joining of constituent parts with minimum processing requirements; (4) suggesting optimized tuning of DVAs for machining process requirements; (5) development of DVAs with the possibility of broad-range tuning; (6) structural optimization of the system; and (7) using a novel concept of a “Torsional Compliant Head”, or TCH, which enhances dynamic stability at high cutting speeds and is suitable for high rev/min applications since it does not disturb balancing conditions. The optimal performance and interaction of these concepts were determined analytically, and then the analytical results were validated by extensive cutting tests with both stationary and rotating boring bars, machining steel and aluminum parts. Stable performance with length-to-diameter ratios up to L/D = 15 was demonstrated, with surface finish 20–30 μm with both steel and aluminum at L/D = 7–11. Comparative tests with commercially available bars demonstrated the advantages of our system.     

Single setup estimation of a five-axis machine tool eight link errors by programmed end point constraint and on the fly measurement with Capball sensor

Five-axis machine tools can be programmed to keep a constant nominal tool end point position while exercising all five axes simultaneously. This kinematic capability allows the use of a 3D proximity sensing head mounted at the spindle to track the position changes of a precision steel ball mounted on the machine table effectively measuring the 3D Cartesian volumetric errors of the machine. The new sensing head uses capacitive sensors to gather data on the fly during a synchronized five-axis motion which lasts less than 2 min. Because the measured volumetric errors are strongly affected by the link geometric errors, they can be used to estimate the link errors through an iterative procedure based on an identification Jacobian matrix. The paper presents the new sensor, the identification model and the experimental validation. The approach allows all eight link errors i.e. the three squarenesses of linear axes and the four orientations and center lines offset of the rotary axes to be estimated with the proposed single setup test. The estimation approach is performed on a horizontal five-axis machine tool. Then, using the estimated link errors, the volumetric errors are predicted for axes combinations different from those used for the identification process. The estimated machine model correctly predicts 52–84% of the volumetric errors for the tested trajectories.     

High-speed rotary cutting of difficult-to-cut materials on multitasking lathe

This paper aims to realize the high-speed rotary dry cutting of an Inconel 718 at 500 m/min on a multitasking lathe which has an additional milling spindle with an X/Y/Z-axis and inclination control. A series of experiments were conducted and are discussed with respect to the tool face temperature analysis by FEM. It was verified that it is necessary to select an optimum inclination angle, tool rotation speed and tool diameter so as to enable the main cutting force direction to align with the highest rigidity direction of an applied rotary tool. Under preferable cutting conditions, the average tool rake face temperature measured by a thermograph camera was about 300 °C even at a high cutting speed of 500 m/min under dry cutting conditions, and the tool wear decreased dramatically compared with the conventional tools.