Hi gizmo,

> Starting from a sphere I can't see how I would be able to precisely
> replicate the shell dimensions I want?

Actually the key thing is not that you have to start from a sphere, but rather that you start from a piece that is extended larger than what the final result will be.

The sphere was just one example of such an extended shape - you can use other tools such as revolve or sweep to build a customized extended shape instead of a sphere.

Here are some steps for how to do this kind of an approach with your shape.

To start with I used these 3 curves. Especially note here that each of these 3 curves are simple planar curves, that is one of the biggest benefits of this approach, it is a lot easier to control and shape curves that are just 2D. The model file of the curves is attached here as pad_extended_trim.3dm



Select the top curve:



Run Construct/Sweep and select the central one as the sweep rail. This will create the extended shape:



One thing to watch out for there is you don't want to have a large profile shape that goes through a tight bend in the rail, which can force the swept surface to bunch up and kind of overlap through itself, you don't want that.

Now select the shape, run Edit/Trim, and select that 3rd profile curve as the cutting object:



That will cut that shape up and then the Trim command will allow you to pick which pieces you want to keep or discard, select this piece to remove:



Now with that trim complete, it will give you your basic form:



Note how that was constructed without having to build curves that swoop around in 3D?

Also another big benefit of this kind of construction is that shapes that are built to an extended simple form and then trimmed back will tend to have a very evenly curved shape throughout them. When you try to surface something to a fancier outline directly it will tend to cause different amounts of curvature in the surface as it approaches corners and pieces where your curve framework comes together.

Generally surfaces constructed in this extended manner will be higher quality in their curvature characteristics.


Now you can use Construct / Offset / Shell to turn this surface into a solid by giving it thickness:



This will cause a piece to kind of jut out past the original trim outline - you can select the shelled solid and use Construct / Boolean / Difference to slice it with that same trimming curve, which will cut the shell into 2 pieces and then you can delete this piece:



That leaves you with this shape:



Now run Construct/Fillet to round off the edges:






So to use this method, you have to construct your initial surface to be somewhat larger than your final shape will be, then you use trimming or boolean tools to cut it back to your final outline, rather than trying to construct your shape initially to the actual final outline like would tend to do with a poly modeler.

It's a much different approach and it may take a little while to get used to it.

Hope this helps show you how this approach can work though!

Once you get used to it, you can create shapes using just a very small number of 2D curves though, which tends to be really quick.

- Michael