G. Valiño

Automatic determination of bending sequences for sheet metal parts with parallel bends

A method for solving the problem of bend sequencing in sheet metal manufacturing is presented. The algorithm developed divides the part into basic shapes (channels and spirals) and determines the partial sequences associated with them. The complete bending sequences associated with the complete part were obtained from the combination of these partial sequences. To make this combination possible, several strategies were defined to reduce the number of solutions and, therefore, the searching time. All sequences (partial and complete) were checked considering possible part-tool collisions and tolerance constraints. The last stage was to order the sequences by taking into account the total process time. To attain the required process time accuracy, a robot was used for the handling operations. Finally, the sequence associated with the lower process time was selected as the optimal solution.

Influence of Surface Position along the Working Range of Conoscopic Holography Sensors on Dimensional Verification of AISI 316 Wire EDM Machined Surfaces

Conoscopic holography (CH) is a non-contact interferometric technique used for surface digitization which presents several advantages over other optical techniques such as laser triangulation. Among others, the ability for the reconstruction of high-sloped surfaces stands out, and so does its lower dependence on surface optical properties. Nevertheless, similarly to other optical systems, adjustment of CH sensors requires an adequate selection of configuration parameters for ensuring a high quality surface digitizing. This should be done on a surface located as close as possible to the stand-off distance by tuning frequency (F) and power (P) until the quality indicators Signal-to-Noise Ratio (SNR) and signal envelope (Total) meet proper values. However, not all the points of an actual surface are located at the stand-off distance, but they could be located throughout the whole working range (WR). Thus, the quality of a digitized surface may not be uniform. The present work analyses how the quality of a reconstructed surface is affected by its relative position within the WR under different combinations of the parameters F and P. Experiments have been conducted on AISI 316 wire EDM machined flat surfaces. The number of high-quality points digitized as well as distance measurements between different surfaces throughout the WR allowed for comparing the metrological behaviour of the CH sensor with respect to a touch probe (TP) on a CMM.

Parametric and associative design of cartridges for special tools

Presents work on the design of special rotary tools using CAD systems. Special tools are used for the simultaneous machining of several parts in the same workpiece, with only one feed motion. The reduction in the number of tool-changes is the main advantage using them. Special rotary tools have a modular feature, and the cutting edges are assembled onto small holders called cartridges. Taking into account the importance of defining the cartridge geometry, it seems to be necessary to point out all the steps followed in the design process as well as those concerning their suitable location and orientation. On the one hand, one needs to make a parametric design of the ISO standard cartridges so that all the sizes and types of cartridges can be obtained from a few basic models. Each cartridge is built joining several solids together: an insert, the cartridge body and the axial adjusting screw. On the other hand, it is necessary to have in mind the location and orientation of the cartridge. With this aim, the cartridge geometry is attached to three points, two of these represent the tool axis direction and the third one is a point on the work surface profile. These three points define the tool reference plane from which all the whole cartridge geometry is generated.

Methodology for set-up planning automation of turned parts

This paper presents an automatic set-up planning module integrated in a CAPP system for rotational parts to be machined on a lathe. The developed system determines the possible set-up combinations that are necessary for a complete machining of the part as well as the order of each set-up and the surfaces to be used for clamping the part. The applied methodology takes into consideration constraints such as the geometry of both the stock and the final part, the geometry and the capacity of the chuck, and the part tolerances. In general, these constraints allow the system to obtain several valid solutions for clamping the part. Some criteria based on the clamping force and the value of tolerances have been considered in order to establish a preference order among these solutions. Finally, the analysis of linked tolerances and the tool approach direction to each surface determine the sets of surfaces to be machined within each set-up. An example part is used in the course of the paper to illustrate conveniently the methodology, and two additional case studies prove that this methodology is adequate for the solution of real cases.

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 star probes in automatic inspection of rotational parts

This article describes a method of accessibility analysis based on the geometrical study of all possible interferences between the probe and the part to be inspected. The parts considered in the analysis are rotational parts and their recognition is automatically made from a feature-based design. The probe considered is a star type, defined by adjustable parameters that allow for the adaptation to different types and sizes. The system analyses the possible collision with the machine column, touch probe and probe stylus. Apart from determining the conditions of collision, the system also calculates any collision-free positions on each surface to be probed. These positions can be used to obtain inspection paths for communication to a CMM or to a global inspection planning system. The collision conditions calculated have been checked and validated using different rotational parts.

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.

Tool compensation by means of touch trigger probes in CNC turning

The current demand for high quality products has lead to the necessity of a total dimensional control of workpieces produced in order to avoid dimensional errors. Inspection techniques were developed and progressively integrated into machine tools leading to the on-machine-measuring (OMM) concept. Among other methods, touch-trigger-probes (TTPs) inspired on CMMs were adopted but their actual on-machine accuracy is not reported by manufacturers. Therefore, this work describes the main applications and characteristics of two types of TTPs for CNC lathes or turning centers and analyzes their utilization for presetting of tools.

Constraints Evaluation and Working Methodology for Laser Scanning of Free-Form Surfaces

This research presents a working methodology for developing an automatic planning system of the scanning process of free-form surfaces. The surface has been modelled using a STL format, that permits the automatic recognizing of any type of surface. This work does only consider collision-free orientations that guarantee the visibility of the zone to scan and that are compatible with the constraints imposed by the process parameters. In order to speed up the calculation of these orientations, different methods like back-face culling and space partitioning techniques, such as kd-trees, are applied. Once the space occupied by the part is partitioned in regions, recursive ray traversal algorithms are used in order to check for intersection exclusively the part triangles (STL) that can potentially be traversed by each laser beam direction

Influence of scanning strategies on measuring and surface digitising by means of a conoscopic holography sensor integrated in a machining centre

A conoscopic holography sensor (CH) integrated in a machining centre was used to analyse how the scanning strategy affects distance measurements between planes and to the reconstruction of a digitised surface. Distance measurement tests were conducted on a stepped specimen whereas the surface reconstruction was studied on a cylindrical surface. Two different scanning strategies were compared. In the first, the height position of the sensor kept constant along the surface scanning so that all the points were located at different positions within the sensor depth of field (DOF). In the second strategy, the sensor was continuously adjusted describing a trajectory equidistant to the surface so that all the points were acquired at a same distance within the DOF. Both strategies were compared by the discrepancies between measurements taken by the CH sensor and those obtained by a touch probe. Some recommendations are provided for distance measurement and surface reconstruction.