Hi!

I'm trying to make a kind of shell by sweeping two pentagons (but I suppose the number of sides doesn't mind) along a curve. But I don't get the desired effects. I don't know if I'm doing wrong or if I could get the same shape witH an alternate method. I think that with the model it's quite easy to understand what I'm trying to get but I ca'nt get. (I'm still a noob with NURBS)

Hi edlhn8, the way that sweep works is that it moves the profiles and rotates them to be perpendicular to that rail curve.

If you have a fairly wide shape, it can get kind of messy and sort of "fold over" itself if the rail has a bend that is sharper than that size.


It's kind of hard to describe in words, here is a visual example showing a similar situation:



One way you can see that happen in your specific example is if you use the Transform/Array/Curve method to replicate your profiles along that path (this just copies the curve instead of building a surface but it is similar in the way it works):






You can see there that as it travels along the rail, that long curve swings downwards when it is positioned perpendicular to the rail.

In your sweep, that shape is also shrinking at the same time as it is rotating, because it is "morphing" into that smaller vertical profile, but the shrinking is happening at a slow enough rate that it does not make the in-between profiles small enough to fit around that tight bend.

So basically the one-rail sweep won't be a good tool to use for this particular situation.

Some other possible tools which may work well are to build more rail curves connecting the points of your polygons there, and then use Edit/Separate to break things into individual lines, and use a 2-rail sweep or Network tool to build it out of smaller segments.

But since this shape has a kind of "pivoting" aspect to it, possibly the Rail revolve tool will actually work the best, I will see if I can give you some good steps for that.

- Michael

Here's an example using rail revolve - You actually just need the bottom profile and the rail for this. Start by selecting the bottom rail, then run Construct / Revolve / Rail revolve. (note that this is a sub-option alongside the regular Revolve command).

Then pick the rail curve, then pick 2 points to define the pivoting axis of the revolve, which you want to snap on to the ends of that polygon:




EDIT: oops - I thought I made a mistake here, but I think that this actually does give you the right shape that you were looking for.

You'll need to build some planar surfaces to finish this shape into a solid. To do that, construct the first one by selecting that bottom profile and running Construct / Planar. That will build a planar surface there. Select that planar surface and the result of the rail revolve and then use Edit/Join to glue them together. Then select that "almost solid" piece and run Construct / Planar again on it to fill in the last remaining hole and make it into a solid.

Let me know if you need any help with these final steps, that is using Construct/Planar in 2 different ways - if you have a planar curve selected it will build a surface for that curve, or if you have a surface or joined surface selected with a planar opening in it, it will seal that off. But it has to be a simple planar opening, that's why you have to seal off one side of this shape manually before that will work.

- Michael

And by the way, the reason why this other one next to it worked fine:




Is because the path bends more gradually there and the size of the profile is smaller than the curvature of the path, so it avoids that "folding over itself" problem that I showed earlier.

Hope that helps to make some sense of what you were running into at least!

- Michael