If the cut plane has a fixed offset from the top plane, how do you make that plane move when the surface it's mated to moves? Won't something be over-defined with this method? Also, it's not clear if the original poster is interested in animating the spring or just being able to draw the spring uncompressed but show it compressed in the assembly. If animation is not required, often you can create different configurations in the spring part file and assembly files. Make a configuration for "Fully compressed" and any other positions you require to show.I offset a plane from the top plane, and create an extruded cut off that new plane away from the top plane, actually making the spring look shorter. Then in the assembly, mate the spring top plane to the stationary surface, and drive the new plane off the moving surface. When the moving surface changes, the driven plane will move, forcing the cut to change positions.
I know it's long time ago, but can you give more instructions? I just cannot make it happenHi, this is another way to create a spring that will update with out the need to have calculations happen. Add a new part in the assembly and sketch a line to join the two points at the spring location ends. Then on the same plane as the path sketch a circle to the wire size and place horizontal to one end of the path at a distance that would be the rad of the spring. Now sweep the circle along the path using twist and the number of turns req. When you move spring anchor point the path will change length and the spring will update. The attached files shows this plus how I finish the ends off
Hope this helps