Constraining the body movement in all the axes for certain time

I want to constrain the motion of a body in all the directions for a certain amount of simulation time. After the time has elapsed I want that body to be moved in 'y' direction with certain velocity. How can I do this in sim mechanics? The axis of the body up to the specific simulation time is varying. As an example, the body is placed on a platform which is being tilted upwards w.r.t a pivot point. I don't want the body to move in any direction at the time of tilting upwards. When the platform is completely vertical, I want the body to move in 'y' direction. Can anyone help me out? I was trying to use a weld joint to fix the movement of the body till the platform becomes vertical and open up the weld joint after that. But its not working.