hmmm now I don't know how to forumlate my question
ok I'll try
a screw and nut combination. the nut is fixed. the screw rotates and at the same time translates along the axis of rotation. with me?
where do I place the joint and where is it's direction? (in the property manager for screw joint)?
for example, do I place it at the egde of a top of the screw and it's direction is the cylinder that forms the screw? or do I place it at the top face of the nut and the direction is cylinder that forms the nut? or a combination?
tutorials and online docs are frighteningly brief on screw joints!!!