I turn the little screw on the bobbin of my Bernina all the time. All you have to do to get it back to the original setting is retighten it a little and test it. To test: put the bobbin in the case, run the thread through the tension, grasp the thread in your hand and see if the bobbin and bobbin case descends from your hand like a spider on it's silk. It should have enough tension that it descends slowly with a little motion from the hand grasping the thread. Put it back in the machine and try it. Some machines like the thread tighter or looser, find your machines favourite setting.
I like to use thicker threads in my case for bobbin work, so adjusting the bobbin is critical.
Last edited by Tartan; 01-16-2012 at 07:27 AM.