Hi Rudl,

> Ok, I come from TC. There I only need to choose three
> points to define the Cplane. The first is the origin, the
> second x-axis, and the third y-axis. That´s it.
>
> In MoI I must define a rotation axis for the third point, to define it.

No, you only need to place 3 points in MoI to define it - the first point is placed at the origin and then the 3 axis picker will show up.

Then you can drag the x axis to the 2nd point, and then placing the y axis to the 3rd point completes the process and it is fully oriented with those 3 picks.

See here for an example:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4239.6


However, if you place the origin point snapped on to an object, it will start out with the z axis already aligned perpendicular to that object, like aligned to the surface normal if you snapped a point on to a surface. If that z axis perpendicular is what you wanted then you can adjust the rotation around it by dragging either the x or y axis to finish the orientation. If you did not want to have the z axis aligned perpendicular to the object the origin is snapped on to, there is a checkbox option that you can uncheck to stop that from happening:



Maybe that automatic z-axis alignment is causing some confusion - if that's the case then uncheck that "Align to objects" box to prevent it.


If you're still having some difficulty placing the cplane could you please post an example file that shows what you are trying to align it to? Then I can give you a demo on your particular file to help illustrate it.


- Michael