Editor's Note: This post was originally published in February 2022 and updated with new information in September 2023.
Top-down design is a methodology used in CAD to simplify the development of products with complex interdependencies and relationships between components.
Take an electric sports car, for example. As a machine, it needs to be functional and practical. The silhouette of the car is smooth and streamlined to lower drag and conserve battery power.
Beneath that exterior is the battery, along with a mess of wiring, computer chips, sensors, screens, and countless other parts.
Top-down design isn’t a literal definition of the method. It merely describes the process of designing from a basic sketch—that sits at the top of the model tree—and then adding more complexity. Your sketch can include whatever base components are necessary to communicate design intent and provide a platform for additional geometry.
The best part is that top-down design methodology can be applied to nearly any complex machine including large-format printers, MRI and other medical scanners, small appliances like hair trimmers, and more.
Take our electric sports car example. The first work is typically conceptual, concentrating on the overall shape and function of the car. Then various teams can work on details and motion, adding the gears and gadgets that give it power and allow for driver controls.
One team adds a rudimentary drivetrain and other interior structural supports while others work on the headlights and fenders. Another team develops the outer skin to ensure the panels all match up properly, while still others add mechanisms and other interior components that sit beneath them and attach to them.
One alternative is, of course, bottom-up design. The name might bring up obvious differences, but they’re more nuanced than you might expect.
As stated above, the top-down approach begins with the overall concept of the product. Think of it as the high-level vision of what the product is supposed to be, what it does, and who it’s for.
Nailing down those details helps you create the roadmap from an initial idea in your head to a product on the market. From there, you begin to break down everything into smaller systems and projects.
By contrast, bottom-up design begins by using or building components that eventually merge into a larger, more complex final system. It’s useful when you understand some pieces, but not the overall picture.
These projects typically begin with several smaller, specialized teams that have the flexibility to spend more time and budget on iterating and validating components.
A variety of industries benefit from top-down design techniques. When it comes to engineering, three of the most relevant are Product Design and Development, Computer Science, and Management and Organization.
Product Design and Development: A bottom-up approach is okay when you have more ready-made components that fit together. However, if your product requires components with custom materials and geometry, and budget-saving simulations, the top-down approach is a great fit.
Computer Science: Modern machines can’t run without capable and reliable software. The time spent writing code modules must be done with a good understanding of the overall purpose of the software and how each module contributes to that purpose. Top-down design ensures that this understanding is fully developed at the beginning of the project before a single module is written.
Management and Organization: Directing a large organization requires a lot of decisions every day. This is a case where a combination of methods contributes to a vibrant, loyal company culture. Leadership, with their strategic vision, make decisions for the good of the organization’s long-term goals. However, the company also benefits when leadership is open to ideas and feedback sent up from employees.
Let’s return to the electric car. When you design the car, you try to imagine the entire experience. Along with the battery and other electronics mentioned before, there are seats for comfort and safety, gauges for information, and the fun extras like built-in satellite radio.
The top-down approach helps you get clarity on all of these systems, the differences between them, and how to break the entire vehicle down into smaller, more easily designed pieces.
Even with various teams working at the same time, you can keep the design intent all in one place. Each of the specialized teams can easily collaborate among each other, while solving problems on their own and sharing results among the entire company.
Managers of the project also have deeper insight into every step, allowing for faster, more informed decisions that keep the project moving, deadlines met, and ensure everything matches up to the organization’s strategic goals.
Heady discussion about approaches is great, but how does it impact your day-to-day work? Glad you asked!
Master models typically have information about the dimensions, surfaces, and overall layout information for assemblies and subsystems. This information can be accessed by each part, ensuring continuity across the entire model.
The terms master modeling and skeletons are sometimes used interchangeably, but there are nuanced differences (more on skeletons below).
Assembly context is critical when you need to understand how the part you’re actively working on fits into the whole, and whether any changes you make will cause ripples upstream and/or downstream. A master model allows data sharing throughout, making it easy to use a single part or assembly in multiple contexts.
Multi-body part designs no longer have to be separate from top-down design. When used together with the master model, multi-part offers faster part conceptualization, simpler assemblies with interdependencies, and more robust data sharing features.
Save time with skeletons: When you start the design, like our car example, the first basic sketch is known as a skeleton. It’s the central object to capture design intent, along with tracking references and interdependencies within the design. Skeletons can also have some early-stage motion, or kinetic features.
Relationships between all the components and features are also managed automatically.
Ultimately, what this amounts to a lot of time and money savings. Without top-down design, a small change to any part of a model becomes a big job, because that change may affect dozens or hundreds of other components in the design. With top-down design, the data is propagated from the skeleton to all design components. Making small or large changes almost anywhere in the design is easier because it will be applied automatically to affected components. If the change affects calculations that are stored in the accompanying PTC Mathcad notebook, those will be updated, too.
Whether you’re working on existing models or starting fresh, there is a powerful set of tools and techniques to help you make your work more efficient. Enhanced team collaboration, shorter development timelines, higher quality products, and reduced wasted can all be realized through the top-down design approach.
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Mike Gayette is a marketing professional and freelance writer based in North Dakota. He writes about engineering software, marketing technology, customer service, and team building. He also spends time at the local humane society as a dog walker and cat entertainer.