Sorry to come late to the party, I read most of them but ran short of time.
Heres is what experiance with 1000's of machines in assembly plants has told me. Many sewers on the shop floors call mechanics over the same thing.
Number one cause of cutting or fraying thread in an industrial machine is Needle installed in wrong orientation and wrong length. Are you sure you have the right needle lenght and it is square to the bobbin case and facing the right way? It only takes a few degrees of turn to do exactly what you are saying. Or not fully nested into the needle bar.
Also make sure you didnt put bobbin in backwards. Bobbin should wind out counter clock wise. The fact that the issue just arose makes me fairly certain the needle wasnt tight and moved when welting. And the boobin comments make me think you replaced a bobbin backwards by accident. Let me know if it helped.
Last edited by creativeinteriors; 07-25-2007 at 08:47 PM.