Coordinate Measuring Machine Measurements Research Articles

Influence of surface roughness on X-ray computed tomography dimensional measurements of additive manufactured parts

In many industrial applications, components characterized by high surface roughness are measured by X-ray computed tomography (CT). This is the case, for example, of additive manufactured parts. Surface roughness has a strong influence on CT dimensional measurements, causing relevant measurement deviations with respect to tactile reference measurements by coordinate measuring machines (CMMs), especially for parts characterized by high surface roughness. It comes that roughness effects on CT dimensional measurements must be quantified.In this work, the influence of surface roughness on CT dimensional measurements, and the relation between tactile CMM and CT measurements are studied. Effects of larger as well as smaller surface roughness are taken into account, by means of three different additive manufactured samples characterized by different roughness. Experimental results prove the presence of a systematic error between tactile and CT measurements; the relation between this error and the Rz roughness parameter of the surface is analyzed.

Knowledge Capture in CMM Inspection Planning: Barriers and Challenges

Coordinate Measuring Machines (CMM) have been widely used as a means of evaluating product quality and controlling quality manufacturing processes. Many techniques have been developed to facilitate the generation of CMM measurement plans. However, there are major gaps in the understanding of planning such strategies. This significant lack of explicitly available knowledge on how experts prepare plans and carry out measurements slows down the planning process, leading to the repetitive reinvention of new plans while preventing the automation or even semi-automation of the process. The objectives of this paper are twofold: (i) to provide a review of the existing inspection planning systems and discuss the barriers and challenges, especially from the aspect of knowledge capture and formalization; and (ii) to propose and demonstrate a novel digital engineering mixed reality paradigm which has the potential to facilitate the rapid capture of implicit inspection knowledge and explicitly represent this in a formalized way. An outline and the results of the development of an early stage prototype - which will form the foundation of a more complex system to address the aforementioned technological challenges identified in the literature survey - will be given.

A Statistical Approach To Prediction Of The CMM Drift Behaviour Using A Calibrated Mechanical Artefact

Abstract This paper presents a multivariate regression predictive model of drift on the Coordinate Measuring Machine (CMM) behaviour. Evaluation tests on a CMM with a multi-step gauge were carried out following an extended version of an ISO evaluation procedure with a periodicity of at least once a week and during more than five months. This test procedure consists in measuring the gauge for several range volumes, spatial locations, distances and repetitions. The procedure, environment conditions and even the gauge have been kept invariables, so a massive measurement dataset was collected over time under high repeatability conditions. A multivariate regression analysis has revealed the main parameters that could affect the CMM behaviour, and then detected a trend on the CMM performance drift. A performance model that considers both the size of the measured dimension and the elapsed time since the last CMM calibration has been developed. This model can predict the CMM performance and measurement reliability over time and also can estimate an optimized period between calibrations for a specific measurement length or accuracy level.

Metrological Overview for Coordinate Measuring Machines

In manufacturing system, 3D Coordinate Measuring Machines (CMM) are widely used for inspection services where the product should be subjected to the verification of design specifications. It is usually necessary to know the accuracy of 3D CMM equipment during the verification process of the product quality in mass production. To insure and increase the accuracy of 3D CMM the international series standards ISO 10360 are used. This paper presents the importance of the establishment of traceability to international standards as a key role for the reliability of 3D CMM measurement results.

Laser Tracker Kinematic Error Model Formulation and Subsequent Verification under Real Working Conditions

A kinematic model of the Laser Tracker (LT) based on the Denavit-Hartenberg method has been developed. In this model, error matrices have been included with error parameters for linear and rotary joints. The calibration method is based on the measurement of a mesh of reflectors measured by a LT from different positions. Error parameters are calculated knowing that distances between every pair of reflectors is the same regardless the LT position. The absence of nominal data prevents us from knowing the calibration behaviour and its suitability. Although synthetic data tests show a good accuracy improvement, it is not possible to know if this will work with under real working conditions. An experiment has been made to check the calibration procedure. A set of 17 reflectors have been placed on a Coordinate Measuring Machine (CMM) and have been measured by a LT from 5 different positions. Reflector positions have also been measured with the CMM to calculate the initial errors. With LT measurements we calculate the error parameters. LT measurements are recalculated considering the kinematic error model and compared with the CMM measurements to get the residual error. Two errors have been calculated; distances error between reflectors and their position error compared with CMM data.

CNC-CMM Measurement Accuracy and Accompanied Uncertainty at Different Alignment Positions of Long GBs

Coordinate Measuring Machine (CMM) is the most important computer numerical control (CNC) system in dimensional metrology. CNC-CMM is a major metrological tool used for guarantee the quality of engineering products due to its accuracy, precision, speed and general versatility in industry. The alignment position strategy of an object or product on CMM affects the measurement results. In this paper, the verification of CMM working performance with a touch trigger probe is presented. The influence of alignment of long gauge blocks (GBs) artifacts and its positions on CMM measurements have been studied and evaluated in the X-Y plane. This method treats CMM axis length errors of GBs measurements resulted due to their alignment at different angles. Parallelism and GD&T of GBs artifacts are also identified and evaluated. The combined standards uncertainty in measurement is estimated and achieved within the CMM maximum permissible error.

In vivo and ex vivo measurement of polyethylene wear in total hip arthroplasty

Background — Determination of the amount of wear in a polyethylene liner following total hip arthroplasty (THA) is important for both the clinical care of individual patients and the development of new types of liners.Patients and methods — We measured in vivo wear of the polyethylene liner using computed tomography (CT) (obtained in the course of regular clinical care) and compared it to coordinate-measuring machine (CMM) readings. Also, changes in liner thickness of the same retrieved polyethylene liner were measured using a micrometer, and were compared to CT and CMM measurements. The distance between the centers of the acetabular cup and femoral head component was measured in 3D CT, using a semi-automatic analysis method. CMM readings were performed on each acetabular liner and data were analyzed using 3D computer-aided design software. Micrometer readings compared the thickest and thinnest regions of the liner. We analyzed 10 THA CTs and retrievals that met minimal requirements for CT slice thickness and explanted cup condition.Results — For the 10 cups, the mean difference between the CT readings and the CMM readings was -0.09 (–0.38 to 0.20) mm. This difference was not statistically significant (p = 0.6). Between CT and micrometer, the mean difference was 0.11 (–0.33 to 0.55) mm. This difference was not statistically significant (p = 0.6).Interpretation — Our results show that CT imaging is ready to be used as a tool in clinical wear measurement of polyethylene liners used in THA.

Wear Measurement and Assessment of Explanted Cross-Linked PE Acetabular Cups Using a CMM

Wear has been considered the main limiting factor in the longevity of hip replacements. Wear analysis is thus essential for determining wear-related failure mechanisms and prediction of wear, which will consequently enable biomedical engineers to improve the design, material, and service life of the bearing components. This article presents wear measurement and assessment of the explanted conventional cross-linked polyethylene (XPE) and second-generation highly cross-linked polyethylene cups (X3) using a coordinate measuring machine (CMM). An expanded uncertainty analysis was performed to assess the performance of wear measurement. Wear measurement using the CMM method was validated with the gravimetric technique. The normalized error between volumetric wear measurement of the CMM method and that of the gravimetric technique was estimated to be always less than 1, suggesting that the CMM method applied to explanted hip wear measurements under the specific conditions was accurate and reliable. The approach to CMM measurement with uncertainty analysis was shown not only to locate 3D wear scar and wear direction but also to accurately quantify linear and volumetric wear with a maximum volumetric uncertainty of ±3.15 mm3 (95% confidence level). It is shown that identifying the key uncertainty components involved in the measurement process including validation, which contributes to an overall expanded uncertainty budget, is crucial to improve the confidence and the reliability of hip wear measurement results using a CMM.

Uncertainty of pin height measurement for the determination of wear in pin-on-plate test

The paper concerns measurement of pin height for the determination of wear in a pin-on-plate (POP) or pin-on-disc (POD) test, where a pin is mounted on a holder that can be fixed on the test rig and removed for measurements. The amount of wear is assessed as difference of pin height before and after the test, using the distance between holder plane and pin friction plane as measurand. A series of measurements were performed in connection with POP testing of different friction material pins mounted on an aluminium holder. Pin height measurements were carried out on a coordinate measuring machine (CMM), achieving an expanded measurement uncertainty (k=2) better than 1μm. A simple dedicated fixture adaptable to workshop environment was developed and its metrological capability investigated, estimating an average uncertainty of measurement in the order of 5μm (k=2). Fixture measurements were compared with CMM measurements using the normalised En value. Acceptable En values (En<1) were obtained in 58% of the measurements, showing a discrepancy which was explained in terms of different probing patterns on CMM and fixture.

Measurement of Radius of Curvature by Coordinate Measurement Machine

Radius of curvature is one of the key parameters of optical components. A variety of methods have been developed for this measurement. This paper describes a novel way, named “CMM spherometry by probe compensation”, to measure the radius of curvature of optical surfaces by coordinate measuring machine (CMM). The measurement combines CMM measurement and iteration calculation scheme is presented. The measurement results of CMM and the trace samples are compared. The effect of temperature compensation is discussed.

Measuring the profile of a convex aspherical surface by solving a bi-objective optimization problem

This paper describes a method based on a bi-objective evolu- tionary algorithm to obtain the profile of a convex aspherical surface, which is defined by a set of synthetic points placed on an xyzcoordinate system. The set of points to be analyzed is constructed considering the sources of measurement error in a coordinate measuring machine (CMM), such as machine, probe, and positioning errors. The proposed method is applied to solve a bi-objective optimization problem by minimizing two objective functions. By minimizing the first objective function the positioning error is removed from the coordinates of each affected point. Once the first goal is achieved, the second objective function is minimized to determine from the resulting data all parameters related to the test surface, such as paraxial radius of curvature, the conic constant and the deformation con- stants. Hence, this method can obtain the correct surface profile even when the positioning error tends to increase the CMM measurement error in the set of analyzed points. The bi-objective evolutionary algorithm (BEA) was tested against a single-objective evolutionary algorithm, and illustrative numerical examples demonstrate that the BEA performs better. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). (DOI: 10.1117/1.OE.51.7 .073607)

Measuring external profiles of porous objects using CMM

Coordinate measuring machine (CMM) has been extensively used in surface measurement and inspection. CMM produces more accurate and reliable results compared with non-contact measurement devices, since CMM measures target surfaces in a tactile way and is not affected by surface reflection quality. However, when there is porosity on the outside surfaces, CMM measurement will have errors because part of the probing stylus will come into the porosity spaces. This paper presents a method of using CMM to measure external profiles of objects with external porosity spaces. The center of a CMM probing stylus will be compensated to avoid porosity spaces and located above external surface areas along surface normal directions. Such a probing strategy can be implemented in both computer-aided design (CAD)-guided mode and non-CAD-guided mode. When the CAD model is available, the probing styli are guided to approach the surface along its normal directions successively to identify and avoid porosity spaces. When the CAD model is unavailable, surface normal directions will be estimated first and then adjusted. The presented method is able to avoid porosity spaces in CMM measurement regardless of the availability of CAD models.

Three-dimensional modeling of coordinate measuring machines probing accuracy and settings using fuzzy knowledge bases: Application to TP6 and TP200 triggering probes

AbstractOne of the fundamental elements that determines the precision of coordinate measuring machines (CMMs) is the probe, which locates measuring points within measurement volume. In this paper genetically generated fuzzy knowledge based models of three-dimensional (3-D) probing accuracy for one- and two-stage touch trigger probes are proposed. The fuzzy models are automatically generated using a dedicated genetic algorithm developed by the authors. The algorithm uses hybrid coding, binary for the rule base and real for the database. This hybrid coding, used with a set of specialized operators of reproduction, proved to be an effective learning environment in this case. Data collection of the measured objects' coordinates was carried out using a special setup for probe testing. The authors used a novel method that applies a low-force high-resolution displacement transducer for probe error examination in 3-D space outside the CMM measurement. The genetically generated fuzzy models are constructed for both one stage (TP6) and two stage (TP200) types of probes. First, the optimal number of settings is defined using an analysis of the influence of fuzzy rules on TP6 accuracy. Then, once the number of settings is obtained, near optimal fuzzy knowledge bases are generated for both TP6 and TP200 triggering probes, followed by analysis of the finalized fuzzy rules bases for knowledge extraction about the relationships between physical setup values and error levels of the probes. The number of fuzzy sets on each premise leads to the number of physical setups needed to get satisfactory error profiles, whereas the fuzzy rules base adds to the knowledge linking the design experiment parameters to the pretravel error of CMM machines. Satisfactory fuzzy logic equivalents of the 3-D error profiles were obtained for both TP6 and TP200 with root mean squsre errors ranging from 0.00 mm to a maximum of 0.58 mm.

Dimensional metrology of bipolar fuel cell plates using laser spot triangulation probes

As in any engineering component, manufacturing a bipolar fuel cell plate for a polymer electrolyte membrane (PEM) hydrogen fuel cell power stack to within its stated design tolerances is critical in achieving the intended function. In a bipolar fuel cell plate, the dimensional features of interest include channel width, channel height, channel parallelism, side wall taper, straightness of the bottom or side walls, plate parallelism, etc. Such measurements can be performed on coordinate measuring machines (CMMs) with micro-probes that can access the narrow channels. While CMM measurements provide high accuracy (less than 1 µm), they are often very slow (taking several hours to measure a single plate) and unsuitable for the manufacturing environment. In this context, we describe a system for rapid dimensional measurement of bipolar fuel cell plates using two laser spot triangulation probes that can achieve comparable accuracies to those of a touch probe CMM, while offering manufacturers the possibility for 100% part inspection. We discuss the design of the system, present our approach to calibrating system parameters, present validation data, compare bipolar fuel cell plate measurement results with those obtained using a Mitutoyo UMAP (see footnote 1) fiber probe CMM, and finally describe the uncertainty in channel height and width measurements.

Influence of feature form deviations on CMM measurement uncertainties

Dimensional quality inspections of high precision parts are often performed by coordinate measuring machines (CMMs) equipped with touch-trigger probes. Features of these parts are then measured with a limited set of points, due to time constraints. This limited sampling has an important influence on the measurement uncertainty because the form deviation of the feature can not be completely assessed. It will not only affect the measurement uncertainty of the form deviation, but also the measurement uncertainties of other feature parameters like size, position and orientation. The quantification of this influence is difficult since the true form deviation is unknown. This paper describes a method, based on Monte Carlo simulations, that allows to determine these measurement uncertainties based on an estimate of the form deviation using a limited set of points. The method is illustrated for circles with theoretical shapes of form deviations, as well as measured shapes.

Dimensional characterization of a quasispherical resonator by microwave and coordinate measurement techniques

We describe the dimensional characterization of copper quasisphere NPL-Cranfield 2. The quasisphere is assembled from two hemispheres such that the internal shape is a triaxial ellipsoid, the major axes of which have nominal radii 62.000 mm, 62.031 mm and 62.062 mm. The artefact has been manufactured using diamond-turning technology and shows a deviation from design form of less than ±1 µm over most of its surface. Our characterization involves both coordinate measuring machine (CMM) experiments and microwave resonance spectroscopy.We have sought to reduce the dimensional uncertainty below the maximum permissible error of the CMM by comparative measurements with silicon and Zerodur spheres of known volume. Using this technique we determined the equivalent radius with an uncertainty of u(k = 1) = 114 nm, a fractional uncertainty of 1.8 parts in 106. Due to anisotropy of the probe response, we could only determine the eccentricities of the quasihemispheres with a fractional uncertainty of approximately 2%.Our microwave characterization uses the TM11 to TM18 resonances. We find the equivalent radius inferred from analysis of these modes to be consistent within ±4 nm with an overall uncertainty u(k = 1) = 11 nm. We discuss corrections for surface conductivity, waveguide perturbations and dielectric surface layers.We find that the CMM radius estimates derived from each hemisphere cannot be used to accurately predict the equivalent radius of the assembled resonator for two reasons. Firstly, the equatorial flanges are flat only to within ±1 µm, leading to an equatorial ‘gap’ whose dimension cannot be reliably estimated. Secondly, the resonator undergoes significant elastic distortion when the bolts connecting the hemispheres are tightened. We provide CMM and microwave measurements to support these conclusions in addition to finite-element modelling.Finally, we consider the implications of this work on a forthcoming experiment to determine the Boltzmann constant with a relative uncertainty below 1 part in 106.

Achieving Traceability of Industrial Computed Tomography

Achieving traceability is crucial for complex measurement techniques, especially for coordinate measuring machines (CMMs). For CMMs using tactile probes, traceability can for certain measurements be achieved using model-based uncertainty budgets. Up to now, uncertainty simula-tions could be used applicable only for tactile CMM measurements of regular geometries, but are available as an add-on for different CMMs. This procedure is accepted by guidelines and inter-national standards (VDI/VDE 2617-7, supplement 1 [1] to GUM). Furthermore, empirical ap-proaches to assess the measurement uncertainty by means of calibrated workpieces or prior know-ledge exist or are under development. These approaches can as a matter of principle also be used for CMMs featuring computed tomography (CT). In this paper, the empirical assessment of the mea-surement uncertainty of the upcoming measurement technology CT [2, 3] will be discussed uniting the present approaches and the current knowledge, with the focus being on the applicability of con-cepts for users in industry. For this purpose, the influences on dimensional CT measurements are analyzed and evaluated, taking the measurement data of a current industrial micro CT system as a basis.

Influence of eccentricity and axis tilt on circularity and cylindricity assessments

Circular and cylindrical features are commonly present in the engineering parts. After manufacturing, measurement of these features can be done using either form measuring instruments or coordinate measuring machines (CMM) and form error value is evaluated with reference to a suitable feature. Misalignment between instrument centre/axis and the actual centre/axis of the feature influences the form error values obtained. Though analysis of form measurement using a truly circular part is reported in literature, an attempt is made in this paper to develop mathematical equations for carrying out simulation on truly cylindrical parts with misalignments and to select suitable reference features for form and CMM measurements. It is established that form measurement requires limacon and limacon-cylinder references, while circle and circular-cylinder references are suitable for CMM measurement for circularity and cylindricity evaluation respectively. Wider ranges of misalignments are considered to bring out the robustness of the references. The effect of misalignment is particularly important, when the tolerances are tighter as in precision manufacturing and metrology.

Experimental evaluation of software estimates of task specific measurement uncertainty for CMMs

Estimation of the uncertainty of Coordinate Measuring Machine (CMM) measurements for real, imperfect parts is a very complex undertaking. Not only are there many contributors to the uncertainty that may interact in a non-linear fashion, making it difficult to mathematically determine an uncertainty estimate, but it is also difficult to predict the long-term variation of these parameters. Our work seeks to provide experimental validation of the uncertainties predicted by two different commercial software packages that purport to predict the task-specific measuring uncertainty of CMM measurement results. The validation procedure uses repeated measurements of calibrated artifacts to experimentally determine measurement uncertainties. These measurements can then be simulated in the commercial software packages. The comparison will allow the software to be tested to see if it appropriately accounts for the influences of the operator, environment, and part placement. This paper reports the results of actual part measurements and the predicted uncertainty provided by commercial simulation packages. Differences between experimental and simulated uncertainties are highlighted, and their causes examined.

Uncertainty due to experimental conditions in co-ordinate measuring machines

Any measurement method of a physical quantity cannot provide an exact unequivocal result owing to the infinite amount of information necessary to characterize fully both the physical quantity to be measured and the measuring process. Therefore, a quantitative indication of the quality of a measurement result needs to be given to enable its reliable use. Uncertainty is one such indication. Provision of incorrect uncertainty statements for measurements performed by a co-ordinate measuring machine (CMM) may lead to very serious economic implications. In this study, the uncertainty of CMM measurements is estimated by a single parameter accounting for both systematic and random errors. The effects that environmental conditions (temperature), discretionary set-up parameters (probe extension, stylus length), and measuring plan decisions (number of points) have on uncertainty of measurements are then investigated. Interactions between such factors are also shown to be significant.