Assembly Motion: Should You Use It?

CAD can be addictive. Some of us just make CAD models on the weekends for fun. We walk through the hardware store or Walmart just looking for cool models to build. I’ve heard sites with a lot of cool CAD images described as “CAD porn”. It’s true. Some of us are ill and just can’t help ourselves.

Assembly motion just for the sake of motion is kind of the same thing. It’s cool. It looks cool. It moves. Oooooh, ahhhhh. Cars, grills, hand or power tools, construction equipment.

But then some of us also use CAD for work, and we have to consider what’s practical. How practical is cursor-dragged motion in production models? Do you really need it? To what extent does cursor-dragged motion represent action real-world motion anyway?

Motion can be expensive when it comes to a production model. Expensive in terms of all the time you spend getting the mates to work.

In a recent article I wrote about making multiple models for downstream processes. Motion – in particular just motion that you get by dragging parts with the cursor around the screen – are CAD porn. They don’t serve a real purpose aside from visualization and wasting a lot of time. But sometimes the visualization itself has some value. That’s what you have to decide – how much value does the ability to move parts around the screen have for your particular project. Maybe moving parts around the screen is one step in creating a simple animation, which can have value communicating how a mechanism works with non-technical people.

Motion, at least 2D type of motion, can often be handled with sketch blocks. Lay out each part as a set of sketch entities, make each set of entities that represents a part into a block, and then you can use the blocks together in the same way that you’d use an assembly, but you’re only using a sketch. It’s smaller, takes less time to put together, and it should run faster and more reliably. Plus, then once you have verified the motion should work using the 2D sketch blocks, you should be able to model the 3D parts around the 2D sketch blocks. You don’t always have to solve problems using the most complicated solutions available. Sometimes the simplest method will be the most economical and the best overall in the long run.

Sketches shown on the assembly can also display how the motion is intended to work. Beyond that, the alternate position assembly views in drawings can explain a lot. Again, though, you need to ask the question if the documentation you’re creating actually requires information about motion or different positions. Certain part manufacturing information doesn’t need motion information, although assembly documents might, and testing or QC docs certainly do.

To manufacture, all you need to know is the part geometry. To assemble the manufactured parts, you have to know what the final product looks like. If your parts are designed, manufactured and assembled correctly, the motion should just be a by product.

But sometimes you need more. In cases like that, you might need motion analysis or simulation or possibly just a real animation. In that case, you probably need a different assembly. Remember from my previous article, you probably can’t make a single assembly file do everything you need it to do with a single assembly. You probably need to have copies of that assembly. Not configurations. Different assemblies.

So give it a thought. Does the documentation you’re currently creating actually need assembly motion data or information about other positions, or are you just adding it to play CAD voyeur?