SolidWorks beginner tutorials – What’s a top down assembly?

SolidWorks Beginner Tutorials – What is Top Down Assembly? 

A common question we hear asked by beginner SolidWorks users is, “What’s a “top-down” assembly?

This working strategy applies to any 3D CAD software modeller, such as CATIA, Inventor and others, not just SolidWorks, but I’ll post it here at our SolidWorks pages for quick reference, as well as in our general blog.

In fact, there are a few methods for building an assembly: “top-down”, “bottom-up”, and “middle-out.” Chances are you’ve been using all three of these methods already without even knowing what they were called. One method is not intrinsically better than another; they are just different, and called upon based upon your design circumstances.

In our SolidWorks video tutorials, we use all three methods of assembly design, so you’ll see many examples of all three approaches.

In a nutshell, a “top-down” assembly means you start out working from the Assembly tab, using the insert or create component tools to design your model, and thus you build your parts right in place, so that you don’t have to “put them together later on.” That’s why it’s called “top-down”: your design intent begins from the assembly level, where you plan how all of your parts will work together, and then you cascade your engineering down to the individual parts.

This is great for when your parts may move or change in size; if the sizes of some parts are defined in relation to other parts, then a change in one part’s size will not required you to modify static dimensions in all the related parts. If your assembly is really big and complicated, with many parts, then the rebuild time is longer with a top-down assembly (that contains many in-place parts).

How you’d generally start (and you can see this in our SolidWorks beginner tutorials) is with a layout sketch that defines critical dimensions, part locations, and then build your 3D parts so they fit the sketch. The layout sketch is a good tool for saving time, for experimenting on what will work well in your design. You can try several versions of the design before you detail the individual parts completely. The layout sketch also lets you make critical changes in one central location, with the related parts updating automatically.

All the components you create this way are, of course, adaptive and associative, so that if you change one relation in your geometry, the appropriate related components will update accordingly. Part that you create are internal to the assembly, but you can also externalize them so that, for example, you can re-use the parts in other models and assemblies. Mates and other constraints are sometimes applied automatically by the software. You need to be careful about creating too many mates as this will create errors in your geometry.

Generally, a top-down approach is handy when you don’t know all the dimensions of your parts and you need to reference a dimension from another part. 3D CAD modeling software like Solidworks, CATIA, and Inventor let you edit parts whilst your working in your assembly, so your source geometry is always available for reference.

As you may have guessed by now, the bottom-up method of assembly design works the opposite way. Our SolidWorks tutorials cover this, too. You start in the part modeling environment, and create separate part files for each element of your model, which you then put together in the assembly environment. There is no automatic mating of parts; you apply the mates and other relations manually. All the parts of the assembly are external references, and can be referenced in other assembly files. Generally you work this way when you know the size or dimensions of your parts.

The “middle-out” method of assembly design makes use of both top-down and bottom-up strategies. So, some parts are created in-place (from the assembly tab), and other parts are created from the part modeling environment, and then later referenced by the assembly. Most of the time you’ll end up working this way, actually. For example, for the parts where you have dimensions, you create in the part modeling environment. For the parts whose dimensions are dependent on other parts, you can create from within the assembly environment.

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