[ Basics Concepts ]
[Additional Information]Responsible : Lucas
Cley Horta; Prof. Henrique Rozenfeld
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the resposible(s)
Source: KERRY, H. T., 1997.
(See additional information)With the continuous technological advance, the addition of information and knowledge, under the engineering domain, grow uninterruptedly. CAD systems seek to assist in the manipulation and creation of this information, systemising the design data involved, making it possible for a fast re-use of this information, when it is necessary.
CAD Systems support any design activity from its creation, modification, backup or documentation. Despite the acronym "CAD" including the term "Design", it is observed that there are few cases where the computer effectively projects something. It serves more as an aid tool to realise the engineering drawings.
Its bigger contribution occurs in the products and components modelling and in the detailed drawings. In some CAD systems the term "design" was changed to "drafting", considering its application as purely directed to the design documentation, which, in some cases, can take the sub utilisation of the system.
Other systems that act in the area of engineering calculations, called CAE ("Computer Aided Engineering "), carry out other kind of activities such as structural analysis based on finite elements (FEM), draining analysis, multi-bodies simulations, tensions analysis, etc. There are some attempts related to the systems development that really can take decisions in the design, the great majority of them are based on Artificial Intelligence techniques.
However, considering the difficulties in capturing the development process logic of a design and the amount of data involved, this task becomes too complex, resulting in few practical results.
Even with these difficulties, the CAD systems are extremely important in design. The advantages offered in its support to design can be proved in many of its stages; a better documentation and presentation of product, quality improvement in drawings, time and cost reduction, and general increase in productivity, resulting in better design management.
On the other hand, the total potential benefit of CAD systems can only occur, justified technically and economically, if they are integrated with the whole production process. In an integrated structure, the CAD provides, beyond the intrinsic profits to the product design, an efficiency increase of the functions related with planning, manufacturing, and quality. In other words, the CAD must be integrated with other systems such as CAPP, CAM and systems of production management (PCP, MRP, ERP).
Currently, to analyse the CAD systems, there are a variety of options that must be considered, amongst them some features include the system functionality or its integration applicability with the other systems. Considering the compilation data nowadays there are some variations in the market. Below is a description of the 2D and 3D systems.
2D systems
One of the advantages of using 2D CAD system is that it involves fast employee training, as most are generally used to using common drawing boards. However, its use is limited, risking transforming the system into a simple electronic drawing board, it is only a little more productive than the common drawing boards.
For some applications, the representation in 2D is enough, as for example, in electric designs, hydraulic, circuits and electronic board projects, where there is not the necessity of having volumetric information. Also, for example, in the creation of some kinds of sketching, with the aim of supporting production, the 2D CAD is more appropriate. In this case, it must work with a CAPP system, which would be responsible for the data management represented on the sketch (as tool list, assembly and/or inspection instructions, etc.)
In the mechanical design, the 2D representation is being adopted to develop set drawings because they can be easily modified. In this phase, a great number of normalised parts are used that are included in the drawing interactively, which confers a great productivity to this activity. Small and medium companies from mechanical sector prefer to use 2D systems. Beyond the low cost of acquisition and employees training, these systems demand less powerful machines. However, there are a series of 2D systems in the market nowadays that aim to fill this gap (see the Related Sites).
The answer for the great comeback of the 2D CAD systems is its potential for information re-use, considering that it is much easier to recoup and to modify an electronic drawing, instead of a drawing carried out in a conventional way.
3D Systems
The 3D modelling presents difficulties typical of the drawing process, considering that the designer needs to simultaneously think in the three dimensions. In some cases, the use of the 3D model is essential, for example, in the application of finite element analysis aimed at the verification of tensions, draining, temperature, etc. It is also essential when it is necessary to calculate volume, mass properties and inertia axle, and interference’s verification (see CAE)
Below are the main methods adopted for 3D representation:
In the past, the wireframe modelling was the main method used by CAD systems, making it possible to link lines between points in the 3D environment. This allowed the creation of space models and guaranteed the consistency of a 2D view derived from 3D and its associated measurement.
With the technological advance and larger processing capacity of computers, the wireframe modelling system started to be substituted by those ones based on the solid modelling methods. This also happened, in part, due the difficulty to use wireframe when it was necessary to incorporate them in softwares of analysis (CAE) or manufacturing (CAM), considering that this kind of model does not contain any information related with the physical features of the real components.
Solid Modelling CSG (Constructive Solid Geometry)
Systems that are capable of conducting solid modelling are much more powerful than simple modellers based on wireframe. These programs are used to construct components that are solid objects, and not only a mesh of plait lines. A CSG model is a binary tree consisting of primitive objects and Boolean operators. The tree leaves represent the primitive objects and the most complex objects are the origin. The tree root represents the whole product. Each primitive object is associated with a 3D reference that specifies the position, orientation, and dimensions. The characteristic of this method is to compose models from solids.
Using solids to shape the components, they start to obtain physical properties, for example volume, and if you specify its density it is possible to obtain other features such as weight and mass. Then the computer can calculate some additional physical properties, of these components, such as gravity centre, inertia moment, etc. These calculations can be used in components with irregular forms, where the manual calculation becomes extremely difficult and laborious. Moreover, it facilitates the use of the model in analysis/simulation softwares.
However, this method has some limitations as a limited set of operations and primitives, limiting the creation possibilities of the designer.
Solid modelling Brep (Boundary Representation)
The Brep modelling is based on the previously existing, surface modelling techniques. The first generation of Brep modellers represented solid objects only for table faces, edges, and verticals. Thus it only supported objects with flat faces. Curved surfaces were shaped by a linear approach called shaping process.
The second generation of B-rep modellers included primitive objects with analytical surfaces such as cylinders, spheres, cones, etc. They allow the creation of much more complex models with "accurate" geometry. To achieve this some much more complex intersection algorithms were necessary.
Other developments in B- rep modelling have been directed to improvements in the effectiveness of Boolean operations. An example is through the use of directories of space occupation, which reduces the number of verifications of geometrical constraints. Another development was the expansion of the number of geometric forms that can be shaped with Brep.
The Brep modelling has some advantages when compared with the CSG, mainly in regards to versatility in the generation of complex models and the speed of verification of topological relations. This is due to the way Brep registers the model information, storing the edge parameters of explicit form.
Hybrid solid modelling
The methods of solid modelling CSG and Brep are frequently aligned to generate component models. Each one of these methods has its limitations; therefore, in the creation of difficult components the idea is to combine both methods for easier modelling.
The majority of the commercial solid modellers systems are hybrid using both the B-rep and the CSG methods.
Solid modelling based on Features
A feature can be defined as a physical element of a component that has some meaning for the engineer e.g. cubes, cylinder, etc. These features must satisfy the following conditions:
- be a physical part of a component;
- be recognised as a generic geometric form;
- be technically meaningful, in an engineering point of view;
- be able to provide the component’s physical properties e.g. volume, mass, etc.
The technical meaning of the feature can involve a lot of things; the practical function of the feature, how it can be manufactured, what actions its presence initiates (e.g. movement, in the case of an axle), etc. A feature can represent some fundamental engineering parts that are very important to an engineering task e.g. a cylinder (feature) can be used as a car axle (engineering part).
The modelling based on features has been increasing in the mechanical engineering area. The method allows creating holes, chamfers, cuts, etc, which will be related with the associated faces and geometrical elements. The feature modelling is based on the idea of drawing using building blocks.
Instead of using fixed geometrical forms such as oblongs, cylinders, spheres and cones as primitives, the user creates the product model using basic drawing elements, such as a line, which are more relevant and flexible for its specific application.
This approach should make the solid modelling systems easier to use. However, the fixed set of features offered by the current modellers is very limited for industrial utilisation, which limits the design possibilities.
This exemplifies that the features must be adaptable to the users and the library must be improved. The efforts to generate a feature specification language promoted a new version of STEP, started in 1990, that allows users to define their own features based on the standard language in STEP.
Parametric solid modelling
The parametric solid modelling allows the creation of product models with changeable dimensions. The dimensions can be linked with mathematical equations. Bi-directional links among the model and the dimensions allow the automatic regeneration of models after dimensions change and the automatic update of the related dimensions. In Figure 3 there is a model with a scaled axle whose smallest diameter dimension depends on the largest diameter through the Da = Db/2 equation.
If the dimension of the largest diameter is modified, the dimension of the smaller diameter is automatically modified. If the dimension of the lesser axle is modified, the dimension of the biggest axle can be automatically calculated through the reverse of the related function. Not all parametric CAD systems have this bi-directionality, due to the complexity involved, which makes it difficult for the designer. The designer needs to think of the dimension link structure first, otherwise, the model alterations may result in the model being redrawn.
SCHÜTZER, K.; GLOCKNER, C.; CLAASSEN, E. (1998). Support of the development process chain by manufacturing features. In: SEMINÁRIO INTERNACIONAL DE ALTA TECNOLOGIA - DESENVOLVIMENTO DISTRIBUÍDO DE PRODUTO, 3., Piracicaba, 1998. Anais. Piracicaba, UNIMEP.
ANACLETO, R. C. (1991). Aumento da produtividade dos sistemas CAD através da utilização de parametrizados. Dissertação (Mestrado) - Escola de Engenharia de São Carlos, Universidade de São Paulo. ( Disponível na biblioteca da EESC - USP ).
KERRY, H. T. (1997). Planejamento de processo automático para peças paramétricas. Dissertação (Mestrado) - Escola de Engenharia de São Carlos, Universidade de São Paulo. ( Disponível na biblioteca da EESC - USP ).
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