I was a sewing machine mech for 20 years on industrial sewing machines like the 31-15. You really need to know how to thread the machind correctly and put a new needle in also. You need a manual. If the machine is broken then you need a mechanic to fix it. You can not do it yourself. There are 2 different 31-15 sewing machines,one looks like a house machine and the other is industrial heavy duty. Upholstery shops use to like the 31 15 machines because they take a beating. But to fix it yourself will just make you frustrated. Not too many sewing machine mech out there that know what they are doing.