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[Southsound]

I’m very new to upholstery and working on my first project (some cushions for my boat). I’ve been practices on lots of scraps and probably 90% of the time I’m good and everything turns out fine, but occasionally there are tangles on the underside of the stitch (see photo). I can’t figure out what I’m doing different each time. Sometimes it looks like the photo and sometimes there will actually be a thread tangled down into the bobbin area and when I pull the fabric away it looks like there are 4 threads connected due to the tangle down in the machine. It seems like it mostly happens when I reverse the machine to lock the stitch. Is there something obvious that would cause this?

I have a Juki 563 that I bought used. It had very little use but did need quite a bit of adjusting to get working well. Is it possible something is slightly out of adjustment?

[JimmieJoe]

I run into that issue when I pause my machine mid stitch and then start sewing again.
Usually in my case it seems like the hook catches the thread twice.

[Southsound]

Ah, that could be related. I’ll pay attention to if I’m doing that when it occurs. Thanks for the idea!

[Adam12]

Of course the basics always apply...pulling the thread tails for a few stitches when you start and making sure the take up lever is at its highest point when you remove the work. Bobbin case opener could need tweaking, check it's action on the bobbin case and make sure it's doing it's job

[Southsound]

I had thought maybe it was a problem with pulling the thread tails at the beginning, but it seems to always occur at the end of a stitch.

How might the bobbin case opener be misaligned for this to happen? Too close or too far from the case? If I end up with the thread tangled down near the bobbin again I’ll pay much more attention to exactly what is tangled where and take some pics.

[Ron Henningsen]

On my Pfaff 545 when this happens it's usually a thread tension issue. Whenever I have any issue with the thread the first things I do if put a different color in the bobbin than what I'm sewing with. Next I stitch a few test pieces and check the thread pattern/tension. If the thread isn't in the middle of the fabric you have an issue. With the two colors it's really easy to see. John has examples in his course or go online to find the three examples. In fact this is so important in my opinion I have the three thread tension examples printed out and taped to the top of my machine table.
Just my thoughts. I always check the basics first. Handles most of the problems. Also I have found that sometimes thread tension issues first show up when using reverse.

[Southsound]

This is great advice! I’ll definitely try using two thread colors and see how everything looks.

[Adam12]

The bobbin case opener taps on the triangular protrusion on the bobbin case. It must move the bobbin case enough to allow the thread to pass cleanly around the hook. The few stitches i can see look good so it can't be too far out of whack if at all. If it checks out, and only have issues at the end back tack, it may just be the position the machine is in when removing. As said, take up lever has to be at its highest point when you remove, sometimes it helps to rock the hand wheel back and forth while you remove the work and it can sometimes pull that loop up into a stitch.

[Southsound]

I think you're correct about the position the machine is in when removing being the most likely cause of my original problem. I've been doing tons of test stitches since posting and paying close attention to that and have not had the problem since. I did try the two thread colors and found that the tension was also a bit off, so that may have added to the problem. I seem to be dialed in quite well now, thanks everyone for the help!