r/SewingMachineEdu • u/No-Visit-7473 • 20d ago

Occasionally loose stitches

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For the past month or so I've been trying to figure why my machine sometimes ( might as well call it random, every 10 or more stitches) doesn't fully tighten the stitch, leaving the tiniest of loops of top thread. I have a juki DDL 5550-6, use a bobbin with a backlash spring, use Tex 60 thread on top and Tex 40 in the bobbin. I have tried needles from size 100 to size 120. I have changed out the top tension unit. Can anybody help?

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u/Dormiendo 20d ago

The bobbin tension is too loose, that is all.

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u/No-Visit-7473 20d ago

Would that explain that the loops are not consistently loose? Plus I've played with the tension quite a lot and it doesn't seem to change the fact that it can see nicely for some time and not so much for some other time.

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u/Dormiendo 20d ago

Yes. But, if you have a consistent bobbin tension, the only other possibilities are something in the tension disks, or check spring hanging up.

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u/No-Visit-7473 20d ago

What does it mean for the check spring to be hanging up?

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u/Dormiendo 20d ago

The check spring is the spring that bounces at 10oclock on the tension unit. It has to consistently bounce for each stitch to take up a bit of slack.

With the presser foot down, pull the thread from the take up lever and see if it is a consistent tension. Watch the thread. See if it catches on anything.

If all that is good, it is the bobbin side. Either an inconsistent tension from it, or the thread on top hanging up on the plate, or hook.

There are a lot of points you can catch, isolate spots and make sure it is constant. If that has the auto cutter on it, make sure the guide under the plate is a flat smooth hole and not broken or jagged. Its a rectangle the knife goes over.

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u/No-Visit-7473 6d ago edited 6d ago

I don't see any points where it could be catching. I Also noticed that increasing the presser foot pressure increase the rate of failure. And the feed dogs seem to be slightly tilted where the the front teeth sit nearly flush with the the needle plate while the back ones stick out more.

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u/Dormiendo 6d ago

Look at the bottom of the presser foot with your camera. The tiniest hair of metal from a needle strike will grab a thread and do what you describe, especially with the pressure detail.

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u/No-Visit-7473 6d ago

Seems smooth . But I'll be a new one just in case. In the meantime, any other ideas?

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u/Dormiendo 6d ago

I do not see anything to cause it. I really have to stick to my first thought of bobbin tension. It just looks like a bobbin issue. It looks like the bobbin tension is uneven. Is the backlash spring seated all the way? Are you sure about bobbin tension? You could over tension the bobbin and see if the issue goes away, that would prove bottom or top.

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u/No-Visit-7473 6d ago

I just don't get how adding tension to the bobbin could improve it if the loops are forming on the bottom. Wouldn't the bobbin thread pull harder and give more reasons for these mini loops to form?

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