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Modelling methodology in CATIA V5 - Part 5 Posted by Alex Fernandes on 01-Dec-2017 12:48:41 Find me on:
The modelling stage
This is the fth article of a series concerning how to implement and use modelling methodology in CATIA V5.
In this article, we will discuss the modelling stage in detail.
This is the second stage of the creation process; we already have all the elements de ned in the Skeleton group, so now is the time to start modelling the solid geometry.
We discussed, in article 2, the Hierarchy rule that strati es all elements in a part le into di erent levels. For the modelling stage, we will be creating elements presented in the last level of the Hierarchy rule; solid geometry features.
The fact they all belong to the last level of the Hierarchy rule is does not mean that we can create features at random without having to worry about their order, far from it in fact. Figure 1 represents the model as it was at the end of the Design stage, discussed in articles 3 and article 4.
Figure 1 - The Design stage of Angle Bracket part
The PartBody is a First Level container. In it, we will insert the solid modelling features that de ne the modelled geometry of a part. Let us have a look at gure 2, where we have represented the modelling stage of the Angle Bracket part we have been using in this series.
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Figure 2 - The Angle Bracket part’s modelling stage
The PartBody is inserted by default in every part le, so we do not have to create a solid body to store our solid geometry. We will leave multi-body modelling for a later article, so for now all the solid geometry is stored inside the PartBody.
The methodology presented here serves the purpose of building robust models without making their features either too brittle or too independent. A brittle model has such an intricate balance of associativity between features that it becomes uneditable. A model with all features independent is built using a methodology called horizontal modelling and gives the user maximum robustness by sacri cing the bene ts we can get from having feature associativity and design intent is lost upon modi cation. The great advantage of CATIA is that it can automatically update all children elements of an existing feature when it is edited, maintaining design intent upon edition. We will use associativity to our bene t, without overdoing it and creating a brittle model in the process.
We are going to create features organized by groups, according to their function in the model and use associativity to our advantage inside those groups. This will help us maintain design intent upon edition but in a controlled way, so we can edit the model without it collapsing in a domino e ect of unresolved features.
The feature groups Figure 3 represents the PartBody structure. It also summarises the renaming convention used for each group as well as typical features and some rules applicable in each group. All groups are presented in their logical order of insertion in a model.
Figure 3 - PartBody structure, with functional groups
Inserting features in a structured logical method will promote correct level of feature dependencies in a model; these feature dependencies are called “parent-child relationships” and are created every time a feature uses inputs that are de ned by previously created features in the tree.
The usage of a correct methodology helps create these dependencies, only and wherever they clearly de ne the design intent. It avoids creating unnecessary feature dependencies, making the model more robust and the tree more exible for reordering if necessary.
Parent-child relationship is sensitive to feature sequence, thus using the correct one will minimize the number of features to edit in a part and minimize unresolved features failures.
The logic used for feature de nition is the same to be used for feature edition, starting from the beginning of the tree and going down. As we edit features in the model, their respective children features will be resolved with the new inputs de ned by their parent features.
A structured speci cation tree is easier to analyse and thus easier to edit as well.
The importance of organizing features by groups has to do with guaranteeing a structured modelling input logic. The groups, presented in gure 3, are to be interpreted as functional blocks; each one will ful l a role according to the prede ned order presented above.
Features are inserted in groups according to function in the part’s modelling sequence. A speci c type of feature does not necessarily belong to a speci c group in every single model, their use can be typi ed inside speci c groups but remember features are inserted in a group according to their function in the model and not by type.
Figure 4 presents the PartBody structure for the Angle bracket part that exempli es this methodology in this article.
All features are renamed, according to the group they t into; considering their function in the part, and renamed according to the geometry they de ne and the location where they are de ned. Additional information, such as diameters and radii, is written in the feature to help model interpretation.
This particular part, the angle bracket, does not require all the groups presented in gure 3 to be correctly de ned; for this reason, the Detail group and the Modify group are not in the speci cation tree presented in gure 4.
The Core group, black group in gure 3, is the rst set of solid modelling features in a part and de nes the model’s main shape, its extents and orientation.
Figure 5 - Angle bracket Core group features
The rst features to be inserted should be the ones that use imported reference geometry as inputs, as these elements drive the geometry of the part when it has to interact with other components at assembly level, so these features take precedence upon all others.
In the example, presented in gure 5, there are no imported elements, all core features were inserted using sketched pro les de ned previously in the Skeleton.
Features in this group can be linked to other elements that were created before them. If you see solid geometry in the background, then you can link your features to it because they will be other core group features or imported solid geometry.
De ne additive features rst and then the ones that remove material, subtractive features.
Edit the core features to make large changes to the model’s main shape.
Core group features are pre xed by letter Cxx-