Bernard Anselmetti

Generation of functional tolerancing based on positioning features

Abstract The aim behind applying functional tolerancing to a mechanism is to widen the tolerances used in parts manufacturing according to the effective functional properties of the product. This step may be performed using CAD systems and geometrical specifications defined by ISO standards. The present paper will describe the complete process involved in functional tolerancing. The CLIC tolerancing method has been implemented within an Excel software environment. CAD models for parts have been imported via a STEP interface. According to this approach, the designer describes the assembly process; the CLIC system then determines the functional requirements corresponding to the joints between parts and generates all datum reference frames and tolerancing of set-up surfaces in compliance with ISO standards. CLIC also determines both the geometrical conditions of minimum distances in order to avoid interference between parts and the conditions for assembling small standard components. The designer next adds other functional requirements. For each such requirement, a tolerancing process creates location and orientation specifications for influential parts using datum reference frames derived during the previous stage. Excel formulae focusing on the sum of tolerances are generated using a three-dimensional statistical approach. Moreover, the tolerance database allows optimizing the tolerances and nominal dimensions of parts.

Deviation of a machined surface in flank milling

The flatness defects observed in flank milling with cutters of long series are mainly due to the tool deflections during the machining process. This article present the results of an identification procedure of the coefficients of a force model for a given tool workpiece couple for the prediction of the defects of the tool during the cutting. The calibration method proposed meets a double aim: to define an experimental protocol that takes the industrial constraints of time and cost into account and to work out a protocol which minimizes uncertainties likely to alter the interpretation of the results (environmental, software or mechanical uncertainties). For that, the procedure envisages the machining of a simple plane starting from a raw part formed by a tilted plane, allowing for the variation of the tool engagement conditions. The tool deviation during the cutting process is indirectly identified by measuring the machined surface. The observed straightness defect conditions can be explained by the evolution of the cutting pressures applied to the cutting edges in catch during the cutter rotation. The precision was considerably improved by the taking into account of the cutter slope defect in the calculation of the load applied to the tool. After identification of the tool-workpiece couple, the prediction model was applied to some examples and allowed to determine the variations of form and position of the surface points with a margin of 5%.

Quick GPS: A new CAT system for single-part tolerancing

This paper depicts a new CAT (Computer Aided Tolerancing) system called Quick GPS (Geometrical Product Specification), for assisting the designer when specifying the functional tolerances of a single part included in a mechanism, without any required complex function analysis. The mechanism assembly is first described through a positioning table formalism. In order to create datum reference frames and to respect assembly requirements, an ISO based 3D tolerancing scheme is then proposed, thanks to a set of rules based on geometric patterns and TTRS (Technologically and Topologically Related Surfaces). Since it remains impossible to determine tolerance chains automatically, the designer must impose links between the frames. The CAT system that we developed here proposes ISO based tolerance specifications to help ensure compliance with the designers intentions, saving on time and eliminating errors. This paper will detail both the set of tolerancing rules and the designers approach. The Quick GPS system has been developed in a CATIA V5 environment using CATIA VBA and CATIA CAA procedures.

Functional tolerancing of complex mechanisms: identification and specification of key parts

The functional tolerancing process for complex mechanisms needs the study of the behavior of mechanical joints. The use of the Computer-Aided Design system helps in carrying out this task. This paper presents an effective new approach for decomposing a global geometric functional requirement of the mechanism into geometric specifications defined on key components (parts and sub-assemblies). A recursive tolerancing method serves to identify and functionally specify the key components related to a tolerance-chain. A geometric variation model, based on the invariant degrees of freedom (DOFs) of the datum reference frames and tolerance zones, enables validating a datum reference system built on the positioning features of a component and then deducing the influential mechanical joints. Formalized simple rules based on the topology of parts have also been developed for validating a datum reference system. A geometric specification defined on a sub-assembly has been introduced as a new geometric functional requirement: this provides the designer with an effective recursive tool for ensuring the functional tolerancing of the entire assembly. The proposed approach is straightforward to implement and has been devised from the concept of standardized specifications, with the particularity that it is capable of treating cases in which the positioning features are not perpendicular.

Optimization of a workpiece considering production requirements

Abstract As a contribution to the future of integrated CAD/CAM systems, this paper proposes a procedure to include the constraints of manufacturing in the dimensioning and the tolerancing of a workpiece. This algorithm is well adapted to consider the unilateral conditions extracted from the functional dimensioning and from the machining requirements in order to obtain a part model in medium dimensions. A rule for choosing dimensions which must be respected in machining is proposed. The system was implemented on an IBM RISC 6000 station and a simple example is presented.

Coupling experimental design: digital simulation of junctions for the development of complex tolerance chains

This paper presents a new functional tolerancing method developed for analyzing tri-dimensional variations of mechanical assemblies. The behavior of a junction is simulated with Microsoft EXCEL to quantify the influence of geometrical variations of a given part on a functional requirement. The solver determines the critical situation of the mechanism with respect to the contact constraints and the location and orientation tolerances. The result of this stage is introduced into an experimental design so as to define a relationship between the variation of the functional requirement and the geometric tolerances specified for the surfaces of the parts. The complete inequality ensuring tolerance synthesis is obtained by adding the defects of all junctions forming the vector loop.

Circular tests for HSM machine tools: Bore machining application

Todays High-Speed Machining (HSM) machine tool combines productivity and part quality. The difficulty inherent in HSM operations lies in understanding the impact of machine tool behaviour on machining time and part quality. Analysis of some of the relevant ISO standards [230. Acceptance code for machine tools. Part 4, Circular tests for numerically controlled machine-tools, April 1998, 10791. Test conditions for machining centres. Part 6, Accuracy of feeds, speeds and interpolation, September 1998, 10791. Test conditions for machining centres. Part 7, Accuracy of feeds, speeds and interpolation, September 1998] and a complementary protocol for better understanding HSM technology are presented in the first part of this paper. These ISO standards are devoted to the procedures implemented in order to study the behaviour of machine tool. As these procedures do not integrate HSM technology, the need for HSM machine tool tests becomes critical to improving the trade-off between machining time and part quality. A new protocol for analysing the HSM technology impact during circular interpolation is presented in the second part of the paper. This protocol, which allows evaluating kinematic machine tool behaviour during circular interpolation, was designed from tests without machining. These tests are discussed and their results analysed in the paper. During the circular interpolation, axis capacities (such as acceleration or Jerk) related to certain setting parameters of the numerical control unit have a significant impact on the value of the feed rate. Consequently, a kinematic model for a circular-interpolated trajectory was developed on the basis of these parameters. Moreover, the link between part accuracy and kinematic machine tool behaviour was established. The kinematic model was ultimately validated on a bore machining simulation.

A recursive tolerancing method with sub-assembly generation

Functional tolerancing of industrial products depends on the assembly sequence and on the relative positioning of parts. The quality of the interfaces between these parts ensures the assembly requirements and the good positioning of functional surfaces. For each geometric functional requirement, we generate a sub-assembly including the influential parts as an attribute graph. A recursive method for tolerancing these parts, based on this graph and positioning tables, is developed. The block and pseudo-block concepts aid the functional tolerancing in complex tolerance chains. An example of an industrial mechanism is used to illustrate the methodology.

Synthesis of tolerances starting from a fuzzy expression of the functional requirements

Functional tolerancing of mechanisms is a recognized industrial approach which comprises four main stages (definition of functional requirements of the mechanism, choice of the specifications of each part according to ISO standards, determination of the inequations corresponding to the chains of dimensions, assignment of a value to the tolerance of each specification).

Machining Strategy Choice: Performance Viewer

Nowadays high speed machining (HSM) machine tool combines productivity and part quality. So mould and die maker invested in HSM. Die and mould features are more and more complex shaped. Thus, it is difficult to choose the best machining strategy according to part shape. Geometrical analysis of machining features is not sufficient to make an optimal choice. Some research show that security, technical, functional and economical constrains must be taken into account to elaborate a machining strategy. During complex shape machining, production system limits induce feed rate decreases, thus loss of productivity, in some part areas. In this paper we propose to analyse these areas by estimating tool path quality. First we perform experiments on HSM machine tool to determine trajectory impact on machine tool behaviour. Then, we extract critical criteria and establish models of performance loss. Our work is focused on machine tool kinematical performance and numerical controller unit calculation capacity. We implement these models on Esprit CAM Software. During machining trajectory creation, critical part areas can be visualised and analysed. Parameters, such as, segment or arc lengths, nature of discontinuities encountered are used to analyse critical part areas. According to this visualisation, process development engineer should validate or modify the trajectory.