Geometric dimensioning, tolerancing and inspection

The introduction of deterministic NC grinding and polishing operations, in the manufacturing of free-form glass components for precision optics, requires the characterization of surface topography evolution as a function of process parameters. In this work, a model based on Reye's wear hypothesis is proposed for the assessment of surface roughness prediction as a function of operating parameters, in the deterministic polishing process of glass moulds. According to Reye's hypothesis, the removed material per unit area is proportional to the work due to the friction force: the removed material per unit area can be computed by adequately integrating the areal material ratio function (Abbott-Firestone curve) of the surface and can be associated with the amplitude roughness parameter; the work due to the friction force per unit area is proportional to the integral of the product of pressure and velocity in the time interval and can be derived from the process parameters by means of the Hertz theory. The model assessment was performed on ground glass flat samples polished with different operating parameters, mapping the surface roughness using an atomic force microscope (AFM). The developed model shows a satisfactory estimate of surface roughness evolution during the polishing process and confirms the experimental results found in the literature for the Preston coefficient. © 2008 Elsevier Ltd. All rights reserved.

Keywords: Abbott-Firestone , Glass , Material ratio , Polishing , Roughness , Surface texture

Incremental Forming is a flexible and innovative sheet metal forming process able to form complex shapes without the need of any expensive die. In this way, expensive fixtures are avoided obtaining a cheaper production, more advantageous for small production batches. Anyway, more than the process slowness, the geometrical accuracy represents the most important drawback today. In particular, two kinds of geometrical errors can be observed on a sheet component incrementally formed: the presence of elastic springback that modifies the imposed final depth, that "moves away" from the designed one, and the undesired bending effect of the sheet, which undergoes to the punch action. Several studies which tend to optimise the equipment and/or the tool path, in order to reduce the profile diverting, were executed. In this paper, an experimental investigation was carried out in order to test and introduce a new approach able to solve the above problem. More in detail, the test were executed applying an additional backdrawing phase, after the conventional negative deformation. Different testing conditions were evaluated during the experimental campaign and critically compared in the analysis. © Springer/ESAFORM 2009.

Keywords: Accuracy , Incremental sheet forming , Sheet metal forming

Geometrical tolerances control of mechanical components requires methods and tools in order to improve the efficiency of process in terms of time. Dedicated software systems in order to plan and simulate the control and hardware tools in order to rapidly acquire the needed 3D information can support the process improvement. In this context it is important to use the 3D CAD (Computer Aided Design) model, as base to plan and pilot the whole process. The aim of present work is to describe an automatic geometrical tolerances measurement system usable during the design stage. It is based on three main tools: a CAD-based modular software tool, in order to plan, simulate, and pilot the whole verification process, a 3D optical digitizer, as shape acquisition system, and a multi-axis Degree of Freedom (DoF) robot arm in order to move the digitizer. This paper is focused on the developed algorithms to optimize the 3D views acquisition planning. Surface Normal Method and Visibility Map concepts have been reworked for range scanner positions determination and the optimal path is computed by a graph of alignable simulated scans. Experimental test cases are reported in order to show the system performance. ©2009 IEEE.

Adhesively bonded multi-layer structures are frequently used, mostly in the aerospace industry, for their structural efficiency. Nondestructive evaluation of bond integrity in these types of structures, both after manufacturing and for periodic inspection during service, is extremely important. A laser-based ultrasonic technique has been evaluated for non-contact detection of disbonds in aluminum multi-layer structures. Two configurations have been used to detect disbonded areas: pitch-catch with unidirectional guided wave scan and through-transmission with bidirectional scan. Guided wave scanning was done with a laser line source and air-coupled transducer sensing at 500 kHz, 1 ;MHz, and 2 MHz. Signals showed attenuation of the main frequency component and frequency shift on disbonded areas, whereas, a regular and standard waveform is seen outside disbonds. In through-transmission the longitudinal wave at normal incidence was monitored with a 1 MHz probe. One sample showed, besides the introduced inserts, other disbonded areas. After the ultrasonic measurements the sample was cut to visually check adhesive and interfaces. The guided wave pitch-catch scan allowed fast inspection and quick indication of disbonded zones, while the through-transmission C-Scan provided better definition of defects but was slower and required access from both sides of the test part.

Keywords: Guided waves , Laser ultrasound , Multi-layer structures

This work describes an automated artificial vision inspection (AVI) system for real-time detection and classification of defects on textile raw fabrics. The tool (software + hardware) is directly attached to an appositely developed appraisal equipment machine (weave room monitoring system) and the inspection is performed online. The developed tool performs (1) the image acquisition of the raw fabric, (2) the extraction of some critical parameters from the acquired images, (3) an artificial neural network (ANN)-based approach able to detect and classify the most frequently occurring types of defects occurring on the raw fabric and (4) a standard image processing algorithm that allows the measurement of the geometric properties of the detected defects. The reliability of the tool is about 90% (defect detected vs. effectively existing defects), that is, similar to the performance obtained by human experts. Once detected the defects are correctly classified in 88% of cases and their geometrical properties are measured with a sub-pixel precision.

Keywords: Artificial neural network , Image processing , Raw fabrics , Real time

This paper develops a method for optimum tolerancing of components in assemblies. This approach, based on the solution of a constrained optimisation problem, has been applied to an industrial case of an opening door system. The equation to minimise is a manufacturing cost-tolerance function that describes the relationship between tolerance values and manufacturing cost. A set of equations, representing the constraints used in the optimisation problem, is formulated based on the design constraints and assembly requirements. The equations obtained for the optimisation problem are solved to determine the optimum tolerances satisfying all the assembly requirements and all tolerance constraints. Copyright © 2008 Inderscience Enterprises Ltd.

Keywords: Constrained optimisation , Dimensional and geometrical tolerance synthesis , Opening door system

The onset and propagation of interlaminar defects is one of the main damage mechanisms in composite materials. This is even more the case when considering layered materials comprising metallic laminae (typically Aluminium) and FRP laminae. Propagation of delamination mainly depends on the initial crack extension and its loading mode. This work presents some results of a combined analytical-numerical-experimental study on the onset and propagation mechanisms regarding interlaminar defects. Two cases have been analysed in particular, the first consisting of a glass-fibre reinforced epoxy resin laminate, and the second consisting of a hybrid laminate where a lamina of aluminium is layered between FRP laminae. © 2006 Elsevier Ltd. All rights reserved.

Keywords: Composite materials , Dissimilar mixed mode bending specimen , Fracture toughness

Current in-field rail track ultrasonic inspection technologies, requiring contact between the probes and the rail surface, show limitations as defect detection is affected by rail surface condition, rail head geometry and defect geometry and orientation. Moreover, inspection speeds are moderate. To improve rail flaw detection, the partnership of the U-Rail project - within the sixth EU framework programme - proposes a new non-contact ultrasonic inspection system, consisting of a pulsed laser and air-coupled transducers. For a proper positioning, the inspection unit includes a laser optic triangulation unit and an automatic positioning system. The first determines the position of the rail, processes the data and drives the positioning system. The U-Rail system allows high speed inspection of the complete rail section. The paper describes the system and results of research activities of two U-Rail partners. © The Eurographics Association 2007.

Surface polishing is a typical example of a machining process based on mixed chemical-mechanical phenomena, as pointed out in the recent literature on the polishing process (CPM - Chemical Mechanical Polishing). In this work, a model is proposed for the assessment of surface roughness evolution in the polishing process of glass moulds, used in the manufacturing of ophthalmic lenses, in order to identify the influence of the operating parameters on the material removal rate (MRR). In this model the evolution of surface roughness during the polishing process is based on Reye hypothesis. According to such hypothesis, the removed material in a specific time interval is proportional to the friction work: the removed material per unit area can be computed by adequately integrating the bearing ratio curve (Abbott-Firestone) of the surface; the friction work per unit area is proportional, according to the dynamic friction coefficient, to the integral of the product of pressure and velocity in the time interval. A similar result can be also obtained adopting other wear models, e.g. the Preston or Archard approaches. The model validation was performed on ground glass flat samples polished with increasing values of MRR. Pressure and velocity distributions on the sample surface were established according to the polishing machine operating parameters by means of the Hertz theory; the surface roughness of the sample was mapped using an atomic force microscope (AFM). The developed model shows a satisfactory estimate of surface roughness evolution during the polishing process and confirms the experimental results found in literature.

It is well known that the geometrical accuracy is a very relevant problem in Incremental Forming operations, since the material is not well sustained and, then, the elastic springback plays a significant role during the process. A number of researches are involved in the study of geometrical precision after the forming stage but considering the sheet clamped to the equipment. However, it is well known that material coupling is carried out after trimming, when it could change its shape after the new equilibrium. In the paper here addressed the above concept is kept in touch and a wide experimental campaign has been set-up in order to acquire experimental information on the effect of unclamping and trimming after incremental forming processes. The obtained results are able to suggest to the process designer some best practices which are accurately discussed in the paper.

Keywords: Incremental forming , Precision , Sheet metal forming

In this paper a statistical method for tolerances analysis and cost evaluation is presented. Statistical tolerances analysis is performed by assuming the Chase and Greenwood mean shift model, providing an original systematic approach to evaluate the mean shift factor. The proposed approach allows for the analysis of manufacturing costs for different confidence levels of the variables to be optimised. The interaction between customer requirements, design parameters and process variables, are taken into account by incorporating the process planning models and conceptual and embodiment design solutions. The cost analysis is performed by selecting the most appropriate driver for each component of the cost due tolerances. Cost driver parameters are evaluated over product life cycle, in different design domains (customer, design and process) and also include the customer satisfaction (quality loss).

Keywords: Cost analysis , Mean shift model , Tolerance analysis

The use of air-coupled sensors to detect flaws in Al welded joints and the advantage of Lamb wave for their characterization has been proved. Ultrasonic waves were generated by a pulsed laser and by contact angle-beam transducer, and received by an air-coupled capacitive transducer. Laser-based configuration was used to quickly locate the defect, whereas the angle-beam contact probe configuration to characterize it thanks to the dispersive behavior of Lamb waves. Guided waves allow inspection of the complete thickness with only one scan, permitting to detect and to size both internal and surface defects; their high flexibility in the measurement is due to the various parameters characterizing their behavior when compared to the bulk waves. Moreover, L-scan configuration can be used to inspect single-side access structures, and compared to the conventional A, B, C and P-scan allows to reduce inspection time, and therefore the related costs. © 2006 Elsevier B.V. All rights reserved.

Keywords: Air-coupled transducer , Lamb waves , Laser generation , Non-contact ultrasonic inspection , Welded joints

The selection of modeling and machining parameters for glass mould fabrication in ophthalmic lenses production, has required the definition of a theoretical-empirical model of the ground surface in order to predict the overall geometry errors of the surface. The accurate control of the geometrical errors and of the surface texture for the mould functional surface is crucial for the subsequent polishing operation, which is responsible for the final geometry, surface finish and cost. The basic hypotheses validation has been accomplished by measuring the micro-geometric parameters P, W and R and by characterizing the macro-geometry comparing the nominal profile and the measured profile. The correspondence among theoretical hypotheses and experimental results allows realistic predictions of the attainable surface texture during a contour grinding operation and the adoption of preventive actions in order to compensate the geometrical errors due to modeling and tool path generation parameters. © 2005 Elsevier Ltd. All rights reserved.

Keywords: Brittle mode , Geometry error , Glass , Grinding , Process modeling , Surface roughness

In the productive process of moulds for ophthalmic lenses, the availability of a specialized tool for the analysis of the optical property of 3D virtual models of ophthalmic lenses and relevant injection moulds, may reduce the tests on physical prototypes during the design phase. The optical properties of interest are usually power and astigmatism of the lens surface. Both of them are proportional to the geometric curvature of the lens surfaces. Therefore the identification of the optical properties of a surface can be brought back to the computation of the minimum and maximum curvature maps on lens surface. Some commercial software tools able to perform curvature analysis on both physical and virtual models exist, but they show some limitations: methods and algorithms used to compute the desired parameters are not declared, the size of the area used to compute the desired parameter cannot be set by the operator and no estimate on the accuracy of the computed results is given. These last two issues are crucial: the area considered in the analysis should be related to the area actually used; the accuracy of the adopted algorithm should be verified and compared with the eye sensitivity to geometric errors of the lens surface. Aim of the present work is to describe the functioning principles of a software tool for curvature analysis and optical properties computation of either virtual models expressed as high resolution meshes or physical prototypes of lenses sampled using a Coordinate Measuring Machines (CMMs), that overcomes the cited limitations. Such tool will be included into an integrated tool for design, analysis, manufacturing and verification of ophthalmic lenses.

The present work presents the devising of a new highly automated artificial vision inspection (AVI) tool for real-time defect detection and classification on circular knitting machines. The tool is based on the combination of statistical analysis, Image Processing and an artificial neural network (ANN) approach. The tool (software + hardware) is directly attached to a circular knitting machine and the inspection is performed on-line during other common operations like laser cutting and ironing. The automatic inspection allows the real-time detection and classification of the most frequently occurring types of defects on knitted fabrics, which are significant for purposes of quality control and fabric grading. The reliability of the detection tool, i.e. the ratio between defect detected and effective defects is about 93%. The AVI system has been developed by the Department of Mechanical Engineering, University of Florence (Italy), and the textile research centre Tecnotessile s.r.l. of Prato (Italy).

Keywords: Image processing , Knitting machines , Neural network , Radon transform , Real time , Skewness

Tolerance allocation influences production costs in a big way. For this reason it is very important to have an accurate study about the effects of manufacturing errors on the functioning and performances of linkages. In this paper, the authors present a computer-aided methodology based on a 3D geometrical approach using the dual-algebra fundamentals. The purpose is to give an useful tool which can be integrated into CAD software in order to evaluate the performances of spatial mechanisms with mechanical errors. The proposed methodology has been validated by means of experimental tests on a Cardan joint mechanism with clearances, misalignments and dimensional errors. Copyright © 2005 John Wiley & Sons, Ltd.

Keywords: Computer-aided simulation , Dual algebra , Spatial linkages , Tolerance allocation

This work provides an automatic and non-intrusive tool to objectively monitoring the raising process by measuring the height and the density of the fibres emerging from a raised cloth (pile). These parameters are assessed by a numerical procedure, which elaborates the images provided by an appositely developed machine-vision system. The proposed approach allows the investigation and the control of the raising process and has been validated by experimental measurements performed on a set of specimens (cloths) with several raising degrees. The comparison between the results obtained by the proposed procedure and the ones coming from a widely accepted textile-measuring device (fabric assurance by simple testing, FAST) is also provided. © 2005 Elsevier B.V. All rights reserved.

Keywords: Cloth raising , Image processing , Machine-vision design , Pile density , Pile height

Incremental forming applications are currently increasing in industry, especially for the production of small batches or single components. In fact, sufficient know-how is now available for the manufacture of simple products. However, further efforts are required to reduce the drawbacks of typical incremental forming processes, which compromise important advantages in terms of costs and flexibility. First of all the duration of the process, usually a few minutes, influences this kind of process, even if the operations are carried out on high-speed digitally controlled units. A tendency to produce inaccurate parts can reduce industrial interest with respect to incremental forming. Different approaches could be proposed to reduce this drawback, and a feasible and easily implemented strategy is the design of modified trajectories able to take into account both springback effects and stiffness reduction owing to specific clamping equipment. In this paper, such a strategy is pursued by integrating an on-line measuring system, composed of a digital inspector and a computer numerically controlled (CNC) open program. The geometry obtained is sampled in particular steps and an appropriate routine modifies the coordinates of the future punch path. This procedure of automatic control has been developed using an effective finite element (FE) code. An experimental design illustrates the potential use of the suggested methodology. © IMechE 2005.

Keywords: Incremental forming , Net shape , Sheet metal forming

Incremental Forming processes have been introduced in the recent past as an alternative to the money consuming stamping technology, when small batches have to be manufactured. Anyway, they introduce some advantages in terms of flexibility and material formability but, also, some problems such as the dimensional accuracy decreasing. In this paper, a particular application is carried out taking into account the development of an innovative technique to produce a customised ankle support. In this way Incremental Forming process has been selected for the sheet profiling, exalting the role that this technology may play when single complex product has to be manufactured. The producing procedure finishes with a measure of the dimensional accuracy that shown a good result for the desired application. © 2005 Elsevier B.V. All rights reserved.

Keywords: Ankle support , Incremental Forming , Reverse engineering

Experimental, analytical and numerical methods of measuring the surface density of the pile of napped fabrics are presented. Surface density of pile is defined as the ratio between the weight of the pile extending above the fabric and the area of the sample to which it belongs. Samples were weighed, shaved and reweighed and the area of the sample was calculated from the number of pixels on a high-resolution image. The density of the fibre removed was estimated as the difference in density of the sample before and after shaving. A non-intrusive analytical method based on the height, diameter and number of pile fibres and a numerical method are described. Calculations based on pure wool and wool blend are presented and compared with the results obtained using the experimental method.