Here are some steps... But this is starting to get a lot fancier and more difficult, if you need to do a lot of more amorphous and really non-mechanical looking stuff, you may be better off using subdivision surfaces for that style of shapes.

But here is one approach using NURBS anyway -

The big thing is you can't just start with a box, you need to more directly create a surface closer to the actual desired shape.

To do that, I'll use the sweep tool to make a more custom shaped surface.

To start with, I drew one quadrant of the sweep rails, using Draw curve / Freeform / Control points. When you use this tool the first and second points define the starting tangent direction of the curve. So if you plan on some kind of smooth mirror image like in this case, you want to make sure that second point comes away from the first right along a perpendicular direction.

So here is what I drew:

smooth_thing1.jpg


Then I selected that, and use Transform / Mirror to create a mirrored copy around the y axis. Then I selected both pieces and used Edit/Join to glue them into a single curve. This curve is made up of 2 segments. Sometimes it is nice to fuse segments together to make single segments because one single segment is guaranteed to be nice and smooth inside of it. So I turned on control points with Edit / Show pts, and selected and deleted the middle point where the 2 segments were touching, that fuses them into one single segment. Then I hit escape twice to turn off points, and selected that one and now mirrored around the xaxis. This gets you to here:




Now for a cross-section for the sweep. I decided to draw one using Draw curve / Freeform / Control points, I turned on grid snap and drew this:



You want to draw that a little off to the left side of the other curves. Now it is time to create the surface - select the last curve, and run Construct / Sweep, then select the 2 longer ones, that will generate this shape:



When you have the profile shape for a sweep off to the side like that, MoI will automatically move it and rotate it perpendicular to the rail curves. You can also have more than one, just have them in left-to-right order (all off to the side of the rails though) and they will get evenly spaced along the rails.

At this point you can also turn on points for the profile or the rails and tweak them to fine tune the shape of the surface.

Then I deleted all the curves, and used ctrl+drag on that shape to make a duplicate , used Transform/Rotate to spin it around a little bit and that gets this:



Then I selected both of those, ran Construct / Boolean / Union to fuse them together into one solid, then selected the solid and ran Construct / Fillet to round the sharp edge where they intersected. If you select an entire object, all sharp edges of it will be filleted - if that's what you want it is not necessary to pick the edges themselves, just the object as a whole. Also another option for selecting things in fillet is to select a face, that will cause all the edges that belong to that face to fillet. Sometimes that is easier than trying to select individual edges.

Ok, so in this case I applied a fillet of radius 5, and then I switched the fillet shape to be "G2 Blend" instead of "Circular". The default fillet shape is circular, but that tends to make a rather mechanical type shape since it is an exact perfect circle cross-section. If you use the blend shape, it is not an exact circle anymore, it is a more organic type gradual curve shape which is what you kind of want for non-mechanical forms like this.

That generates this shape, shown here in OBJ export with display:shaded




So the key thing different from what you were showing, is to more directly draw the shape that you want, that's kind of how the overall approach works.

That gives you a lot of more explicit control over your shape, but in some situations it is hard to draw a controlled shape of something that is totally blobby and amorphous, so the more that things tend towards that direction, the less suited the NURBS toolset can become.

Eventually at some point in the future it will become more common for CAD programs to combine both "traditional" NURBS for its accuracy and quick mechanical design stuff, along with subdivision style cage modeling. There is a program called "T-Splines" that you might want to look into if this type of design is important to you, it is kind of a fusion like that. For quite a while MoI will be more focused on the more "explicit, drawing" type methods though.

- Michael