Pascal Hernandez

A new calculation method for the worst case tolerance analysis and synthesis in stack-type assemblies

Productivity and industrial product quality improvements entail a rational tolerancing process to be applied as early as product design. Once functional conditions are defined, an optimal specification for each component in a mechanical system is to be developed. Despite numerous studies in this area, the problem is still far from solved. It may be decomposed into two stages: development of specifications based on standards, or qualitative synthesis, and calculation of tolerances. To the extent that these two sets of problems are related, we propose to address them in parallel. In this paper, we present an original method that enables us to solve these two problems for the case of serial assembly (stacking) without clearances. This method is based on the use of influence coefficients to obtain the relationship between the functional tolerance and the tolerances associated with the geometry of the mechanisms interface surfaces. We will describe a calculation algorithm that helps obtain influence coefficients solely from the assemblys geometric definition. Then, we will show that under our working hypothesis, this relationship is piecewise linear.

Virtual Gauge Representation for Geometric Tolerances in CAD-CAM Systems

Abstract: The CAD software seeks to represent the syntax of the geometric tolerances, i.e. their writing on the drawings. We propose to represent their semantics, i.e. their meaning with respect to the part. We show that the meaning of the geometric tolerances can be defined thanks to a model of virtual gauges. These gauges concern geometrical entities of the part which are represented on the three-dimensional geometrical model of the part (CAD model). The topology of a gauge is related to that of the part. Recording these attributes is sufficient. The advantages of this representation are its simplicity, the semantic coherence which can be guaranteed, the independence from the standards, their limits and their evolutions, and the extension of the tolerancing possibilities for the designer. Key words: Tolerancing, Virtual gauge, CAD-CAM.1. INTRODUCTIONThe subject of this paper is to present the bases of a data-processing representation of the geometric tolerances. The tolerances which are considered are those which are allowed by the ISO and ASME standards. Nevertheless, we will show that the suggested representation allows to specify functional tolerances which are difficult and even impossible to express with the writing rules of the standards. Indeed it is necessary to distinguish the

The pilot dimension method: Reconciling Steering and Conformity in Workshops

In machining workshops, workpieces are produced according to dimensions known as manufacturing dimensions. For the same workpiece and the same manufacturing plan, several sets of manufacturing dimensions can be used but none satisfies simultaneously the two main missions workshops need to fulfil: (a) ensuring conformity of products to their design dimension tolerances (also called blueprint tolerances); and (b) steering machines in order to compensate for tool wear. The set of manufacturing dimensions obtained from the design dimensions using the minimal chain of dimensions method is optimal for a conformity check of workpieces but is practically unusable for steering machines because of the complexity of its relationships toward the tool correctors and tools dimensions. The pilot dimensions method consists of, on the one hand, identifying and representing these tool correctors and these tool/program dimensions on the production drawings (besides the manufacturing dimensions) and, on the other hand, determining their correction values through a mathematical set of relations after having measured the manufacturing dimensions on a workpiece. Doing so will strongly reduce adjustment time, reduce the number of workpieces used for adjustments and greatly enhance the quality of workpiece batches.

Rigid and Elastic Precision Domains of Ball Bearings

Ball bearings are complex components where local deformations are the main factor on the global behavior. One problem is the relation between a contact configuration and the load level. When those components are assembled, position errors have important effects on those contact configurations. A rigid ball bearing has very small clearances that give tolerances impossible to achieve. How can the designer write geometric specifications on the assembly taking into account bearing elastic behavior?. We propose a method to identify the limits of those errors according to the ball bearing limits and the clearances. The built model has been compared with industrial references with good accuracy.

A New Method for Integrated Design and Tolerancing

The dimensional and geometrical tolerancing of machine elements is an important step in the design and manufacturing of a product. Unfortunately, tolerancing takes place late in the current design processes. Generally, it is only in the detail drawings of the parts that the tolerances are determined qualitatively and quantitatively. Some design problems appear which could have been detected upstream if the tolerances had been introduced from the very start. In the proposed design process, the mechanism is defined from a minimal kinematics structure to a detailed geometry. The tolerancing method is directly integrated into this design process. There is an inevitable growing complexity of the mechanical structure. Some technical choices are carried out at each level and it would be interesting to evaluate their geometrical influence on the expressed conditions. Therefore, we propose to deal with the problem of tolerance in an integrated manner with the process of design. The recursive top-down design and tolerancing process is general. The different design solutions, and technological choices, directly influence the dimensional and geometrical tolerances. We present a graph tool, which allows definition of the topology of the mechanism, during all phases of its design. The tolerancing graph is translated into ISO standards conforming tolerances. Different views are possible depending on the detail level needed by the designer. During the design process, the graph is simultaneously updated. An example is studied with the different steps to illustrate this integrated method. The influence of different possible design solutions on the tolerances is compared in order to validate these choices.