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Are the feed dogs fully up and clean?
With the unthreaded machine transport a piece of paper with the feed dogs?
Is there a presser foot pressure adjustment? -- if so, it may need tweaking.
Do you have a tangle of thread under the needle plate (= throat plate) or hook and bobbin case area?
Do you have the stitch length set to greater than 0?
First thing to check is if you've got the right needle system: http://partsbooks.sewingmachine.com/Instructions/Juki/Industrial/DDL-5550%20&%205530%20Instruction%20Manual.pdf and that the needle is correctly oriented... the scarf has to be precisely set.
The next things I'd check are the stitch cycle -- hand crank the machine through the normal stitch cycle and see what's hitting. Then fix that. I've got my suspicions this may be a needle bar height adjustment. Don't thread up for this test... misthreading can also cause the needle to hit something and break, so start with the fewest possible variables first.
bobbin case should be stationary, bobbin itself is rotated as thread unwound, check hook timing. Simply turn hand wheel towards you and watch needle go down, when it reaches lowest point turn it slowly until needle rises 3/16" or so. Hook should be behind needle and slightly above eye.
Take the foot off the machine Remove the needle plate Remove the feed dogs rotate by hand until the needle is at the lowest position continue rotating until the needle rises 2 mm the hook assembly should be passing exactly by the scarf of the needle if not loosen the two screws that secure the the hook assembly and rotate the hook assembly by hand until it is exactly at the scarf of the needle and the distance between the hook and the scarf is .1 mm (almost touching the needle) tightened the two screws, replace the feed dogs and foot and it should sew perfectly
can you please tell me your machine model? i can help
any way if lockstitch machine you must first make sure you installed needle correct way and you are use new needle
can you check hight od needle bar is ok if you move it up and dwon?
can you check timing between needle and tip point for hook?
i can send page from instruction manual how to adjust hight of needle bar and timing with hook.
ashraf abozid firstname.lastname@example.org
Make sure the feed dogs are dropped (or feed dog cover is on - I can't remember which way this machine works). The stitch length on the machine has no function so ignore it. (I set it to zero so the machine doesn't have to work as hard). Hopefully this is all that's wrong. If not, use a shorter setting for the stitch length on the regulator, oil the hook and move slowly and smoothly. Good luck!
Basically on any sewing machine to time it you will need to separate the two gears that drive the hook. Usually loosen the set screws that attach the gear to the shaft and separate them.
Time the hook first. On center needle position, set the hook so that the point is behind the needle when the needle bar has risen about 2.5mm.
2) Set the needle bar height. Choose the needle position which is the last for the hook to pass.
(Left on most machines, right on older Vikings) At this point, place the point of the hook behind
the needle and set the needle bar height so that top of the eye is just barely below the tip of the
3) Check feed timing. Set the stitch length to the longest stitch. The needle should enter the
fabric just as the teeth drop below the surface of the plate.