Assembly Building

Back in the day, when Inventor was still in diapers and I was going through initial training on the application, we were taught to start building our 3D models not in the Part environment, but rather in the Assembly environment. The reasoning was more often than not, you’re going to be adding that part into an assembly anyways, so why not build your parts while in the assembly?

This process worked fine and dandy at the time and worked fairly well. The problems we began to run into dealt with the automatic associativity of those parts to the other parts in said assembly. Now, don’t get me wrong, that may work very well for you, but you must keep in mind, if and when a part changes, it changes everywhere, in every assembly that part resides in. Again, this is typically what you may want, but not always. This issue needs to always be in the back of your head of course and you need to plan accordingly.

The simple fix was to break that associativity (if desired) once the final design of the parts are completed. During the initial design process, this associativity is a huge time saver and a very powerful tool. But at the same time, I’ve seen numerous instances where this wasn’t understood and created havoc during revisions to some parts. This made a lot of engineering/Design groups to shy away from such and build the piece parts as stand-alone and then add them into the assemblies as needed.

These assembly building processes are known as Top-Down and Bottom-Up respectively. But there’s a 3rd option, Middle-Out. This is where you have an existing assembly, sub-assembly, or components and you create the remaining components to that assembly from within the assembly file itself. Confused? I hope not. If you are, I hope this article will clarify things.

In my experience, the most common process tends to be a combination of these 3 options. Chances are you’ll already have an existing assembly that is very similar to the one you’re needing to create. You’ll want to use that as a starting point for what you need. If you use Vault, take advantage of its Copy Design process. I used the Copy Design function for about 15 yrs or so now and I can’t imagine the cost/time savings it has provided. Seriously, it is worth the price of Vault itself. Maybe I’ll do a blog on such if there isn’t one done already or by then.

So, without this article getting too long, let’s look at each in a nutshell.

The Top-Down Process:

This is best used when you have a 2D layout sketch/Skeletal Model (that is a subject I will no doubt cover soon here) of the assembly design criteria. This sketch can then be used as a base to create the individual components within the assembly file.

This layout/2D sketch can include profiles of surrounding components for which your design my interact with or even need to stay away from, such as other machines, walls, columns, etc. allowing you to design within the required envelope and avoiding clashes, neat huh? The best way to do such is create that sketch in a part file and place it into your assembly file. This is an example of Skeletal Modeling.

Keep in mind, this process creates components that are associative and changes to any part can cause changes to one or more parts depending on the associativity between them, especially any parts that are connected to each other.

The result is an assembly of inter-related parts.

Bottom-Up

The Bottom-Up assembly process is when you have created the individual components as stand-alone parts (or sub-assemblies) and then add them into an assembly file, applying needed assembly constraints (mate, flush, etc) to replicate the actual process of building the assembly as you would on the assembly line.

Note: since the individual components were created outside of the assembly model and thus not adaptive, if you need them to be adaptive to another component in the assembly, you can make the part adaptive within the context of the assembly.

Middle-Out

This process is where you have an existing assembly or components and design other components as required to complete the design. This process lends itself to where you can analyze design intent and insert or design new components in place as required.

Pictures were taken from the Autodesk Knowledge Base pages.