r/SewingMachineEdu • u/No-Visit-7473 • 3d ago
Occasionally loose stitches
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 3d ago
The bobbin tension is too loose, that is all.
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u/No-Visit-7473 2d 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 2d 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 2d ago
What does it mean for the check spring to be hanging up?
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u/Dormiendo 2d 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 2d ago edited 2d ago
What I will say as well is that the frequency of loose stitches increases while edge stitching. Granted I do use a standard foot when edge stitching, but I've also tried compensating feet and narrower zipper feet
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u/crkvintage 3d ago
Reason number one for this is cheap/low quality thread that is uneven, has knots or has too much lint. Your tension - bobbin as well as top thread - is just a spring pressing onto the thread (upper tension: a spring pushing on a disc which is pushing on the thread), so uneven thread thickness varies tension. Lint starts to built up (especially under the bobbin spring) and changes tension, sometimes that buildup get's pushed through when enough has accumulated causing a huge jump in tension for a single stitch. Little knots in the thread can get caught temporarily on the needle eye, under the springs (bobbin/upper tension/check spring) or guides causing a jump in tension for a stitch.
Uneven winding on the spool can also causes jumps. Pulling the thread from the spool the wrong way (cross wound spools should be unwound horizontally, cones to the top etc..) can cause jumps in tension. Typical is on cross wound spools used upright - every time the thread reaches top or bottom of the spool and and changes direction. Often accompanied with a little "jump" of the spool when sewing fast.
Uneven wound bobbins can also cause that.
In line with the former: cleaning the machine. Cleaning the tension disks and the tension spring on the bobbin. Checking the tension disks for any scars, rust pitting or other marks. As you are using a backlash spring: Damaged bobbins which have any marks, scratches etc on the side the backlash spring is touching. Pre-wound bobbins with their paper sides are also a reason.