thread becoming slack on a wound bobbin & thread breaking

[thobgood]

Hi folks!

I joined a number of years ago when I got my first industrial sewing machine, as when you google problems with industrial machines, all the best advice comes from leatherworker.net! Now after being a member for a few years, I'd like to ask for advice about a problem I'm having.

This is on my Juki 5550N, which I bought new a few years ago. I'd say I use it a moderate to low amount, maybe ten hours a week? So it hasn't been run into the ground or anything. I'm actually not using it to sew a lot of leather, mainly spandex and vinyl (I make pro wrestling masks, yeah, weird, I know.) I'm currently using size #46 bonded nylon thread. I wind my own bobbins on a standard on-table bobbin winder. They seem to wind evenly and tight, quite nice bobbins. I occasionally ( a few times a week maybe) have an issue where I'll be sewing and the bobbin thread will suddenly break, and when I pull the bobbin out to get the thread going again, a bunch of the thread on the bobbin is very loose (where it was nice and tight before). I unwind the loose thread (it's usually about ~15-20 times around the bobbin, what's come loose), rewind it around the bobbin by hand, put it back into the machine, and I'm good to go for hours and hours usually, and then the same thing'll happen. I *think* it only happens when I go to backstitch to reinforce my stitching, but it definitely doesn't happen EVERY time I backstitch. I've examined the bobbin case for burrs or inconsistencies, but don't see anything obvious. Ditto for the enclosure where you insert the case in the machine. The bobbin tension appears to be fine, in that I do that test you read about where you put the bobbin in the bobbin case and you can hold the weight of the case by the thread coming out, but if you bounce it a bit, thread does flow out of the case. (I think that's how you test to make sure bobbin tension is ok, yes?) Like I said, I can sew just fine for hours, occasionally backstitching just fine, and then unpredictably, I'll get the thread break and the odd loosening of the bobbin thread. It's been so frustrating, as you might imagine! I've tried googling the issue and searching for it in the forums here, but when I search for "loosening of bobbin thread," I just turn up the basic bobbin tension issues that commonly happen, not this actual loosening of the thread on the bobbin itself. I've attached some pix of what the thread on the bobbin looks like when this happens, as well as the broken thread.

Has anybody ever had this happen to them? Any idea what might be causing this? Many thanks in advance!

EDIT: Forgot to mention: I thought that the issue might be the direction my bobbin was spooling when I inserted it into the bobbin case. I checked my manual, and indeed I have been putting it in backwards in this regard for a long time now, but the issue continues to happen even now that I'm placing the bobbin in the case with the correct orientation.

Edited April 26, 2020 by thobgood
forgot to list a fact about how I was inserting the bobbin