Problem with a motion script

Hello everyone,

What i'm trying to do, is to make a trap for my game that works like a spinning saw. It's constantly spinning through it's z axis and I want it to go Up and down within a certain amount of time, what I tried to do is this:

 function UpAndDown()
     transform.position = Vector3.Lerp(up_transform.position, down_transform.position, Time.time);
         
     yield WaitForSeconds(1);

     transform.position = Vector3.Lerp(down_transform.position, up_transform.position,Time.time);
 }

The up_transform and down_transform variables contain the two transform where the saw will move within.

What I wanted to do is when the saw reached its lowest position (down_transform), wich is, after one second of movement, start going up, instead of down, but it doesn't work. At the moment it reaches the down_transform position it starts going up and down very fast, instead of returning at the same speed.

What am I doing wrong?

Thank you