Parametric CAD Systems Research Articles

Strategic knowledge-based approach for CAD modelling learning

Strategic knowledge is a differentiating factor for conveying the design intent when modelling parts in parametric feature-based CAD systems. Nevertheless, it is rarely considered in training or, when it is tackled, it takes a traditional pedagogical approach that is far from ideal for acquiring knowledge with highly cognitive requirements. This paper demonstrates a contextualised instructional approach that is developed in activities following active learning principles. These activities are structured on a modelling procedure that provides trainees with guidelines to think about the modelling rationale and consider alternative strategies. The paper describes one activity in detail, as an example, so that our proposal can be replicated and adapted to other scenarios. Finally, experimental research has been carried out that validates the proposal as opposed to the traditional teaching focus. Its results support the idea that it can have a favourable impact on developing strategic knowledge for CAD modelling.

A new procedure for the automated design of ejection systems in injection molds

In this paper a new method of automatic design for the layout and sizing of the ejector pins of the plastic injection mold, based on the discrete geometry of the plastic part, is presented. The proposed algorithm consists of a first phase of geometric analysis in which a node mesh corresponding to the surfaces to be molded by the lower mold cavity is generated. Then the variations of the difference in thickness of the part for each pair of mesh nodes are evaluated, locating areas with varying thicknesses and resembling this geometric value at discrete nodes that are stored in arrays. Two sub-algorithms of discrete geometric recognition enable the location of the nodes which can be points of expulsion. Ejection points will correspond to those nodes which have a maximum in the parameter regarding concentration of nodes near walls, ribs, protrusions, etc. The dimensioning of the ejector pins is performed by an optimization algorithm with three objective functions, ensuring that the system of forces in the ejection of the part will be balanced. This new method improves on the methods used so far as it does not require heuristic methods to achieve the result and does not use the method of identifying features for the geometric recognition of the surface of the plastic part, avoiding the problems of dependency from the modeler and analysis of complex features. It takes the part in discrete format as input data, analyzing in this way the geometry externally, obtaining the advantage of the possibility of implementing the algorithm in any CAD modeler. The proposed algorithm is applicable to any geometry because it works regardless of the CAD system in which the piece has been designed. Finally, the system provides as a result the layout with the location and the diameter of the ejectors on the part while ensuring a suitable distribution of balance of ejection forces. The solution of the algorithm is shown superimposed on the part as a map of ejection. The results of the algorithm can be exported and stored for use in other applications and parametric injection mold CAD systems. A node mesh corresponding to the surfaces to be molded by the lower cavity is analyzed.The algorithm locates zones with varying thicknesses resembling this value at discrete nodes.A geometric recognition method places the ejection points close to rigid workpiece areas.Ejectors are dimensioned with optimization algorithms ensuring that the forces are balanced.Ejector design is performed on any system no other solid internal information is required.

Explicit reference modeling methodology in parametric CAD system

Today parametric associative CAD systems must help companies to create more efficient virtual development processes. While dealing with complex parts (e.g. the number of surfaces of the solid) no CAD modeling methodology is existing. Based on the analysis of industrial designers' practices as well as student practices on CAD, we identified key factors that lead to better performance. Our objective in this article is to propose a practical method for complex parts modeling in parametric CAD system. An illustration of the performances and the results obtained by this method are presented comparing the traditional method with the proposed one while using an academic case and then an industrial case.

3D Parametric Modeling for Complex Parts Generation

The complex parts, like block engine or inside panel door, represent a real challenge for 3D modeling with modern parametric 3D CAD systems if we intend to reuse the modeling for future modifications. The complexity of the part generates a large number of parameters and parametrical dependencies that is difficult to manage and even difficult to change for modifying the part, often, for big changes, making it easier to remodeling the part from the beginning. In this paper is presented a new method of parametric and hierarchical 3D modeling that allows easy future modifications of the part. This paper also presents the implementation of this new method in Catia V5 and the validation for a number of CAD models.

Geometric Constraint Solving With Solution Selectors

Current parametric CAD systems are based on solving equality types of constraints between geometric objects and parameters. This includes algebraic equations constraining the values of variables, and geometric constraints constraining the positions of geometric objects. However, to truly represent design intent, next-generation CAD systems must also allow users to input other types of constraints such as inequality constraints. Inequality constraints are expressed as inequality expressions on variables, or as geometric constraints that force geometric objects to be on specific sides or have specific orientations with respect to other objects. The research presented here investigates whether the frontier algorithm can be extended to solve geometry positioning problems involving systems of equality- and inequality-based declarations in which the inequality-based declarations are used as solution selectors to choose from multiple solutions inherently arising when solving these systems. It is found that these systems can be decomposed by the frontier algorithm in a manner similar to purely equality-based constraint systems, however they require tracking and iterating through multiple solutions and in many cases may require backtracking through the solution sequence. The computational complexity of the new algorithm is found to be the same as the frontier algorithm in the planning phase and linear in the execution phase with respect to the size of the problem but exponential with respect to the distance of solution selection steps from the satisfaction steps they control.

A New Method of Visualization and Documentation of Parametric Information of 3D CAD Models

AbstractThe modeling process of complex assemblies with modern parametric 3D CAD systems generates a large network of various parametric dependencies and parameter information that is difficult to manage. In this paper, a new method and a software solution for extraction and visualization of all parametric information within 3D CAD models are presented. The requirements for the visualization of complex 3D CAD models are discussed and an overall solution concept is described. Based on the general structural analysis of the parametric 3D models, a CAD system-independent graphical language with two forms of visualization will be introduced. A data model to store parametric information from CAD models, as well as a method to export them has been developed. This paper also presents the implementation of the method in CATIA V5 and the validation on a number of CAD models.

Tracking topological changes in parametric models

In current parametric CAD systems, the relation between the values of the parameters of a model and the topology of the model is often not clear to the user. To give the user better control over the topology of the model, this relation should be made explicit. A method is presented here that determines the parameter values for which the topology of a model changes, i.e. the critical values of a given variant parameter. The considered model consists of a system of geometric constraints, which relates parameters and feature geometries, and a cellular model, which partitions space into volumetric cells determined by the intersections of the feature geometries and represented by topological entities. Our method creates a new system of geometric constraints to relate the parameters of the model to the topological entities. For each entity that is dependent on the variant parameter, degenerate cases are enforced by specific geometric constraints. Solving the resulting constraint systems yields the critical parameter values. Critical values can be used to compute parameter ranges corresponding to families of objects, i.e. all parameter values which correspond to models that satisfy a given set of geometric and topological constraints.

Modelling of Solid Part Using Multibody Techniques in Parametric CAD Systems

In this paper, the multibody approach to design in part environment in parametric CAD systems is described. Different kinds of multibody methods and tools which apply this approach are discussed. Some examples of solid part design performed in SolidWorks 2007 system are presented.

Solving Interval Constraints by Linearization in Computer-Aided Design

Current parametric CAD systems require geometric parameters to have fixed values. Specifying fixed parameter values implicitly adds rigid constraints on the geometry, which have the potential to introduce conflicts during the design process. This paper presents a soft constraint representation scheme based on nominal interval. Interval geometric parameters capture inexactness of conceptual and embodiment design, uncertainty in detail design, as well as boundary information for design optimization. To accommodate under-constrained and over-constrained design problems, a double-loop Gauss-Seidel method is developed to solve linear constraints. A symbolic preconditioning procedure transforms nonlinear equations to separable form. Inequalities are also transformed and integrated with equalities. Nonlinear constraints can be bounded by piecewise linear enclosures and solved by linear methods iteratively. A sensitivity analysis method that differentiates active and inactive constraints is presented for design refinement.

A constructive approach to calculate parameter ranges for systems of geometric constraints

Geometric constraints are at the heart of parametric and feature-based CAD systems. Changing values of geometric constraint parameters is one of the most common operations in such systems. However, because allowable parameter values are not known to the user beforehand, this is often a trial-and-error process. We present an approach for automatically determining the allowable range for parameters of geometric constraints. Considered are systems of distance and angle constraints on points in 3D that can be decomposed into triangular and tetrahedral subproblems, by which most practical situations in parametric and feature-based CAD systems can be represented. Our method uses the decomposition to find critical parameter values for which subproblems degenerate. By solving one problem instance for each interval between two subsequent critical values, the exact parameter range is determined for which a solution exists.

Modelling with constraints: theoretical foundation and application

Modelling with constraints is a modern approach to product modelling. Engineering knowledge is associated with geometry and topology in the product model. Together with the technique of feature based modelling, modelling with constraints has widely affected the development of new cad-systems. The architecture of so called parametric cad-systems reflects the impact of these new technologies. Besides the new hybrid modellers of geometry and topology, the sketcher and the constraint solver have become key components in a parametric cad-system. The sketcher may be regarded as a designer interface for modelling with constraints. Moreover rule-based methods of automatic constraint detection are applied in sketchers. Constraint solvers evaluate the network of constraints formulated by the sketcher as a set of equations or predicates using numeric or symbolic algorithms or rule based approaches. The article closes with a summary of the advantages and risks of designing with constraints, showing typical application fields of this modelling technique and discussing some open issues.

Constructive constraint-based model for parametric CAD systems

The paper discusses proposed solutions for constraint-based modelling, with special emphasis on constructive approaches. A new constructive scheme is proposed that is based on a nonevaluated, constructive solid model. The model definition language is presented and discussed, with the general architecture of the system and the structure of the internal model representation. The proposed approach supports the instantiation of predefined models, parametric geometric operations 1D, 2D and 3D, variable topologies, and operations with structural constraints.