Machine Design

Modeling smart assemblies

Intelligent modeling software makes building complicated assemblies a snap.

Mike Hudspeth
Senior Designer
Tyco/Healthcare Hazelwood R&D
Hazelwood, Mo.

If the red component in this smart assembly moves, the vertical pipe will change its length to keep everything aligned.

Care must be taken when setting the conditions for the assembly. However, modelers that can "flip mates" keep the task from becoming a nightmare.

With intelligence built into an assembly, retrieving a bill of materials is almost automatic. Most systems will even insert item bubbles if required.

Today's modeling software lets designers pretty much create anything they want. But unless the product — be it a battleship or mp3 player — is a one-piece item, an assembly will be needed. Building a digital assembly model was once an arduous task. But thanks to advancements in "smart" 3D-modeling tools designers can now build assemblies quicker and more efficiently than ever before.

The Assembly Dark Ages
Before "intelligent" modeling, designers created separate files for each component then manually positioned renderings in place. The result was a pretty good snapshot of how the assembly would look. But the technique often required downloading enormous (several megabytes) files for each component. These files could easily overwhelm the computer's RAM making it slow going to do simple operations such as rotating a part to reposition it.

Additionally, if one part needed modification, its mating components would also have to be manually modified to accommodate dimensional changes that would impact how the parts went together. Likewise, if a hole size or location changed, a bolt or shaft that was to go through it might no longer fit, or could be left hanging on the screen in midair.

There was also a constant risk of error. Before the assembly could be used, the designer had to check every dimension to make sure the most recent or correct models had been imported. One little slip-up and production schedules would possibly be thrown out the window. Verifying each part was a labor-intensive proposition. As changes were made to each component the assembly file would have to be reopened and all the changes repeated or reimported, then it would have to be manually replaced in the rendered assembly. If numerous parts changed, the workload could increase tremendously.

An Assembly Renaissance
Thankfully, most recent 3D modelers have "smart" assembly capabilities. These packages let designers bring components together in one file and constrain them in such a way that each part "knows" where it will go and to which other components it will mate.

The software also predicts how each component will act in the final assembly.

File sizes are much smaller in a "smart" assembly because the components no longer reside within the assembly file. Instead a small pointer tells where the file may be found. This means a lightweight representation of the component can be loaded, displayed, and manipulated without actually having to load the entire part into memory. This improves functional performance across the board. Because the assembly file is intimately connected to each component part file, any changes made are reflected immediately in the assembly. There is only one source of information being referenced, thus the possibility of an error is significantly reduced.

Smart assemblies also give designers a dynamic picture of their device. In most smart 3D modelers, designers can click onto a single component and drag it to a new location. As the part moves, all its mating components move with it. Because the piece parts are constrained to move only in prescribed ways, the intended range of motion can be seen and tested. It's possible to quickly see interference between parts. If the software also supports collision detection, the motion will stop at the point of contact with another component. In short, the assembly can function on screen as it is intended to do in the field. This can save time and expenses associated with the physical testing of the assembly prior to building the first prototype.

Some modelers capture the assembly's motion as a movie that can serve in an online instruction manual if needed. The software also lets designers program assembly behavior to meet application demands it might be exposed to. For example, designers can show a customer or vendor how the device will function under different operating conditions without physically performing the test themselves.

Drafting also benefits from smart assemblies in ways the older methods never could. True, designers could always build an exploded view of an assembly, but they would have to employ multiple copies of each component in several positions. This would drastically increase file size.

With recent 3D-modeling techniques employing constraints, designers need only "tell" components how to behave in a particular view. Some software can even put in phantom lines to show where parts go. Additionally, smart assemblies keep track of the component part files, so they can automatically construct a bill of materials (BOM). They can even keep track of part quantities. So when a design updates, such as by eliminating a screw, the BOM updates automatically. Most 3D modelers will keep track of what components are used where and can attach a standard bubble callout to each. The software, however, lets designers eliminate certain bubbles to clarify complicated assemblies. And there's no worry about whether every part is tagged because the assembly automatically does it.

With careful effort up front and knowledgeable application of mating conditions, designers can construct a model that captures just about anything the assembly needs to know. Mating conditions are constraints (surface-to-surface contacts, component separation, tangencies, and alignment) applied between components that tell the assembly what it needs to do. These mating conditions add intelligence to the assembly. Designers can apply constraints to a component before it's added to an assembly. By telling the part what surfaces or edges it's intended to mate to, designers can give it a predisposition to automate its placement. This saves time when building with commonly used parts such as bolts.

But new capabilities usually drag along new eccentricities. Smart assemblies are no exception. For example, when trying to mate one part to another, the software will sometimes flip the part around. This can result in a bolt whose head is buried inside the part while its threaded end sticks out. The ability to "flip" mates lets designers quickly rectify this situation. Software without flip-mate capability forces designers to delete the mating component and then reapply it in the correct orientation.

Another capability of smart assemblies is its extensive reporting. The software compiles a list of components in the assembly along with their file names, revision level, and directory location. Of course, smart assemblies don't ensure product success or tell if a part is well designed. They will, however, make it a lot easier to accomplish those things.

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