Part creation process
This is the first article of a series concerning how to implement and use modelling methodology in CATIA V5.
In this article, we will cover the creation process for a part file. We will break it down in two stages; design stage and modelling stage.
Let us imagine you need to edit a part, changing some parameters in existing features, and that the geometry update process fails after modification. Models that fail after editing are brittle and they work fine if you do not edit them!
|Figure 1 -Design and modelling mixed together, brittle model||Figure 2 - Corrupted model after parameter edition|
In figures 1 and 2, we have an example of a very simple model that is brittle. A parameter edition completely corrupted the geometry of the part.
Next step, you decide to make a thorough evaluation of the model and you conclude that you cannot interpret, decipher, the modelling process flow because the tree is not ordered properly, design intent is not captured correctly because features have unnecessary dependencies and elements are not properly identified. After this, you would have realized you had three options:
- Correct the entire model, which would certainly be very time consuming;
- Model over existing geometry, using what we call undo features, these are features that eliminate geometry created by other features to create the desired final shape;
- Start a new model from scratch that respects the design intent you need to define in the part and guarantee the necessary final shape.
Usually a designer in a hurry tends to go the second option and this only contributes to make the model even harder to interpret the next time someone needs to modify it, creating a snowball effect. In the end, this leads to uneditable models that have no design flexibility, pretty much like a dead solid; they work but you cannot modify the features that were used to create them.
Trying to correct the model will often imply redefining design intent elements as well as redefining the existing features to consider those elements. This correction process can take too much of the designers’ time.
The third option is the one that is risk free because we create an independent file and we can guarantee design intent capture as well as the desired final shape, while going through the creation process.
The first stage in the creation process is the design stage. It concerns the implementation of a thought process in a part, effectively a plan to create the part. It encompasses the definition of all the needed features and their sequencing, to achieve the desired final shape, while guaranteeing an editable part, correctly parametrized that remains valid after editing.
During the design stage, the designer will define all necessary sketch elements; all reference elements needed and will insert all the external reference elements necessary to create the features in the part during the modelling stage. These elements will capture and define design intent clearly and should be located in a specific geometrical set, outside the PartBody.
Design elements are hierarchically superior to modelled geometry, this means that design elements can only depend on other design elements and must not be dependent on modelled geometry. The only way to guarantee this is to create them first, before any modelled geometry.
Figure 3 - The design stage elements
In figure 3, we can see all the elements created during the design stage. Notice how all elements are organized, renamed and numbered, all existing outside the PartBody.
The second stage in the creation process is the modelling stage. It is the 3D creation stage; we will add physical reality to our part by creating all the features that constitute that part’s body, respecting the feature sequence defined in the design stage without creating any unnecessary relations between features.
The modelling stage will create features in the specification tree that are dependent on the design elements and finalize with features that modify existing geometry; dress-up features, transformations, patterns.
Figure 4 - The modelling stage elements
In figure 4, we can see all the elements created during the modelling stage. Notice how all elements (features in this case) are organized, renamed and numbered, all existing inside the PartBody but depending on sketch elements stored inside the skeleton geometrical set.
In this article, we discussed that any editable part created in CATIA V5 is a mixture of design elements and modelling elements. Design elements are hierarchically superior to modelled elements and should be created first. This will happen naturally if a designer understands that design elements must be created before modelling geometry and not during the modelling stage. This method will definitely improve model robustness while maintaining design intent and editability.
In the next article, we will talk about part structuring and specification tree organization.