Showing papers in "Precision Engineering-journal of The International Societies for Precision Engineering and Nanotechnology in 2010"

Synthesis of multi-degree of freedom, parallel flexure system concepts via freedom and constraint topology (FACT). Part II: Practice

Abstract: In Part II of this paper we demonstrate how to use freedom and constraint topology (FACT) to synthesize concepts for the multi-degree of freedom, parallel precision flexure systems that fall within the scope of Part I. Several examples are provided to demonstrate how the Principle of Complementary Topologies and geometric entities from Part I are (i) relevant to flexure system characteristics, (ii) used to visualize the possible layout of flexure constraints to achieve a desired motion and (iii) used to select redundant constraints. A synthesis process is presented, and then used to visualize and construct a flexure system concept with the requisite kinematic characteristics and redundant constraints that provide increased stiffness, load capacity, and symmetry. The output of the process is a flexure concept that would then be modeled and refined by existing modeling and analysis methods.

Three flexure hinges for compliant mechanism designs based on dimensionless graph analysis

Abstract: This paper presents the dimensionless empirical equations and graph expressions of three flexure hinges for compliant mechanism designs. The in-plane and out-of-plane stiffnesses of the flexure hinges are developed. The rotational precision, denoted by the midpoint stiffness, is derived for the purpose of optimized geometric design. Based on the developed methodologies, the influences of the geometric parameters on the performance of the flexure hinges are investigated, and the performance comparisons among the flexure hinges are conducted to further understand the characteristics of these kinds of compliant mechanisms.

Closed-form compliance equations of filleted V-shaped flexure hinges for compliant mechanism design

Abstract: This paper presents the closed-form compliance equations for the filleted V-shaped flexure hinges. The in-plane and out-of-plane compliances of the flexure hinges are developed based on the Castigliano's second theorem. The accuracy of motion, denoted by the midpoint compliance of the flexure hinges, is also derived for optimized geometric design. The influences of the geometric parameters on the characteristics of the flexure hinges are investigated. It is noted that the filleted V-shaped flexure hinges have diverse ranges of compliance corresponding to different filleted radius R and angle θ . These types of hinges can provide both higher and lower stiffnesses than circular flexure hinges. This makes filleted V-shaped flexure hinges very useful for wide potential applications with different requirements. The finite element analysis is used to verify the established closed-form compliance equations for these filleted V-shaped flexure hinges.

Machining tests to identify kinematic errors on five-axis machine tools

Abstract: The machining of a cone frustum as specified in National Aerospace Standard (NAS) 979 is widely accepted as a final performance test for five-axis machining centers. Although it gives a good demonstration of the machine’s overall machining performance, it is generally difficult to separately identify each error source in the machine from the measured error profile of the finished workpiece. This paper proposes a set of machining tests for a five-axis machine tool to identify its kinematic errors, one of its most fundamental error sources. In each machining pattern, a simple straight side cutting using a straight end mill is performed. The relationship between geometric errors of the finished workpiece and the machine’s kinematic errors is formulated based on the kinematic model of a five-axis machine. The identification of kinematic errors from geometric errors of finished workpieces is experimentally demonstrated on a commercial five-axis machining center, and the estimates are compared to those estimated based on ball bar measurements.

A review, supported by experimental results, of voltage, charge and capacitor insertion method for driving piezoelectric actuators

Abstract: A piezoelectric actuator consists of ceramic material that expands or contracts when a positive or a negative potential voltage signal is applied. The displacement of a piezoelectric actuator is commonly controlled using a voltage input due to its ease of implementation. However, driving a piezoelectric actuator using a voltage input leads to the non-linear hysteresis and creep. Hysteresis and creep are undesirable characteristics which lead to large errors when a piezoelectric actuator is used in positioning applications. The amount of hysteresis and creep could be minimized to a large extent when a piezoelectric actuator is driven using a charge input. Another method which substantially reduces hysteresis and creep involves the insertion of a capacitor in series with a piezoelectric actuator which is driven using a voltage input. A review of voltage, charge and capacitor insertion methods for driving piezoelectric actuators is presented in this paper. Experimental results, for a piezoelectric actuator driven using the above three methods, are presented to validate the facts presented in this review.

Design and construction of a two-degree-of-freedom linear encoder for nanometric measurement of stage position and straightness

Abstract: This paper presents a two-degree-of-freedom (two-DOF) linear encoder which can measure the position along the moving axis (X-axis) and the straightness along the axis vertical to the moving axis (Z-axis) of a precision linear stage simultaneously. The two-DOF linear encoder is composed of a reflective-type scale grating and an optical sensor head. A reference grating, which is identical to the scale grating except the scale length, is employed in the optical sensor head. Positive and negative first-order diffracted beams from the two gratings are superposed with each other in the optical sensor head to generate interference signals. The optical configuration is arranged in such a way that the direction of displacement in each axis can also be detected. A prototype two-DOF linear encoder is designed and constructed. The size of the optical sensor head is about 50 mm (X) × 50 mm (Y) × 30 mm (Z) and the pitch of the grating is 1.6 μm. It has been confirmed that the prototype two-DOF linear encoder has sub-nanometer resolutions in both the X- and Z-axes.

An effective pseudo-rigid-body method for beam-based compliant mechanisms

Abstract: Beam-based compliant mechanisms have been widely used in many precision applications due to their large-displacement characteristic Meanwhile, a simple and accurate analytical tool is no doubt useful to assist design and optimize these mechanisms The pseudo-rigid-body (PRB) model is such a good alternative The PRB model in a traditional sense, however, is too approximate to calculate some precise characteristics such as center-shift of a flexural joint In this paper, a new method is proposed to easily establish simple and accurate PRB models for a variety of beam-based compliant mechanisms As is well known, for most of the beam-based compliant mechanisms, there is usually an instantaneous center of rotation (ICR) on each beam, and the beam can be considered as a fixed-guided beam with one end fixed while the other end moves along a certain curve that usually is an approximate arc around the ICR In this case, the behavior of beams can be imitated by a rigid bar with two pin-joints One of the pin-joints is directly added at the end of the rigid bar The position of the other is determined by the characteristic radius factor, and a torsional spring is added at the position to represent the beam's stiffness The modeling method is used to analyze two commonly used beam-based compliant mechanisms as examples, ie a parallel linear spring stage and a cross strip pivot The performances of precision, stiffness and stroke for the two mechanisms are investigated, and the method is validated by comparing the results with other research work presented in the literatures Both cases show the method is intuitive and accurate, and it will be useful in the early phases of the design for beam-based compliant mechanisms

Electrical discharge machining of submicron holes using ultrasmall-diameter electrodes

Abstract: We have carried out the electrical discharge machining (EDM) of submicron holes using ultrasmall-diameter electrodes. Two types of electrode were used: tungsten electrodes fabricated by the combination of wire electrodischarge grinding and electrochemical machining, and silicon electrodes originally designed as probes for scanning probe microscopes. The diameters of the former and latter were 1 μm or less, and less than 0.15 μm, respectively. Holes were drilled using a relaxation-type pulse generator at an open-circuit voltage of less than or equal to 20 V with the machine's stray capacitance as the only capacitance. Using tungsten electrodes, holes of less than 1 μm in diameter and more than 1 μm in depth were successfully drilled. A 1.3-μm-wide slot was also fabricated by drilling many holes with a small pitch. It was possible to drill holes of approximately 0.5 μm diameter using silicon electrodes because the electrode diameter was less than those of the tungsten electrodes. These holes have the smallest reported diameter for holes drilled by EDM, indicating the possibility of submicron- and nanoscale machining by EDM.

Design and control of a long-traveling nano-positioning stage

Abstract: In this paper, a Dual-Axis Long-Traveling Nano-Positioning Stage (DALTNPS) is presented. In order to extend the traveling and increase the accuracy, the two sorts of stages, a traditional ball-screw stage and a three-degrees-of-freedom (3-DOF) piezo-stage, were composed. The traditional ball-screw stage which is composed of two guide-ways and a ball-screw at each axis is a long-travel stage, and the 3-DOF piezo-stage, which is composed of three piezoelectric actuators and four translation–rotation mechanisms, is a high precision stage. In addition, a 3-DOF measuring system and a PID controller are composed of a 3-DOF closed-loop controller and applied to implement the DALTNPS. The measuring system which is composed of two laser interferometers and two plane mirrors is a 3-DOF optical measuring system. Thus, the position at the x and y axes and the rotation around the z axis can be obtained and they are the responses of DALTNPS. Finally, the experiment results evidence that the DALTNPS is characterized by long-travel, high linear accuracy, high rotation accuracy, high contouring accuracy and high motion speed.

Design and modeling of a six DOFs MEMS-based precision manipulator

Abstract: In this paper a design is presented for a precision MEMS-based six degrees-of-freedom (DOFs) manipulator. The purpose of the manipulator is to position a small sample (10 μm × 20 μm × 0.2 μm) in a transmission electron microscope. A parallel kinematic mechanism with slanted leaf-springs is used to convert the motion of six in-plane electrostatic comb-drives into six DOFs at the end-effector. The manipulator design is based on the principles of exact constraint design, resulting in a high actuation compliance (flexibility) combined with a relatively high suspension stiffness. However, due to fabrication limitations overconstrained design has been applied to increase the stiffness in the out-of-plane direction. The result is a relatively large manipulator stroke of 20 μm in all directions combined with a high first vibration mode frequency of 3.8 kHz in relation to the used area of 4.9 mm × 5.2 mm. The motion of the manipulator is guided by elastic elements to avoid backlash, friction, hysteresis and wear, resulting in nanometer resolution position control. The fabrication of the slanted leaf-springs is based on the deposition of silicon nitride (SixNy) on a silicon pyramid, which in turn is obtained by selective crystal plane etching by potassium hydroxide (KOH). The design has been analyzed and optimized with a multibody program using flexible beam theory. A previously developed flexible beam element has been used for modeling the typical relatively large deflections and the resulting position-dependent behavior of compliant mechanisms in MEMS. The multibody modeling has been verified by FEM modeling. Presently only parts of the manipulator have been fabricated. Therefore, a scaled-up version of the manipulator has been fabricated to obtain experimental data and to verify the design and modeling.

A comparison of reversal and multiprobe error separation

Abstract: This paper demonstrates the application of two methods of separating spindle error motion from artifact roundness on a spindle with <5 nm radial error. Two error separation methods, reversal and multiprobe, were each applied to data taken on two different test stands allowing direct comparison of the four combinations of hardware and separation algorithm. Because the theory of both separation methods is well documented, this work focuses on their implementation for nanometer-level measurements. As will be seen, a number of issues must be addressed to obtain repeatable results at this level of precision in spindle metrology. Ultimately, the results show that sub-nanometer features in both spindle error and artifact form can be reliably and repeatably resolved by both techniques.

Design and characterization of MIKES metrological atomic force microscope

Abstract: An interferometrically traceable metrological atomic force microscope (IT-MAFM) has been developed at MIKES. It can be used for traceable atomic force microscope (AFM) measurements and for calibration of transfer standards of scanning probe microscopes (SPMs). Sample position is measured online by 3 axes of laser interferometers. A novel and simple method for detection and online correction of the interferometer nonlinearity was developed. Effect of the nonlinearity in measurements is demonstrated. In the design, special attention has been paid to elimination of external disturbances like electric noise, acoustic noise, ambient temperature variations and vibrations. The instrument has been carefully characterized. The largest uncertainty components are caused by Abbe errors, orthogonality errors, drifts and noise. Noise level in Z direction was 0.25 nm, and in X and Y directions 0.36 nm and 0.31 nm, respectively. Standard uncertainties for X , Y and Z coordinates are u cx = q [0.48; 0.04 x ; 0.17 y ; 1.7 z ; 2 time] nm, u cy = q [0.45; 0.31 x ; 0.07 y ; 0.14 z ; 4 time] nm and u cz = q[0.42; 3 x ; 7.2 y ; 0.18 z ; 2 time] nm where x , y , z are in μm and time in h. Standard uncertainty for 300 nm pitch is 0.023 nm,and for 7 nm step height measurement is 0.35 nm. Uncertainty estimates are supported by an international comparison.

Influences of operational conditions and geometric parameters on the stiffness of aerostatic journal bearings

Abstract: The stiffness of various geometric designs of aerostatic journal bearings for high-speed spindles was investigated under different operating conditions. First, the stiffness of the front and rear journal bearings was evaluated experimentally using the relationship of force and displacement at different supply pressures (6–9 bar). A numerical model was then developed to simulate journal-bearing stiffness under the same pressure range to validate the results. The model was extended to calculate the stiffness of the designs with various geometric parameters, including bearing length/diameter (L/D) ratios of 0.5, 1.0, and 1.5; different types of restrictor designs (e.g., pocketed orifice versus inherent orifice); supply orifice diameter; radius gap; supply pressures of 6, 7, 8, and 9 bar. These parameters are believed to be crucial to the performance of gas bearings. It was found that gas-bearing geometries had a significant effect on stiffness. The results also provide helpful design guidelines for gas-bearing spindles in high-precision machine applications.

Model reference adaptive control for a piezo-positioning system

Abstract: Piezoelectric (PZT) actuators having the characteristic of infinitely small displacement resolution are popularly applied as actuators in precision positioning systems. Due to its nonlinear hysteresis effect, the tracking control accuracy of the precision positioning system is difficultly achieved. Hence, it is desirable to take hysteresis effect into consideration for improving the trajectory tracking performance. In this paper, a model reference adaptive control scheme based on hyperstability theory is developed for a moving stage system driven by a PZT actuator. It is worth emphasizing that the controller can be constructed without a nonlinear hysteresis dynamic equation to compensate the hysteresis effect. According to simulation results, the tracking error was only nanometer order. Through experimental examinations, the tracking performance was obtained as precision as ten nanometers order which is the resolution limitation of the measurement system. The effectiveness of the proposed adaptive control scheme was validated.

Burr height model for vibration assisted drilling of aluminum 6061-T6

Abstract: Vibration assistance has seen increasing application in metal removal processes. One application of this technique is vibration assisted drilling. This method typically induces high-frequency and low-amplitude vibration in the direction of drill feed during drilling, and has the potential to reduce thrust forces and reduce exit burr height. Note that this cutting process is distinct from ultrasonic machining/drilling. Predicting exit burr height accurately is important for determining the favourable vibration conditions for burr height reduction. This paper presents a novel analytical burr height model to predict the exit burr height in vibration assisted drilling of aluminum 6061-T6. This model also improves upon the existing analytical burr height model for conventional drilling. The results of 72 drilling experiments with TiN coated standard twist drills are reported. The predictions from the developed burr height model are compared with the experimental results. The results demonstrate that the proposed model improves the accuracy of the existing burr height model for conventional drilling by up to 36%, and also predicts the burr heights for VAD within a 10% deviation from the mean values of the experimental results. A fast analytical method for predicting the favourable vibration conditions for minimizing burr height is also presented.

A tolerance interval based criterion for optimizing discrete point sampling strategies

Abstract: In order to compute geometric tolerances, the distance between the actual geometry and the nominal one has to be computed, and this computation requires the selection of a sampling strategy. Sampling strategy consists in deciding the number and the locations of the points that have to be measured on the actual surface. This paper presents a new approach for selecting the optimal locations of the points for any given sample size. The proposed procedure is based on estimating the “manufacturing signature”, that is the systematic pattern left by the manufacturing process on the machined items. In particular, the proposed approach is based on using regression for estimating the manufacturing signature and then selecting the measurement point locations by minimizing the distance between the maximum and the minimum points of the regression-based tolerance interval. Performance of the proposed procedure are compared with those obtainable by selecting different strategies with reference to a case study related with roundness data. Given that the approach proposed outperforms the competitor ones, a robustness study is finally presented to show that the method presented is robust to violation of the assumption behind linear regression models.

A multiphysics model for optimizing the design of active aerostatic thrust bearings

Abstract: Aerostatic bearing solutions avoid problems related to friction and enable high precision linear guiding. However, their relatively low stiffness hinders the accuracy when disturbance forces, such as inertial effects or machining forces, are present. An active compensation strategy based on air gap shape control with piezoelectric actuators has been proposed, which can achieve nanometre position control and high bandwidth disturbance compensation. Suitable design strategies and tools still need to be developed to optimize the performance and enforce the industrial application of these systems. This paper proposes a new multiphysics finite element model which considers the interaction between the air flow dynamics, the structural flexibility of the bearing, the piezoelectricity for the actuators and the control with strongly coupled formulation. A setup for high bandwidth and resolution experiments has been built and used to test the prototypes. The experimental results prove the validity of the model and the relevance of the fluid–structure interaction, and thus the need for such a coupled multiphysics model for optimizing the design of active air bearings.

On the removal of critical cutting regions by trochoidal grooving

Abstract: Particularly in high-speed roughing by a straight end mill, contour-parallel paths often cause an excessive tool load in a critical cutting region such as a sharp corner and a narrow slot. Trochoidal grooving can be used to remove such regions safely, prior to high-speed contour-parallel cutting. This paper present a systematic tool path generation strategy for high-speed 2.5-dimensional end milling with the removal of critical cutting regions by trochoidal grooving. We first present a scheme to extract critical cutting regions for an arbitrary two-dimensional pocket contour by using its medial axis. A trochoidal tool path to remove an arbitrary contour can be generated also by using the medial axis. Two experimental case studies are presented to show the efficiency and tool wear progress in the roughing by a straight end mill with trochoidal grooving to remove critical cutting regions, in comparison with conventional contour-parallel roughing by a ball end mill.

A new, high precision, quick response pressure regulator for active control of pneumatic vibration isolation tables

Abstract: Pressure regulators are important elements in pneumatic systems. Relief-type precision pressure regulators are commonly used to control the supply pressure to actively controlled pneumatic vibration isolators. Herein, a high precision, quick response pneumatic pressure regulator is proposed. This consists of an isothermal chamber, a servo valve, a pressure sensor, a quick response laminar flow sensor (QFS), and a pressure differential sensor (PD sensor) as developed by the authors. Slight changes of pressure in the chamber can be detected by the PD sensor and are fed back to the servo valve to maintain the pressure at a desired value. The performance of this regulator was confirmed experimentally in comparison with one available commercially. The regulator was then applied to the supply pressure regulation of an actively controlled pneumatic vibration isolation table. The superior performance of the regulator is clearly shown in the experimental results, especially in terms of avoiding effects from upstream or downstream disturbances.

Fabrication of tungsten microelectrodes using pulsed electrochemical machining

Abstract: The manufacture of the micro-structure of MEMS (Micro Electro-Mechanical System) becomes more and more important in recent years. Electrochemical micromachining has many advantages over other machining methods. However, a tiny electrode is needed to manufacture the micro-structures. Accordingly, in this study pulsed electrochemical machining is applied to fabricate a cylindrical microelectrode. The influences of working parameters (such as applied voltage, pulsed period, duty factor and temperature) on the fabrication of the microelectrode are discussed. Results also show that cylindrical tungsten microelectrodes with a diameter of 100 μm and various lengths can be fabricated by a linear decay of applied voltage or duty factor.

Precision surface finish of the mold steel PDS5 using an innovative ball burnishing tool embedded with a load cell

Abstract: A load-cell-embedded burnishing tool has been newly developed and integrated with a machining center, to improve the surface roughness of the PDS5 plastic injection mold steel. Either the rolling-contact type or the sliding-contact type was possible for the developed ball burnishing tool. The characteristic curves of burnishing force vs. surface roughness for the PDS5 plastic injection mold steel using the developed burnishing tool for both the rolling-contact type and the sliding-contact type, have been investigated and constructed, based on the test results. The optimal plane surface burnishing force for the PDS5 plastic injection mold steel was about 420 N for the rolling-contact type and about 470 N for the sliding-contact type, based on the results of experiments. A force compensation strategy that results in the constant optimal normal force for burnishing an inclined surface or a curved surface, has also been proposed to improve the surface roughness of the test objects in this study. The surface roughness of a fine milled inclined surface of 60 degrees can be improved from Ra 3.0 μm on average to Ra 0.08 μm (Rmax 0.79 μm) on average using force compensation, whereas the surface roughness was Ra 0.35 μm (Rmax 4.56 μm) on average with no force compensation.

3D linear and volumetric wear measurement on artificial hip joints—Validation of a new methodology

Abstract: Artificial hip replacement joints have been in use to treat osteoarthritis and rheumatoid arthritis for decades. These devices consist of a metallic/ceramic ball articulating against a polyethylene or metallic/ceramic hemispherical cup. However, these artificial joints do wear and their clinical performance starts declining after 10–20 years of successful use. In the case of hard-on-hard bearings, the wear depth and volume remain very small and their measurement has remained a challenge to current technologies. A novel device, the non-contact RedLux Artificial Hip Profiler, which is based on the chromatically encoded confocal measurement method, has been developed so that these joints can be measured accurately, reproducibly and in a short timescale. The system outputs the component radius for the unworn part, the linear depth of wear and also provides the wear volume. The design principles are presented and the capability of the equipment to measure worn parts, with linear wear between a few microns and tens of microns is hereby characterised, and correlated with laser micrometer diameter measurements, roundness measurements of linear wear and gravimetric wear assessment. The Artificial Hip Profiler is capable of measuring spherical bearing surfaces with a resolution of 20 nm. The correlation to benchmark methods for the three parameters considered is very high, with correlation coefficients of 1.02 and 0.95 for the linear and volumetric wear, respectively. The R values were 0.988 and 0.996, respectively.

CAD-based environment to bridge the gap between product design and tolerance control

Abstract: Mechanical product quality is strongly influenced by the respect of Geometrical Tolerances (GT). On the other hand, competitiveness forces companies to improve their productivity making the tolerance verification process faster and faster and more flexible. Component control by 3D full field optical digitizing systems and specific CAD-based (Computer Aided Design) inspection software tools are important steps forward for the achievement of the above-mentioned goals. However, the adoption of these solutions in industry is minimal. This may be due both to technological factors, i.e. poor systems usability, and organizing factors, i.e. clear separation between design department and quality control department. In this context, our research aims at developing a new easy to use CAD-based tool for simulating, driving and optimizing the GT inspection process. Once a component has been digitized, the developed software system automatically realizes the tolerances virtual control. Hence, the designer can prescribe tolerances, pilot the measurement system and verify the component conformity. The implemented tool is based on Full of Information (FoI) CAD models, which contain tolerance data, linked to a knowledge database, where measurement strategies and verification rules are stored. A computation engine calculates the measurement paths and performs the tolerances verification. The prototypal system has been tested on different real cases. Experimental results showed high performances in terms of timesaving and robustness.

Conicity and cylindricity error evaluation using particle swarm optimization

Abstract: The measurement data obtained from the Coordinate Measuring Machines (CMMs) have to be further processed and analyzed to evaluate the form errors of manufactured components. Particle swarm optimization (PSO), which is based on a metaphor of social interaction, searches a space by adjusting the trajectories of individual vectors, called “particles” as they are conceptualized as moving points in multidimensional space. Each particle represents a point in the d-dimensional search space and has a velocity to adjust its flying direction according to the particle's best position ever found and the best position of all particles. PSO is proposed to implement the minimum zone evaluation of conicity and cylindricity errors simultaneously. The method takes an ideal cone as a particle which is decided by vertex, central line and vertex angle of the cone. And the particle’ velocity is modified by constriction factor approach. Compared to other evolutionary computation such as traditional genetic algorithm (GA) and immune algorithm (IM), PSO is easier to implement and there are fewer parameters to adjust. Then, the minimum zone conicity method is formulated which unites conicity and cylindricity errors evaluation as one. Then, the objective function calculation approach of conicity is developed which conforms to the ISO/1101 standard. Finally, the experimental results evaluated by different methods confirm the effectiveness of PSO. Compared to conventional evaluation methods, it not only has the advantages of algorithm simplicity and good flexibility, but also improves conical and non-strict cylindrical parts error evaluation accuracy. And it is well suited for form error evaluation on CMMs.

Evaluation of modelling approaches for machine tool design

Abstract: In order to evaluate the configuration of machine tools, the IWF Axis Construction Kit (ACK) has been developed. This paper describes the evaluation of this approach. The ACK supports rigid body simulations and simple elastic body simulations. The ACK is compared with commercial FEM software to investigate its usability and reliability. Required time was compared in modelling of a machine tool. The ACK needed 30% of the total required time for the FEM because of its modularity in machine modelling. Then, in order to investigate the reliability of the ACK, static and dynamic simulations of both approaches were compared with each other and with analytical calculations on basic beam models. The result showed that the ACK provided equivalent results to the FEM. Static and dynamic simulations were also compared with measurements on an actual machine tool. The ACK obtained almost equivalent results to the FEM. Almost all lower structural mode shapes and their natural frequencies could be reproduced with the ACK when crucial parts were modelled using elastic bodies.

Bundle adjustment of multi-position measurements using the Mahalanobis distance

Abstract: Precision three-dimensional metrology frequently involves the measurement of common points by several three-dimensional measurement systems. These can be laser tracking interferometers, electronic theodolites, etc. A new analysis technique has been developed that fully takes into account the uncertainty ellipsoid for each measurement point. This technique solves for the positions and orientations of the measurement systems while determining the optimal fit to each physical point. No a priori knowledge of the location and orientation of the measurement systems is required. An initial estimate for the location and orientation of the measurement systems is derived from the measurement data by assuming equal uncertainties for each data point. Then a merit function is minimized to determine the optimized location and orientation of the measurement systems and the weighted mean for the position estimates of the physical points. This merit function is based on the Mahalanobis distance from multivariate statistics and takes into account the particular shape and orientation of the three-dimensional uncertainty ellipsoid of each data point from each measurement system. This technique can utilize data from differing types of three-dimensional measurement systems including distance only and angle only measurements, evaluate the “strength” of a measurement configuration and accommodate missing data points from some of the measurement systems.

Investigation of Laser and Process Parameters for Selective Laser Erosion

Abstract: The process of Selective Laser Erosion (SLE) was investigated to study the effects of different process and laser parameters on the process outputs such as surface quality and erosion rate. The SLE process is a direct method to remove material in a layer-by-layer fashion due to high energy densities provided by the laser beam. In addition to its direct use as a subtractive manufacturing method, SLE may be used in combination with layer-additive techniques such as Selective Laser Melting (SLM). Such combination mainly makes sense when both processes can be performed with the same laser. However, one of the major problems involved in SLE process is the high number of the laser and process parameters (laser power, pulse frequency, scan speed, scan spacing, ambient atmosphere, etc.) and the complexity of the relations between them which has not yet been investigated completely. This paper presents an overview of the laser erosion process with nano-second Nd:YAG laser pulses and the results of several single-factor experiments that were carried out to determine the influence of the major parameters on the depth of erosion per layer and surface roughness. Additionally, the relations between the parameters are studied to investigate the interactions between them. The results from single-factor experiments showed that some relations were highly governed by the power intensity of the laser beam and also that cross interactions between the parameters play an important role on the output characteristics. The paper explains how multiple parameters (spot size, pulse frequency, scan speed, scan spacing) can be combined to define two indirectly controlled geometrical parameters, namely the scan and pulse overlap factors. Those two parameters allow calculating the number of hits of the laser beam on a same location on the workpiece possible which is the first step in physical modeling the topography of the surface left behind.

Uncertainty analysis of point-by-point sampling complex surfaces using touch probe CMMs DOE for complex surfaces verification with CMM

Abstract: The paper describes a study concerning point-by-point sampling of complex surfaces using tactile CMMs. A four factor, two level completely randomized factorial experiment was carried out, involving measurements on a complex surface configuration item comprising a sphere, a cylinder and a cone, combined in a single assembly. An investigation into the source and effects of different uncertainty contributors during this complex surface measurement was carried out. The factors involved were machine, probe, operator, and procedure dependent. The results obtained from the experiments were analysed in terms of deviations from CAD model, reproducibility, and measurement uncertainty. Statistical analysis and uncertainty budget are presented.