Thread forming knots on back of leather - Juki 1510

[JuniperLeather]

Hey everyone, this is my first post on this wonderful forum but after much searching I haven't been able to find any posts that address this specific issue. I feel like I've tried so many things to fix the problem but its a bit of a mystery to me at this point so if anyone has advice I'd love to hear!

For context I work mostly with soft  upholstery type leather, with a small business doing custom jobs - bags, accessories, products with decorative appliqué and such. Working with a Juki model LU1510N-7 and Cansew Tex 70 bonded nylon thread. The needles are generally 135 x 15 R size 19 and are changed regularly. 

I have been having the problem of intermittent 'knots' appearing on the underside of the leather - these are made of the needle thread over-twisted around its self to form a tight ball of thread which when pulled and unravelled leaves a loop of several stitch lengths worth of thread on the underside of the leather. There is no fraying or catching of the thread as it goes along the thread path from cone to needle, and the bobbin thread seems to be running smoothly as well.

I've tried different cone placements and thread paths as well to try to get the thread to feed as smoothly as possible off the cone. We have tried adjusting the tension depending on the material being sewn and even when the tension appears balanced between the needle and bobbin threads it doesn't eliminate the knotting altogether.  Some days it seems like the machine will behave and sew perfectly and other times there will be 6-8 knots within a 10 inch line of stitching. Sometimes I will rethread the machine and wind another bobbin and it might go away for a while and other times that doesn't help at all. Also have tried with no real improvement: Needles from size 18-22, silicone thread lubricant, changing bobbin tension, different tension when winding bobbins, different heights for the thread cone. 

I have a suspicion that after switching from A&E brand thread to Cansew this was when the over twisting and knotting really became an issue -  the quality between orders of thread varies and some colours of the cansew thread seem to be very stiff and tangle easily where others almost feel like a lighter weight even though they are both tex 70? Some colours in particular are bad for this, especially the darker ones. 

The knotting happens more frequently on some machines than others (they are the same model of Juki) which makes me think it isn't only a thread quality issue but something that is happening with the machine it's self. 

Here are a few pictures of the knotted thread before and after the knot is untwisted. 

Has anyone else run into this problem before? Hoping someone could shed a little light on the situation, at this point I don't know what else I can be doing to try to improve the stitching.

[Wizcrafts]

41 minutes ago, JuniperLeather said:

I have been having the problem of intermittent 'knots' appearing on the underside of the leather - these are made of the needle thread over-twisted around its self to form a tight ball of thread

It could be that the top thread is twisty and is causing the twisty balls to form randomly. I have lots of twisty thread spools and counteract the twist by feeding the thread through a top post that has two holes, thusly: feed through the top hole inline. Pull the thread around the front on the left. Feed the thread through the bottom hole from right to left. This creates a counterclockwise (CCW) bias that counteracts the clockwise twist that exists in most bonded thread spools. Try this if your top post allows it.

BTW: you said you use 135x15 needles. Did you mean 135x17? Those are the standard needle for sewing vinyl and cloth on walking foot machines like yours.