Singer 31-20 - bobbin comes unthreaded

[wsmeador]

Hello,

The machine is properly threaded and makes near perfect stitches through, both multiple layers of interfaced cotton fabric, and leather.  Test swatches are about 9 inches long.  After about four passes (straight stitching, then lifting presser foot and changing direction) the machine begins to malfunction.  Stitches are no longer forming properly and the bobbin tension has gone haywire.  After cutting the thread and removing the bobbin case, it is apparent that the thread has come unthreaded from the bobbin case.

We've cleaned and oiled the machine including removing and cleaning the shuttle driver and race to remove old thread dust and gummed oil. This problem started before removing the race. Attached is the test swatch top and underside.

Any thoughts on why the thread would come unthreaded from the bobbin?

Thanks!

Scott

[catskin]

Just a guess but MAYBE you put the bobbin in the bobbin case  the wrong way.  the thread coming off the bobbin clockwise when it should be counter or visa versa. Turn it over and try again

 

[Uwe]

Make sure you're winding the bobbin nice and evenly. It seems like a simple thing, but I've seen bobbins that were a total mess straight from the winder.

Is the thread still under the bobbin tension spring after the problems start? It may pop out from under the spring for some reason.

The thread itself also may affect things. Some thread is super stiff and acts like a spring itself - some may unwind from any spool or bobbin unless held in place by something.  

Also make sure you have the basics right for threading the bobbin in the bobbin case. There's a few variations of the manual floating around. I like this Singer 31-20 manual - it shows diagrams with HUGE bobbins:

   

Edited March 8, 2017 by Uwe

[RockyAussie]

Just check also that the dished spring is still in the bobbin case back as it helps to stop the bobbin running too loose and untangling and so on. Brian

[Uwe]

This seems to be the week for bobbin case springs. I had to go hunt down one of those bobbin case springs for horizontal axis hooks. I found two samples with different style bobbin case springs in my parts stash, but neither of these are Singer 31-20 bobbin cases. Below's a picture of the two I found. The springs are designed to put some friction on the bobbin rotation so it doesn't spin too freely and unwind itself. These springs are rarely present in vintage machines in my experience. The style on the left falls right out, the one on the right has little tabs that hold it in place inside the bobbin case. 

  Alas, the Singer 31-20 may not even use one of these bobbin case springs. At least they're not called out separately in the Singer 31-20 parts manual (31-15 and 31-20 Parts list.pdf )

Edited March 8, 2017 by Uwe

[RockyAussie]

Excellent work Uwe as always. I had only assumed that it would or should have one. The one you have pictured on the left I have never seen the like of before and it is a scary looking little beast. Any chips on the outer edge of the bobbin would I think be a real headache.Thanks for your kind input.

[wsmeador]

Thank you all for your quick replies. Unfortunately, my wife and I got too busy with work and life to get back to the machine until now. Catskin's suggestion ended up being the solution. Flipping the bobbin worked even though it seemed to be the opposite of what the manuals showed. After the semester ended this week, we happened to be in our local sewing machine shop picking up some needles and thread and my wife described the issue to the well-seasoned machine repairman there and without a hesitation he said flip the bobbin. Convinced, she tried it and it sews like a champ.

[catskin]

Glad my suggestion helped,  Sometimes simple is all it takes.