When working with CAD, the ability to create an adaptable model is important as it directly relates to its accuracy and functionality. Thus, one of the key concepts to consider when speaking about a model’s adaptability is its design intent. If you understand and apply design intent, the quality of your models will improve, and they will be easier to maintain.
With CAD software, such as SOLIDWORKS, it is much easier to consider design intent. In this article, I will explain why and offer a mnemonic device to help keep the principles of design intent in mind.
From a CAD perspective, what is design intent?
If we look at the SOLIDWORKS help page, we get the following statement on design intent:
“Design Intent is how your model behaves when a dimension is modified.”
In other words, from a CAD perspective, when a part or an assembly is created you should have some sort of plan for how the model should behave whenever you change a dimension or a feature within it. This understanding is based on your design intent. It ensures that the consequences of any changes to the model are predictable and will not present any surprises.
For instance, in Figure 1 I have a plate with two holes that appear to be at the center of the plate horizontally and equal distance vertically.
Figure 1. Two holes on a plate equidistant from the edge vertically and in the middle horizontally.
If I change the height dimension to the same value as the width dimension, what I want is something like in Figure 2.
Figure 2. The intended geometry when the height of the plate is equal to the width.
However, instead I got something like in Figure 3.
Figure 3. How the geometry actually looks when the height of the plate is equal to the width.
A similar issue happens when changing the width dimension. What I wanted is shown in Figure 4.
Figure 4. The intended geometry when changing the width.
But what I got was Figure 5.
Figure 5. The actual geometry when changing the width.
Looking at the original sketch, it is clear that setting the dimensions from edge to hole is the reason I got this result.
Figure 6. A detailed look at how the plate was designed.
What I could have done instead was use relations and constraints to get the result I wanted.
Figure 7. The design of the plate using relations and constraints.
Design intent is meant to help you understand how the design adapts to modifications without having to do extensive rework of the model. Using these principles not only ensures that the product meets the functional requirements, but also that it can be easily adjusted as needed.
Design intent is not a new concept; it has been around for centuries. In the past, most design intent was based on practical knowledge, empirical methods and plans. With CAD software, however, it has never been easier.
Keeping your model PRIMED
Keeping track of design intent can require a lot of different things. When you are just getting started it can seem like a huge task to keep track of it all. This is why I have created a mnemonic device to help me (and now you) with remembering the principles of design intent: PRIMED.
While creating your CAD model, these letters will help remind you of the following concepts behind design intent:
- P for parametric modeling.
- R for relations and constraints.
- I for intention of the design.
- M for manufacturing.
- E for effective use of configurations.
- D for defined sketches.
What does this all mean? Let’s break it down.
Parametric modeling
Parametric modeling is a work method in CAD in which your CAD model is defined by variables and parameters. For instance, in Figure 8, I have a model that has height, width and depth.
Figure 8. A plate defined by three variables, height, width and depth.
These three dimensions are the variables of the model. These allows you to quickly change the dimension of the model by simply changing a variable value, making the model adaptable.
Another aspect of parametric modeling is using a feature tree (Figure 9) to shape your model.
Figure 9. A feature tree can be used to shape a model.
This will not only give you an overview on how the model is created, but also allows you to roll back to a previous state, add to the model or remove parts of the model.
Relations and constraints
This part of the mnemonic is a reminder to reduce the number of dimensions in a sketch.
As an extreme example I have this sketch with 14 dimensions, each of which needs to be updated if the plate size or hole size changes.
Figure 10. This model has defined 14 dimensions. Each needs to change if one variable changes.
Instead, I can use relations and constraints to reduce the number of dimensions to four.
Figure 11. Thanks to relations and constraints, the number of dimensions in Figure 10 have been reduced to 4.
This will give me fewer dimensions to update, and will also reduce the number of update errors.
Intention of design
This item is where you plan your model. For instance, if you change the size of the plate, you should know what is supposed to happen to all of the holes.
Figure 12. Defining a hole to be at the center of a plate.
Should it still be at the center of the plate like in Figure 12? Or is the distance from an edge what you want to consider, as in Figure 13?
Figure 13. Defining a hole to be at a certain distance to an edge.
Every time you change something in the model, it can change numerous other parts of the model. These changes can also cascade into other models. Based on how you have created your finished product, you should understand these changes upfront.
Manufacturing
Keep the manufacturing process in mind when creating your model. While a part may seem simple on a screen, it can be troublesome when you are in the shop. For example, Figure 14 shows an example of what NOT to do when designing a product.
Figure 14. Keep manufacturing in mind when designing a product.
Often when designing a product, I consult with the people who are going to produce it. This helps ensure that what I have in mind is possible to manufacture. More often than not, this leads to design changes, but also points me in the right direction on how to make sure that the manufacturing will be easier and less costly.
Effective configurations
Configuration are great tools to use when working with design intent. As a designer, you can often find yourself in a situation where you have a CAD model that is “the same but different.” For instance, in Figure 15, the basic shape of the model is the same, but the size differs.
Figure 15. These models are all very similar but different.
If this is the case, I would recommend using configurations to help make it easier to produce these similar designs.
Figure 16. Configurations help simplify making similar but different designs.
This has multiple advantages as you don’t end up with as many files, and you don’t have as many places you need to make adjustments.
Just keep in mind that with too many configurations you can lose the big picture, which can also affect the performance of the model.
Defined sketches
Finally, it’s important to fully define your sketches. Within, SOLIDWORKS, a default color scheme is used where a fully defined sketch is shown as black and an underdefined sketch is shown as blue.
Figure 17. An underdefined sketch in blue.
Figure 18. A fully defined sketch in black. A hole might move outside of the solid model.
An underdefined sketch can cause problems when a part of the sketch is changed. For instance, a hole might move outside of the solid model, or an edge could move to an undesired location.
Figure 19. A hole might move outside of the solid model when changing an underdefined sketch.
Clearly, design intent is a very important part of CAD modeling as it helps ensure adaptable designs. If you apply the PRIMED principles to your models, it will ensure that your models will stand the test of time. Whether you are working on a simple part or a complex assembly, incorporating design intent into your workflow will lead to more efficient, accurate and cost-effective designs.