bernd wrote:I still do not understand what the Rigid body is useful for. May be I need some visual example ...
Have a look here: https://forum.freecadweb.org/viewtopic.php?f=18&t=20381
The rigid body is used in dynamic analysises in Calculix. Only the mass of the solid is taken into account. The solid moves relative to a reference-node, which can be any point in 3D-space. A fixed reference node let the rigid body only rotate around this reference node. The solid is rigid, means it does not deform due to forces. So all those equations regarding elasticity of the material are not solved for a rigid body.
The rotation axis of the two cubes in the example is defined via a rigid body. The rotation point is given as an extra node in space (reference node), which does not need to be within the solid. In this example I defined a node in the 3D-space for each cube, with the coordinates of the rotation point.
The cubes itself can deform, ony a few nodes are part of the rigid body.
In order to use this directly in FreeCAD, one would need an input dialog, which lets one either click a vertice in 3D-space or give the coordinates as data.