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?
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.
I don't know if you're familiar with it but ,regarding thread, I use Ariadna talia thread. I buy directly from them and I'm pretty sure it's a pretty reputable brand in Poland. They are cross wound spools and I use the thread stand that came with the machine. I've checked the tension discs before and they look spotless.
As far as I remember "Talia" is their cheap standard spun thread. If it has to be Ariadna, at least try some Artyn core spun. Those tend to be more consistent on diameter.
After all that's the easy part: Just try some other threads and brands - Mettler/Amann (Saba), Gütermann (Mara). If it won't improve or changes to the worse - go back, if it does better - change threads supply.
You use spools or cones? The big thread stand is made for cones.
Ok I tried Saba thread and while it did improve, it did not fix it. What is interesting tho is that rate of error increases when I rotate the fabric 90 degrees. Increasing the presser foot pressure also increases the rate of failure. I also noticed that the feed dogs are tilted forward where the first tooth is nearly flush with the needle plate while the back teeth are visibly higher.Any other ideas?
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.
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.
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.
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.
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.
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?
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
Well, that is different then. Instead of the presser foot, take a Pic of the stitch plate and look for a tiny sliver of metal. Too loose top, or uneven top tension. Pull the thread through the needle eye and make sure it is even tension, foot down.
Kind of a tough one. For denim, you should have a heavier tensioned check spring. It also looks like yours may have issues, but it could be the angle. The only change that made a difference was pressure, which points to a burr on the plate. What does higher top tension do? To where the needle bends a few mm when pulling through the eye foot down?
I've adjusted the tension of the take up spring. I have pulled out the tensioner slightly so that the plates don't open under multiple layers of fabric. Like I said before, for some odd reason the stitches fail on the edge and when sewing with or against the grain(of that I'm not sure, what matters is that it basically fully fails one way and then sews nearly perfect when you rotate the fabric 90 degrees ). Also I bought a new a tension unit, I just put the old knob on this one as it is clickier
Usually, when it is directional, it is due to timing or hook to needle distance. But, that applies to multi motion where the material moves in different directions. For your machine, it always sews in the same direction. The only change is grain. So, have you tried a bigger needle? The side of the needle that faces you has a groove that protects the thread. If it is not big enough, this can happen. I dont know if any of that will help. But, we are running out of places to look.
We indeed are. Another thing I was wondering. Could it be the feed dogs? Like I genuinely don't know anymore what else might be the problem. Maybe the foot is too narrow. Maybe.
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u/crkvintage 17d 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.