Braulio José Álvarez

A new concept of feature-based gauge for coordinate measuring arm evaluation

Articulated arm coordinate measuring machines (AACMM or CMA) have conquered a market share in the actual dimensional metrology field, overall when their role implies the inspection of geometrical and dimensional tolerances in an accurate 3D environment for medium-size parts. However, the unavoidable fact of AACMM manual operation constrains its reliability to a great extent, avoiding rigorous evaluation and casting doubt upon the usefulness of external calibration. In this research, a dimensional gauge especially aimed at AACMM evaluation has been developed. Furthermore, the operator skill will be revealed through the use of this gauge. A set of geometrical features, some of them oriented to evaluate the operator and others the equipment, have been collected for the gauge. The proposed evaluation methodology clearly distinguishes between dimensional and geometrical tolerances (with or without datum references), whereas actual verification standards only consider the former. Next, quality indicators deduced from the measurement results are proposed in order to compare AACMM versus coordinate measuring machine (CMM) performance, assuming that CMM possess the maximum accuracy that AACMM could reach, because CMM combines maximum contact accuracy with minimum operator influence. As a result, AACMM evaluation time could be significantly reduced since this gauge allows us to perform a customized evaluation of only those specific tolerances of interest to the user.

Feasibility Evaluation of Photogrammetry versus Coordinate Measuring Arms for the Assembly of Welded Structures

The present paper studies the technical feasibility of Photogrammetry and Coordinate Measuring Arms (AACMM or CMA) for the geometrical inspection of welded pins used for the assembly of medium-sized sheetmetal structures. As a result of this evaluation it was found that the deviations (measurement recorded by the device minus the nominal value of the measurement) obtained by means of Photogrammetry and the CMA were equivalent. Moreover, the number of measurements found to be out of tolerance by both methods was statistically equivalent. According to our results both methods are feasible for performing the dimensional inspection of the position of the welded cylinders of the structure as they are able to distinguish measurements inside and outside of tolerance and are equally capable of detecting the data variance.

Comparison between a Laser Micrometer and a Touch Trigger Probe for Workpiece Measurement on a CNC Lathe

The current requirements for an efficient dimensional inspection of manufactured parts have lead to development of different in process and on-machine measurement (OMM) techniques. Touch trigger probes (TTP) are the most common technologies utilized, inspired on contact probes used on coordinate measuring machines (CMMs). The on-machine accuracy of TTPs depends upon precision of the tool-machine control as well as upon the procedure for TTP presetting. Taking this into account, a different OMM technique is considered in this work, which consists on a laser micrometer (LM) that is commonly used for in-process measurement of continuous products. The behaviour of TTP and LM is analysed and discussed in terms of repeatability and reproducibility. Results obtained by both techniques are compared each other by measuring a cylindrical workpiece and by checking the results with those obtained on a CMM.

Accessibility analysis for automatic inspection in CMMs by using bounding volume hierarchies

Accessibility analysis represents one of the most critical tasks in inspection planning. This analysis determines those probe orientations that can touch an inspection point without collision. This paper presents a methodology based on part discretization and the application of space partitioning techniques (kd-tree) in order to reduce the number of intersection tests between probe and part. A STL model has been used for discretizing the inspection part in a set of triangles, which permits the application of the developed system to any type of part, regardless of its shape and its complexity. Likewise, a recursive ray traversal algorithm has been used in order to speed up the traversal of the kd-tree hierarchical structure and to calculate exclusively the intersection of each probe orientation with those part triangles that can potentially interfere with it. In a further step of the analysis, the real geometry of the probe has been considered. Hence, a simplified model has been developed for each probe component (column, head, touch probe, stylus and tip) using different basic geometrical shapes. Finally, collision-free probe orientations are clustered for minimizing the orientation changes during the inspection process. Furthermore, the applied algorithm allows for determining different valid combinations of clusters. The developed system was applied to two example parts in order to prove that this methodology is adequate for the solution of real cases.

Testing coordinate measuring arms with a geometric feature-based gauge: in situ field trials

This work describes in detail the definition of a procedure for calibrating and evaluating coordinate measuring arms (AACMMs or CMAs). CMAs are portable coordinate measuring machines that have been widely accepted in industry despite their sensitivity to the skill and experience of the operator in charge of the inspection task. The procedure proposed here is based on the use of a dimensional gauge that incorporates multiple geometric features, specifically designed for evaluating the measuring technique when CMAs are used, at company facilities (workshops or laboratories) and by the usual operators who handle these devices in their daily work. After establishing the procedure and manufacturing the feature-based gauge, the research project was complemented with diverse in situ field tests performed with the collaboration of companies that use these devices in their inspection tasks. Some of the results are presented here, not only comparing different operators but also comparing different companies. The knowledge extracted from these experiments has allowed the procedure to be validated, the defects of the methodologies currently used for in situ inspections to be detected, and substantial improvements for increasing the reliability of these portable instruments to be proposed.

Implementation of decision rules for CMM sampling in a KBE system

This work presents a methodology for setting the number and spatial distribution of inspection points in the automatic inspection of parts using Coordinate Measuring Machines. This methodology collects the existing knowledge about sampling rules in automatic inspection. The final purpose is to store this knowledge in a KBE platform oriented to the inspection planning process and based on the MOKA methodology.

Influencia del Acabado Superficial en el Digitalizado con Sensores de Triangulación por Láser

The present work deals with the measurement of the influence that surface finishing has on the accuracy and quality of the scanning performed using laser triangulation sensors. Amongst the great number of parameters that influence the scanning quality, the effect of roughness is analysed. To achieve this, a procedure for constraining the variables while providing an excellent repeatability and reproducibility was proposed. This procedure has been applied to several manufacturing processes that produce parts with very different quality of surface finishing. As a result, certain correlations have been extracted that quantify the influence of the roughness on the quality of the cloud-point, the spatial distribution of the cloud-point dispersion and the optimal laser intensity for each manufacturing process.

Development of a Behaviour Curve for Quality Evaluation with Optoelectronic Profilometers

This work shows an experimental procedure aimed to generate a graph with the optimal roughness parameters in order to obtain the best roughness measurements of an optoelectronic profilometer. The optimal parameters have been determined taking into account the grade of agreement between the optical roughness values and the equivalent values of traditional contact devices. The working parameters of the optoelectronic profilometer are based on computational filters which are controlled by software working with a 3D stratified colour map (chromatic fragmentation of the white light). However, these parameters substantially differ from the usual contact profilometers that work with 2D roughness profiles (cut-off, evaluation length and contact stylus radii). This work pursues to find the optical profilometer parameters, and its values, that ensure the best quality measurement for a wide range of machining process and testing several ISO roughness intervals.

Models for stiffness characterization of the spindle-chuck system in a CNC lathe for prediction of deflections in CAPP

Precision machining of components with tight tolerances requires not only to do a fine presetting of tools, but also to carry out on-line tool-path corrections in order to compensate the form deviations that arise in the workpiece derived from the action of cutting forces. Advanced and expensive technology, such as machine-tools equipped with multiple sensors, are commonly utilized for this purpose. However, if an early prediction of deviations were analyzed, it would be possible to take them into account in planning stages so that necessity for on-line compensations would be minimized. With this purpose, two math models based on strain energy analysis and a FEM model are proposed in this work and compared each other and also with respect to experimental machining tests, showing satisfactory coincidence of results.

Conceptual principles and ontology for a KBE implementation in inspection planning

The inspection process planning with coordinate measuring machines involves repetitive and well-known decisions to make about the different strategies to use, although much of this knowledge is today implicit in the expert mind. Therefore, the inspection planning is a good activity for implementing knowledge-based engineering (KBE) systems. However, the origin of KBE is in the design activity and, traditionally, the different methodologies have been applied to that activity. In this paper, the focus is the application of a known methodology traditionally used in the design process to the inspection process with coordinate measuring machines. The paper considers the knowledge elicitation phase, that is, the knowledge identification before its formalisation and implementation in a platform. The identification of knowledge is done in a high-abstraction level using a combination of IDEF0 diagrams and a text analysis application. The knowledge extracted was represented in a first approximation by means of a modified ontology to adapt the original MOKA ontology to the inspection process.