Impossible Models: Draft

Sometimes the modeling difficulty is the draft.

I had forgotten about this project. It was a quickie stuck in between a couple of other larger projects about 8 years ago. Of course in this one, the model, draft and the shell were kind of tricky. Here’s how it was made:

I started off with a lopsided closed loop boundary surface. This was supposed to be one of those tchotchkes you find on a bar, with a bottle of champagne or in this case vodka in it, being cooled in the ice bucket. Chilled vodka? Who am I to question the premise? Anyway, this is another case of using a computer to make something look like a natural process. It’s difficult.

Next step was a big 3D spline on surface sketch that went all the way around the splash surface, to imitate the tendrils of the splash. Somewhere between dramatic and cartoony. The real trick here was the combination of the back curvature of the splash and the outward curvature of the boundary surface. They had to combine in the right way to make it moldable. If the tendril recurved too much, it would be undercut. No lie, this was a lot of back and forth. Running draft analysis, tweaking splines, shifting the boundary surface curves, etc.

Next was a ruled surface that went all the way around the trim. This one was a little more determinate, because a ruled surface will allow you to specify the angle against a certain direction. So if the ruled surface worked, that meant I was part way there. This also helped me specify some sort of thickness for the part.

From there I created a series of Freeform features that allowed me to bulge each tendril as if it had some fluid pressure in it or something. Just another method to add shape to a predictable computer model. I had to make one of these features for each tendril around the part. Another way to do this, if you’re interested, is the Fill surface with a control curve or control point. I think the Freeform allows more control, which is probably why I went with that method here.

Then I actually used a real Shell feature on this part. I was skeptical that it would work. I selected the outside and bottom to be the open faces. I’m not sure I’ve ever used a shell feature quite like this one before.

Fillets and shell and draft have this rock-paper-scissors sort of relationship. Each effects the others, and you’ll hear all sorts of ridiculous best practice claims about which comes first. In various situations, they are all wrong. Somehow in this situation, I got away with a single constant radius fillet around the entire outside edge. I would predict at least a variable radius fillet, or maybe a set of constant width fillets.

I also added some engineering/fit type features to limit the thickness, and connect this to the bucket it was supposed to attach to. All the while checking the draft with the Draft Analysis to make sure it didn’t go out of range.

In the end, this was a fun part that was over before I knew it. It required some unconventional modeling to get the shape of the product, and also to make sure that it was moldable, and stackable for shipping.

Here’s another one. This was a plastic part with some internal turbine type blades. Of course the mold had to open and close in a straight line. In this case, I left the faces colored with the draft analysis colors so you can see the interior parting line on the part, both front and back views.

This one was a bit of a mind bender at first. I was working from scanned PDF hand sketches. The science behind the part was a little “sketchy” itself. I’ve never seen this part surface in the wild mart of commerce, so I’m not sure what happened to it after my involvement. I suspect someone found a simpler, cheaper method to achieve the goal of this product.

The main modeling challenge here was of course the draft. SolidWorks kept losing track of selections every time a change was made, so I spent more time on this model than I should have just due to bugs, or quirks, or software crankiness, or whatever you want to call it. This was one of those cases where model colors for draft was an absolutely essential technique for keeping track of everything.

The parts are moldable. I can only imagine what kind of a mess it was trying to design the mold. All you really needed to do was design the parting surface for one vane, and mostly copy that around. The PL was nearly symmetrical all the way. To me, mold design is probably the most difficult work there is, from a visualization point of view. Everything is inside out, and you have to imagine an interface between parts that you will never see. When the parting surfaces are non-planar and asymmetrical like they are here, it’s that much worse. And then you get some smartass part designer trying to make something like this in a single part instead of two or three parts – sheesh.