The Importance Of Parametric Solid Modelling For Engineers
Parametric solid modelling is using software to create objects by modelling their parts and elements with real world parameters. These can vary from the simple dimensions of the model to the density of the material that is being modelled. The software is often specialized for engineering design, in our case we used PTC CREO 4.0. A parametric modeler is aware of the characteristics of components and the interactions between them.
As the model is changed and given more detail it keeps the relationships between the model’s components constant. This is achieved partly by using constraints which are related to parameters. They are relationships between some of the object’s geometric features, which the user specifies. For example, if the angle of a roof is changed, the walls will adjust to be in line with the new roof perimeter. The software would ensure that, if a hole is defined as offset three centimetres from the edge, this will always be the case no matter what other changes are made to the rest of the model.
Parametric solid modelling is a very effective way for engineers to create a working model without having to prove the boundary relationships of its components mathematically first. This major benefit allows the designers lots of freedom and speeds up the design the process. The use of parameters and constraints to define an object, create what are known as boundary representations.
Another way of creating shapes in CREO and similar commercial programs is by using Constructive Solid Geometry (CSG). CSG defines a model by combining basic (primitive) and generated (using extrusion and sweeping operation) solid shapes. It uses Boolean operations to construct the model. These allow the shape of an object to be described as the intersection of other objects, or as the union of objects. Essentially this means the resulting shape can be joined, or intersected with others, to describe 3D models of any complexity.
The final shape is an aggregate or overlap of simple shapes. So, which is better, boundary representation or CSG? Well boundary representations are more robust and are used normally for generation of complex geometries. But, CSG is very useful for simpler, especially symmetrical shapes. CSG is easy, concise and requires minimum storage. When designing these shapes and models in CREO another useful feature to engineers is its bi-directional associativity. This means that if you set the diameters of two holes in a model to be equal, if one diameter is changed the other will update automatically.
Also, if you want to make changes to the model’s dimensions at some time during the project, you don’t have to go back and manually rescale the whole model, due to the previous defining of the constraints and bi-directional associativity. Very often in industry a chief engineer or project manager will send back a design and recommend modifications. When these modifications are implemented, the whole model will adapt instantly, which makes software like CREO extremely popular.
A very useful feature bi-directional associativity creates is that when the drawings of the model are made you can change the dimensions of the object by altering the dimensions of the drawing. This relationship also applies when changing the dimensions of the model. This feature is a huge time saver because the dimensions (of which there could be hundreds) never have to be updated manually. You can imagine the uncountable hours this saves on a commercial project, where lots of individual parts of a model are changed by different engineers all the time. The drawings that CREO create include a plan, elevation and side view of the model.
There is also an option to add an isometric view. The drawings are fully dimensioned, and the user can then decide which dimensions to leave on the drawing and which to remove. This helps ensure that the drawing is not over or under dimensioned. If it is too cluttered it is difficult to extract the necessary information from the drawing. If there is not enough information provided, the engineers will not have the required dimensions to create the model. After you have completed your model or part using CREO along with its drawings, you can make use of its archive feature. For example, if you are designing an engine, you can take parts you have previously designed for similar projects and use them again. An old part maybe of great use in a new design.
Furthermore, having access to a depository of knowledge and information from other engineers can really accelerate and streamline the design process. The archive allows engineers to learn from each other and see how similar problems and challenges have been dealt with before. The archive allows engineers to collaborate with each other. Being able to archive the designs allows you to design a part of an engine separately, while a different engineer designs another part. You can then assemble the entire model at the end.
This approach is time efficient as engineers can work individually instead of them all working on the same component at the same time. As mentioned before, with a parametric approach, it is possible to add constraints to parts. This means that they cannot be changed by accident later-on in the design process. Basically, constraints are a way of making sure that any modifications made to the design are done so with design intent in mind. Design intent describes how the model should be created and how it should behave when it is changed.
With a parametric modeller it is very important to plan out the design before modelling. Design intent is built into the model based on how the dimensions’ bi-directional associativity and constraints are defined. This can lead to a lot of intelligence in a model. But, these constraints can often have references that conflict with each other, they can be very limiting if the design intent is not considered carefully. A change to a feature very early in the design can have a significant effect on how the final model turns out. Therefore, it is very important to keep the desired final model in mind at all stages of the design process.
Finally, another key concept of CREO is its Model Based Definition (MBD). This is the concept of using 3D models combined with other data, such as 3D dimensions and constraints within CREO to provide a definition for individual components. In MBD the model itself defines the part, not the dimensions. The dimensions are not necessary, and the engineer decides whether to include them or not based on their personal preference. Therefore, the accuracy of the model is paramount. The goal of MBD is to create 3D technical data packages to be used for manufacturing and other things like logistics and acquisition. These packages are readable by software which helps with time efficiency and labour costs.
