Hi Rudl - sometimes it can be helpful to create some vertical circles that can then be intersected to find the precise amount to rotate around an axis to align to some other geometry.

If you've got 2 circles they will generate an intersection snap point where they meet, and if you want to snap to the intersection between a circle and a surface you can select the surface and the circle and run the Construct > Curve > Isect command to generate a point object where they intersect each other and then that point can be snapped on to.

See this previous post for an example:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2140.46

There 2 flat profiles are rotated by the precise amount to have their points touching by creating 2 vertical circles and snapping on to their intersection. The vertical circles are created perpendicular to the rotation axis that will be used for rotating each shape.

However, I'm not entirely certain if that applies to what you are asking for because that kind of thing involves rotating a shape around a axis, and a rotation axis is a line. In your picture you seem to almost be indicating a curved segment of the curve as the rotation axis? But rotation does not naturally occur around a curved segment like that, a normal kind of rotation happens around a straight line axis...

- Michael

Hi Rudl, so looking at your shape I think I understand what you want.

First create a point at the pivot origin point (use Draw curve > More > Point), and then hide the big sphere piece to get it temporarily out of the way.

Then run the View > CPlane command, and place the origin of the cplane on to that point - it will align the z axis of the cplane perpendicular to that planar end cap surface and then right-click to accept the default rotation placement around that z axis to set the cplane, the screen should now look like this:



Now go to the Top view - you can't use the 3D view for this next step because it ends up using the wrong plane if you snap on to a point, I think this may be a bug that I need to look into (or is it possible those end pieces are not quite exactly planar?). But if you use the Top view it will work fine.

So go to the top view and draw a circle with the center at that point, and the radius of it going through the tip of the shape, like this:



That will create something that looks like this in the 3D view:



So that circle traces out the path that I think you want to pivot the shape by. Unhide the sphere and then select the sphere and the circle and run Construct > Curve > Isect, which will generate a point where the circle intersects the sphere, like this:



That point can now be snapped on to - duplicate the edge curves as regular curve objects by selecting them and doing a ctrl+C and ctrl+V, and then you can go back to the Top view and use the Transform > Rotate command to rotate - pick the rotation origin at that first point that was created, and the first reference point at the tip of the shape and the last reference point at the circle/sphere intersection point.

Then to restore the cplane to default, do a right-click on the View > CPlane button.

Hope this helps!

- Michael

Hi Rudl, and just one quick note - those steps pivot the shape around that center balance point, it puts the sharp tip on the sphere but not the middle of the other end - to put the middle of the other end also right on the sphere you'd basically want to repeat the steps again but this time with a circle centered at the tip.

- Michael

Hi Rudl - I've attached a version that I think has the profile positioned how you wanted it.

I first rotated it around the point where it was centered on the rail to get the pointy tip on to the sphere, then a second rotation around the point tip to get the other side's middle point on the sphere.

- Michael