Jean-François Fontaine

3D part inspection path planning of a laser scanner with control on the uncertainty

This article concerns the measurement process of mechanical parts using laser scanners. From the point of view of industrial applications, the objective is to guarantee the measurement accuracy during the scanning with regard to the geometrical product specifications. The proposed method can be summarized as follows: the first step consists of analyzing the interval of tolerance for the different specifications and to attribute to every geometrical entity a maximal uncertainty of measurement. This uncertainty depends on the angle of incidence between the laser plane and the scanned surface. In the second step, an approach based on the concept of visibility is used from the CAD model of the inspected part to find correct sensor guidance in a metrological point of view. A few position-points from this set are used to define the scanning path. Finally, the measurement can be carried out and the specifications can be controlled after the segmentation of the point clouds. An example illustrates the approach.

Influence of roughness on characteristics of tight interference fit of a shaft and a hub

Tight interference fits are very widely applied in industry, because of their simple manufacturing process. But, in all proposed models for determining their characteristics, one supposes that the contact interfaces are perfect. This is not the case in reality. The objectives of this study are to analyse the contact surfaces behaviour under the effect of the assembly pressure. We show for a given surface texture typology that the roughness has a noticeable influence on the fit strength. Our process uses an experimental approach correlated with numerical modelling of the assembly. The aim is to justify a tightening definition based on the maximum matter concept and to introduce, for this particular case, a prediction of loss due to the deformation of the surface asperities.

Contact pressure between two rough surfaces of a cylindrical fit

Abstract Contact pressure, an important parameter for analysing and evaluating the contact behaviour, is always an interesting subject. The greater part of studies and results concentrates upon the contact, friction, wear, tear and lubrication for the cases of the rough contact. The tight fit is a current process of assembly for transmitting a force or a torque between a shaft and a hub with the help of friction effects. Contact surfaces play an important role in accounting for their capacities. The standards are based on the thick cylinder theory and does not take into account the surface textures and the asperity behaviour, although it is quite normal and useful to have rough surfaces, one is obliged to specify the very smooth contact surfaces. One obvious factor is that the smoother the surface is, the higher the manufacturing cost is. Moreover, having smooth surfaces does not result in a large friction coefficient. This paper, based on different precedent studies on roughness contact, bears, in mind on speciality of the tight fit problem, the distribution, the evolution of pressure, the plastic deformation of asperity and its influence on the pressure, in the objective to build a new method for computing the tight fitting pressure in a cylindrical tight fit [Influence de l’etat de surface sur les caracteristiques d’un assemblage frette, These de l’ENSAM, France, 1998].

Methodology for the assessment of measuring uncertainties of articulated arm coordinate measuring machines

The Articulated Arm Coordinate Measuring Machines (AACMM) have gradually evolved and are increasingly used in mechanic industry. At present, measurement uncertainties relating to the use of these devices are not yet well-quantified. The work carried out consists on determining the measurement uncertainties of a mechanical part by an Articulated Arm Coordinate Measuring Machine. The studies aiming to develop a model of measurement uncertainties are based on the Monte Carlo method developed in Supplement 1 of the Guide to Expression of Uncertainty in Measurement [1] but also identifying and characterizing the main sources of uncertainty. A Multi-level Monte Carlo approach principle has been developed which allows characterizing the possible evolution of the Articulated Arm CMM during the measurement and quantifying in a second level the uncertainty on the considered measurand. The first Monte Carlo level is the most complex and is thus divided into 3 sub-levels, namely characterization on positioning error of a point, estimation of calibration errors and evaluation of fluctuations of the “localization point”. The global method is thus presented and results of the first sub-level are particularly developed. The main sources of uncertainty, including AACMM deformations are exposed.

Form defect influence on the shrinkage fit characteristics

Abstract Today, manufacturing products must meet more and more severe specifications. The different parts composing the product often necessitate high dimensional precision, which increases the difficulties for a large series production. Then it it necessary to optimize dimensioning of the different components in an economic context. In the case of small dimensional fits, there is an influence of the micro-geometry (form of defect, roughness) and time of the process on the geometrical characteristics of the assembly. At the time of conception, it is necessary to obtain a good specification that relates the product functionalities with the best cost. The objective study of this is to simulate a shrinkage fit process and provide the fit characteristics according to the microgeometrical defects. Only the form of the defects are taken into account. In the first part, a method a solution of a three dimensional contact involving two cylindrical bodies is presented. The contact is closely conformed (non-Hertzian). The stress field and the strain field, in both bodies, are calculated with the hypothesis that the interface pressure is a sum of different concentrated loads superposing and depending locally on the defects after rendering the contact area discrete. The different basis loads are formulated by Fourier series and modified Bessel functions. Several cases are simualated for different types of defects that can show the influence of the microgeometry on the cylindrical characteristics fit. Finally, an experiment is presented which shows a good correspondence with theory and predicts the extract strength.

Experimental and numerical modelling of the effects of process parameters in the aquadraw deep drawing

Abstract The determination of the main parameters that influence the aquadraw deep drawing process is studied in order to control and to drive the process. Experimental and analytical investigations are carried out concerning the pressure in the cavity and under the blankholder as functions of the process geometry, material parameters and fluid parameters. A finite element modelling of the process is proposed to access local quantities. Our investigations is limited to the axisymmetric cases.

Optimization of the contact surface shape of a shrinkage fit

Amongst many processes existing today for the assembly of two cylindrical parts, the shrinkage fit is commonly used. However, in the calculation methods, the contact surfaces are assumed to be perfect. Therefore, in order to apply these, the standards necessitate very small geometrical tolerances. The objective of the present paper is to show that it is possible to take into account form defects by closely conformed contact modelling and to have a good representation of reality: form defects always exist to an increasing extent and are inherent in all kinds of manufacturing processes. Finally, it is shown that the presence of defects can be beneficial, allowing optimization from both a mechanical and an economical point of view.

Comparison of the strain field of abdominal aortic aneurysm measured by magnetic resonance imaging and stereovision: a feasibility study for prediction of the risk of rupture of aortic abdominal aneurysm.

The prediction of the risk of rupture of abdominal aortic aneurysm (AAA) is a complex problem. Currently the criteria to predict rupture of abdominal aortic aneurysms are aneurysm diameter and growth rates. It is generally believed that study of the wall strain distribution could be helpful to find a better decision criterion for surgery of aortic aneurysms before their rupture. The wall strain distribution depends on many biological and biomechanical factors such as elastic properties of the aorta, turbulent blood flow, anatomy of the aorta, presence of thrombus or not and so on. Recently, numerical simulations to estimate rupture-potential have received many attentions. However, none of the medical imaging tools for screening and monitoring of AAAs were studied in terms of mechanical behavior and experimentally to demonstrate their capability to measure relevant variables. The aim of this study was to develop a metrological approach for deployment testing of the ability of techniques for measuring local in-vitro deformations based on comparison of stereovision and MRI. In this paper, we present the implementation approach and results of the study based on cylindrical phantoms with or without AAA representing, respectively, healthy and unhealthy artery. Through this study, an experimental device was developed for the behavior study of AAA during a cardiac cycle. The results show that the stereovision techniques used in laboratory is well suited and is qualitatively and quantitatively equivalent with MRI measurements.

3D digitizing path planning for part inspection with laser scanning

If the first work relating to the automation of the digitalization of machine elements goes back to approximately 25 years, the process of digitalization of parts with non-contact sensor remains nevertheless complex. It is not completely solved today, in particular from a metrological point of view. In this article, we consider the determination of the trajectory planning within the framework of the control of dimensional and geometrical specifications. The sensor used in this application is a laser planner scanner with CCD camera oriented and moved by a CMM. For this purpose, we have focused on the methodology used to determine the best possible viewpoints which will satisfy the digitizing of a mechanical part. The developed method is based on the concept of visibility: for each facet of a part CAD Model (STL) a set of orientations, called real visibility chart, is calculated under condition of measurement uncertainties. By application of several optimisation criteria, the real visibility chart is reduced to create a viewpoint set from which the path planning is built.

Validation of the Strain Assessment of a Phantom of Abdominal Aortic Aneurysm: Comparison of Results Obtained From Magnetic Resonance Imaging and Stereovision Measurements

Predicting aortic aneurysm ruptures is a complex problem that has been investigated by many research teams over several decades. Work on this issue is notably complex and involves both the mechanical behavior of the artery and the blood flow. Magnetic resonance imaging (MRI) can provide measurements concerning the shape of an organ and the blood that flows through it. Measuring local distortion of the artery wall is the first essential factor to evaluate in a ruptured artery. This paper aims to demonstrate the feasibility of this measure using MRI on a phantom of an abdominal aortic aneurysm (AAA) with realistic shape. The aortic geometry is obtained from a series of cine-MR images and reconstructed using Mimics software. From 4D flow and MRI measurements, the field of velocity is determined and introduced into a computational fluid dynamic (CFD) model to determine the mechanical boundaries applied on the wall artery (pressure and ultimately wall shear stress (WSS)). These factors are then converted into a solid model that enables wall deformations to be calculated. This approach was applied to a silicone phantom model of an AAA reconstructed from a patients computed tomography-scan examination. The calculated deformations were then compared to those obtained in identical conditions by stereovision. The results of both methods were found to be close. Deformations of the studied AAA phantom with complex shape were obtained within a gap of 12% by modeling from MR data.