Top-Down or Bottom-up?


Over the vast 8 years that I have had in the drafting and design industry, I have noticed how my processes and practices have changed. I can remember back to high school and college drafting classes; getting assigned a part to 3D model from a 2D multi-view drawing and rolling with it. I would start with a part file, model the part, make drawing of said part, put said part into an assembly, make drawing of assembly, so on and so forth. For you out there that was as naive as I was, evidently I was using a process called "bottom-up design". I learned this after getting more into the teaching of CAD and also this is the process that we use at my full time job; actually most places of business use this method.

It was only a mere two years ago when I started SLDD, that I happened upon the other method of design that is used a lot less: Top-Down Design. It was a little technique that I learned from our VAR for our CAD packages.

So what exactly is the difference between the two? The end result is the same; you will end up with what you need to get drawings out the door for manufacturing/assembly, but it's the process and modeling techniques that are totally different.

As mentioned, traditionally, a part is modeled, drawn, and placed in assemblies using Mates or Relationships; there is no "link" or tie between the part and other parts in the assembly other than the mates that define them. You create the part in the part environment with no regards to the other parts that fit in with it other than the sizes that it needs to be to fit.

In the Top-Down method, I usually create a base part traditionally; the part that ever