Dynamic UCS

                     You can use the dynamic UCS to create objects on a planar face of a 3D solid without manually changing the UCS orientation.

                     During a command, the dynamic UCS temporarily aligns the XY plane of the UCS with a planar face of a 3D solid when you move the cursor over the face.

                     When the dynamic UCS is active, specified points, and drawing tools, such as polar tracking and the grid, are all relative to the temporary UCS established by the dynamic UCS.

1. Click                         the DUCS icon  on the status bar                                                                                                                                           (or)
2. Press                   CTRL+D
3. Type                    any draw command                                                                                                                                             Command: Circle
4. Move                   the cursor to the face of the 3D object on which you would like to draw. 
   
5. Click                    to begin drawing your new object.