The modelling stage
This is the fifth 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 defined in the Skeleton group, so now is the time to start modelling the solid geometry.
We discussed, in article 2, the Hierarchy rule that stratifies all elements in a part file into different 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 define the modelled geometry of a part. Let us have a look at figure 2, where we have represented the modelling stage of the Angle Bracket part we have been using in this series.
Figure 2 - The Angle Bracket part’s modelling stage
The PartBody is inserted by default in every part file, 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 sacrificing the benefits we can get from having feature associativity and design intent is lost upon modification. 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 benefit, 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 effect 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 defined by previously created features in the tree.
The usage of a correct methodology helps create these dependencies, only and wherever they clearly define the design intent. It avoids creating unnecessary feature dependencies, making the model more robust and the tree more flexible 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 definition 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 defined by their parent features.
A structured specification 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 figure 3, are to be interpreted as functional blocks; each one will fulfil a role according to the predefined order presented above.
Features are inserted in groups according to function in the part’s modelling sequence. A specific type of feature does not necessarily belong to a specific group in every single model, their use can be typified inside specific 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 exemplifies this methodology in this article.
All features are renamed, according to the group they fit into; considering their function in the part, and renamed according to the geometry they define and the location where they are defined. 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 figure 3 to be correctly defined; for this reason, the Detail group and the Modify group are not in the specification tree presented in figure 4.
The Core group, black group in figure 3, is the first set of solid modelling features in a part and defines the model’s main shape, its extents and orientation.
Figure 5 - Angle bracket Core group features
The first 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 figure 5, there are no imported elements, all core features were inserted using sketched profiles defined 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.
Define additive features first 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 prefixed by letter Cxx-
The Detail group, presented in blue in figure 3, is the second set of modelling features in a part and defines smaller geometrical entities that will help define the part without significantly altering its size and shape.
Features in this group are independent from each other, this way they can be edited, deactivated or deleted without affecting other features inside the detail group.
Features in the detail group can be linked to features created previously in the model. If you see solid geometry in the background then you can link your features to it as long as they are either core group features or imported solid geometry
Just like in the core group, define features that add material first and then the subtractive features.
Edit the detail features to make small changes to the model’s shape.
Detail group features are prefixed by letter Dxx-
The Hole group is the third set of solid modelling features in a part’s specification tree, it is presented as the green group in figure 3.
Figure 6 - The Hole group features inserted in the model
The Hole group is a specification of the Detail group which has hole features only. It is defined as an independent group only to emphasise the importance hole features have in a part.
Features in this group are independent from each other, this way they can be edited, deactivated or deleted without affecting other features inside the Hole group.
Features in the Hole group can be linked to features created in previous groups; if you see solid geometry in the background then you can link your features to it as long as they belong to either Core group, Detail group or imported solid geometry.
Holes should be renamed with descriptive information of hole type, location, size and threading.
Hole group features are prefixed by letter Hxx-
The Modification group, orange group presented in figure 3, is the fourth set of solid modelling features in the tree. This group defines the final features of the part.
Apply replication features are applied first, usually patterns and mirror features when needed. Face refinement features are applied after replication; usually thickness, draft angle, thread.
Avoid replicating core features as these control the main dimensions of a model, apart for symmetrical parts.
Modification group features are prefixed by letter Mxx-
This group is the fifth and final group of features in a part. It is the grey group in figure 3. It has volatile features that are failure-prone after model editing; for that reason, they are left for last.
Figure 7 - The Quarantine group features in the model
Quarantine features are the last features in a model and should not be linked between them although they are always linked to previously created geometry.
Typical elements include all chamfers, rounds and fillets. Start with the largest for each type and proceed to the smaller ones.
Transformation features (that change the location of geometry or re-scale it) also belong to this group.
Quarantine features should be renamed with descriptive information of type, location and size.
Quarantine group features are prefixed by letter Qxx-
In this article, we discussed the PartBody’s structure, naming rules for features and feature sequencing and creation order.
In the next article, we are going to discuss some basic rules for feature definition while we are in the modelling stage of a part.