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 everything attaches to. I model it and save it, done. Then UNLIKE Bottom-Up design, where I would move to the next part and do the same; I actually bring that into an assembly next. Once I bring it into the assembly, I save it.
Depending on the software I am using, whether it be Solid Works or Solid Edge, my next step in the Top-Down process is to create the next part "in place"; Both of these systems can do this. When you create a part in place, the rest of the assembly will fade out to a transparent color. This way you can actually use and grab existing geometry whilst creating your new part. I found this method is FANTASTIC for creating tooling or nest fixtures.
Solid Edge uses a command called "Inter-Part Copy" to grab geometry and bring "surface" models of that geometry into the part you are creating. This geometry can be a face, feature, or body of another part. One thing to keep in mind is that this surface geometry is "linked", meaning that if that assembly changes then the surfaces update to reflect that; it is very much parametric like the rest of the CAD system.
In the picture above you can see the purple "Surfaces" that I grabbed from parts in the assembly to aid in modeling this sheet metal panel in place where it needs to be,
Solid Works can do the same, albeit, using a different command of course. It uses something called :Convert Entities". Simply put, once you have in-place edited a part, create a 3D sketch and click on the "Convert Entities" button. Then start picking edges and faces that you may need to help define the new part. At this point I usually get out of in-place editing, and open the new part in its own environment and start modeling using the geometry that I grabbed.
In either program, once the part is modeled, and you open the assembly, you will find that the part will be placed exactly where it needs to be in the assembly. It will be grounded or fixed in that spot since that is where it was designed. You will not want to un-ground or unfix it and re-mate it; doing so may have adverse consequences since you would be moving it in relation to where it was designed.
Another method that I use quite often when designing tooling or nest fixtures is to place a "Part Copy" of the part needing placed or modified into a new part document. When placing it, I choose "as construction body", which then puts it in as a surface body. I can then build my fixture around it and not constantly have to look back at drawings or the other model for dimensions.
Top-Down is a very powerful tool, but it can also be very frustrating when a design changes; it will require design intent on the designers part. It is great for the initial design stages or for a design that does not have variations or options. In my full-time position, we use parts that are used across the board and in various different models of our product. A Top-Down process would not be very good for our engineering dept as changes would not be able to be predicted.
So give it a whirl, you will not regret it. I love using Top-Down design practices for SLDD, but I also know when there is also a time that Bottom-Up is the way to go!
Keep in mind I was very broad with this subject, if you have any specific questions as how to do this, please shoot me an email at email@example.com