Comparison of Volumetric Analysis Methods for Machine Tools with Rotary Axes

Confidence in the ability of a production machine to meet manufacturing tolerances requires a full understanding of the accuracy of the machine. However, the definition of “the accuracy of the machine” is open to interpretation. Historically, this has been in terms of linear positioning accuracy of an axis with no regard for the other errors of the machine. Industry awareness of the three-dimensional positioning accuracy of a machine over its working envelope has slowly developed to an extent that people are aware that “volumetric accuracy” gives a better estimation of machine performance. However, at present there is no common standard for volumetric errors of machine tools, although several researchers have developed models to predict the effect of the combined errors. The error model for machines with three Cartesian axes has been well addressed, for example by the use of homogenous transformation matrices. Intuitively, the number of error sources increases with the number of axes present on the machine. The effect of the individual axis geometric errors can become increasingly significant as the chain of dependent axes is extended. Measurement of the “volumetric error” or its constituents is often restricted to a subset of the errors of the Cartesian axes by solely relying on a laser interferometer for measurement. This leads to a volumetric accuracy figure that neglects the misalignment errors of rotary axes. In more advanced models the accuracy of the rotary axes are considered as a separate geometric problem whose volumetric accuracy is then added to the volumetric accuracy of the Cartesian axes. This paper considers the geometric errors of some typical machine configurations with both Cartesian and non-Cartesian axes and uses case studies to emphasise the importance of measurement of all the error constituents. Furthermore, it shows the misrepresentation when modelling a five-axis machine as a three-plus-two error problem. A method by which the five-axis model can be analysed to better represent the machine performance is introduced. Consideration is also given for thermal and non-rigid influences on the machine volumetric accuracy analysis, both in terms of the uncertainty of the model and the uncertainty during the measurement. The magnitude of these errors can be unexpectedly high and needs to be carefully considered whenever testing volumetric accuracy, with additional tests being recommended

Multiple-sensor integration for efficient reverse engineering of geometry

This paper describes a multi-sensor measuring system for reverse engineering applications. A sphere-plate artefact is developed for data unification of the hybrid system. With the coordinate data acquired using the optical system, intelligent feature recognition and segmentation algorithms can be applied to extract the global surface information of the object. The coordinate measuring machine (CMM) is used to re-measure the geometric features with a small amount of sampling points and the obtained information can be subsequently used to compensate the point data patches which are measured by optical system. Then the optimized point data can be exploited for accurate reverse engineering of CAD model. The limitations of each measurement system are compensated by the other. Experimental results validate the accuracy and effectiveness of this data optimization approach

Investigation of a new method for improving image resolution for camera tracking applications

Camera based systems have been a preferred choice in many motion tracking applications due to the ease of installation and the ability to work in unprepared environments. The concept of these systems is based on extracting image information (colour and shape properties) to detect the object location. However, the resolution of the image and the camera field-of- view (FOV) are two main factors that can restrict the tracking applications for which these systems can be used. Resolution can be addressed partially by using higher resolution cameras but this may not always be possible or cost effective. This research paper investigates a new method utilising averaging of offset images to improve the effective resolution using a standard camera. The initial results show that the minimum detectable position change of a tracked object could be improved by up to 4 times

Efficient estimation by FEA of machine tool distortion due to environmental temperature perturbations

Machine tools are susceptible to exogenous influences, which mainly derive from varying environmental conditions such as the day and night or seasonal transitions during which large temperature swings can occur. Thermal gradients cause heat to flow through the machine structure and results in non-linear structural deformation whether the machine is in operation or in a static mode. These environmentally stimulated deformations combine with the effects of any internally generated heat and can result in significant error increase if a machine tool is operated for long term regimes. In most engineering industries, environmental testing is often avoided due to the associated extensive machine downtime required to map empirically the thermal relationship and the associated cost to production. This paper presents a novel offline thermal error modelling methodology using finite element analysis (FEA) which significantly reduces the machine downtime required to establish the thermal response. It also describes the strategies required to calibrate the model using efficient on-machine measurement strategies. The technique is to create an FEA model of the machine followed by the application of the proposed methodology in which initial thermal states of the real machine and the simulated machine model are matched. An added benefit is that the method determines the minimum experimental testing time required on a machine; production management is then fully informed of the cost-to-production of establishing this important accuracy parameter. The most significant contribution of this work is presented in a typical case study; thermal model calibration is reduced from a fortnight to a few hours. The validation work has been carried out over a period of over a year to establish robustness to overall seasonal changes and the distinctly different daily changes at varying times of year. Samples of this data are presented that show that the FEA-based method correlated well with the experimental results resulting in the residual errors of less than 12 μm

Towards obtaining robust boundary condition parameters to aid accuracy in FEA thermal error predictions

Finite Element Analysis (FEA) is used as a design tool within engineering industries due to the capability for rapid summative analysis accompanied by the visual aid. However, to represent realistic behaviour, FEA relies heavily on input parameters which must ideally be based on true figures such as data from experimental testing which sometimes requires time-consuming testing regimes. In the case of machine tool assemblies where complex structural joints and linkages are present, access to those areas can be a primary constraint to obtaining related boundary parameters such as heat flow across joints, for which, assumptions are incorporated to the FEA model which in effect increase the uncertainty in the FEA predictions. Similarly, in the case of thermal error modelling, simplifications are made when representing thermal boundary conditions such as the application of a uniform convection parameter to an assembly with parts assembled in both horizontal and vertical orientations. This research work aims to reduce the number of assumptions by providing experimentally obtained thermal boundary condition parameters. This work acknowledges experimental regimes that focus on obtaining thermal parameters related to the conduction across assembly joints (Thermal Contact Conductance-TCC) and measures the convection around areas such as belt drives and rotating parts to obtain convection parameters as inputs to the FEA. It provides TCC parameters for variable interfacial behaviour based on the varying contact pressure and the heat flow through dry and oiled contacts such as the conduction from spindle bearings to the surrounding housing and conduction from guideways into the associated assembly through carriages and contact bearings. It provides convection parameters across the test mandrel rotating at different speeds and around stationary structures such as convection parameters observed during TCC tests. It also provide details on the methods used to obtain all these parameters such as the use of thermal imaging, sensors placements and methods to obtain these boundary condition parameters. The significance of this work is to improve dramatically FEA thermal predictions, which are a critical part of engineering design. Although the focus is on machine tool design, the process and parameters can equally be applied to other areas of thermodynamic behaviour

FEA-based design study for optimising non-rigid error detection on machine tools

Non-rigid-body behaviour can have a considerable effect on the overall accuracy performance of machine tools. These errors originate from bending of the machine structure due to change in distribution of its own weight or from movement of the workpiece and fixture. These effects should be reduced by good mechanical design, but residual errors can still be problematic due to realistic material and cost limitations. One method of compensation is to measure the deformation directly with sensors embedded in a metrology frame. This paper presents an FEA-based design study which assesses finite stiffness effects in both the machine structure and its foundation to optimise the sensitivity of the frame to the resulting errors. The study results show how a reference artefact, optimised by the FEA study, can be used to detect the distortion

Glutathione conjugation of a cocaine pyrolysis product AEME and related compounds

Free-base ( crack ) cocaine smoking continues to be a popular method of drug abuse in the United States. Pyrolysis of free-base cocaine readily occurs, forming primarily benzoic acid and anhydroecgonine methyl ester (AEME), which structurally contains an alpha,beta-unsaturated carbonyl functional group. This electrophilic structure is suggestive of chemical reactivity with ubiquitously occurring cellular nucleophiles, such as glutathione. Glutathione adducts formed in vivo are metabolized to N-acetylcysteine derivatives by the enzymatically driven mercapturic acid pathway. AEME and its transesterification product in the presence of ethanol, anhydroecgonine ethyl ester (AEEE), were synthesized from cocaine hydrochloride. Glutathione (GSH) and mercapturic acid (NAC) conjugates of AEME were synthesized, and the stereochemistry of the AEME-NAC conjugate was structurally elucidated by NMR and mass spectrometry. Using Ellman\u27s method to monitor glutathione depletion over time, AEME reacted with GSH at a slower rate (6.8 x 10 -3 mM-1min-1) than equimolar concentrations of arecoline or ethacrynic acid (2.2 x 10-1 mM-1min -1 and 4.1 x 10-1 mM-1min-1, respectively). AEME also reduced the chemical formation of the DCNB-SG conjugate. In pooled human liver cytosol, the incubation of AEME with GSH at 37°C for 60 minutes at different pH levels was analyzed by LC-MS. We found unequivocal data suggesting enzymatic catalysis of AEME to AEME-SG in the presence of reduced GSH. However, AEME (10 mM) significantly reduced GST activity (p \u3c 0.005) following a 20 hour incubation with HLC (1 mg). Additionally, AEME exhibited mixed-linear inhibition (Ki = 334 muM) towards cytosolic GST activity. A solid phase extraction (SPE) method was developed to extract cocaine and cocaine metabolites from urine. We identified approximately 50 ng/mL of AEEE by LC-MS in the urine of a known freebase cocaine/ethanol abuser, and additionally confirmed the structure of the metabolite by LC-MS/MS. This SPE method is useful in detecting AEME conjugates in biological fluids. We studied the cytotoxicity of pyrolysis products on A549 lung fibroblasts using trypan blue exclusion and flow cytometry. To summarize, our work suggests that AEME plays a potential role in the sequelae of abused cocaine related toxicities

Iluzija nagiba mjeseca

The moon tilt illusion is the startling discrepancy between the direction of the light beam illuminating the moon and the direction of the sun. The illusion arises because the observer erroneously expects a light ray between sun and moon to appear as a line of constant slope according to the positions of the sun and the moon in the sky. This expectation does not correspond to the reality that observation by direct vision or a camera is according to perspective projection, for which the observed slope of a straight line in three-dimensional space changes according to the direction of observation. Comparing the observed and expected directions of incoming light at the moon, we derive a quantitative expression for the magnitude of the moon tilt illusion that can be applied to all configurations of sun and moon in the sky.Iluzija nagiba mjeseca zapanjujući je raskorak između svjetlosne zrake koja osvjetljava mjesec i smjera sunca. Ona se povećava, jer promatrač pogrešno očekuje da zraka svjetla između sunca i mjeseca bude pravac konstantnog koeficijenta smjera s obzirom na položaj sunca i mjeseca na nebu. Ovakvo očekivanje ne odgovara stvarnosti kod koje je promatranje s direktnom osi pogleda, ili s kamerom, u skladu s perspektivom (centralnim projiciranjem) za koju se promatrani koeficijent smjera pravca u trodimenzionalnom prostoru mijenja s obzirom na os pogleda. Uspoređujući promatrane i očekivane smjerove zrake usmjerene na mjesec, izvodimo kvantitativan izraz za veličinu iluzije nagiba mjeseca koji se može primijeniti na sve položaje sunca i mjeseca na nebu

Evaluation of measurement technique for a precision aspheric artefact using a nano-measuring machine

A precision aspheric artefact is measured in 3D by a commercially available nano-measuring machine (NMM) integrated with a contact inductive sensor as the probe. The mathematics of 3D compensation of the error caused by the probe radius is derived. The influence of the probe radius measurement uncertainty on the compensation errors for the 3D measurements is discussed. If the calibration uncertainty of probe radius is 1m and 0.1 m respectively, the compensation errors for a paraboloid artefact are within 100 nm and 10 nm respectively, and the artefact measurement uncertainties are 103 nm and 26 nm respectively. The artefact calibration uncertainty depends more on the uncertainty of the probe radius calibration than the probe radius

Five-Axis Machine Tool Condition Monitoring Using dSPACE Real-Time System

This paper presents the design, development and SIMULINK implementation of the lumped parameter model of C-axis drive from GEISS five-axis CNC machine tool. The simulated results compare well with the experimental data measured from the actual machine. Also the paper describes the steps for data acquisition using ControlDesk and hardware-in-the-loop implementation of the drive models in dSPACE real-time system. The main components of the HIL system are: the drive model simulation and input – output (I/O) modules for receiving the real controller outputs. The paper explains how the experimental data obtained from the data acquisition process using dSPACE real-time system can be used for the development of machine tool diagnosis and prognosis systems that facilitate the improvement of maintenance activities