Juki 1508NH : Problems with corner stitches - bobbin thread coming up

[showtime]

Hello there Ladies and Gentlemen,

I'm hoping one or more of you marvelous sewing machine magicians would be so kind to lend me some advice on this problem that has me at my wits end.

 

Background:

I have been sewing nylon webbing using bonded nylon thread, everything has been working fantastically for quite some time until a few days ago.  I had a needle break and the machine got jammed, I cleared the jam and got everything going again not thinking much of it.  Next time I was back on the machine I noticed skipped stitches and needle thread being cut on occasion. 

To resolve this, I checked the timing and noticed the hook rotation was retarded according to the factory manual.   I went ahead and corrected the hook rotation, made a small tweak to the needle guard, and generally got everything in-line with the factory specs (rotation, hook tip to needle distance, needle guard deflection amount.)  I'm not sure if the rotation got knocked out of spec from the jam or if that's how it came set up from the dealer (had them set the machine up after purchase a few years ago.)

Testing out the machine after the timing change seems to have resolved the previously mentioned issues, however while testing I have now noticed another problem...

 

Problem:

The bobbin thread is pulling up to top of material on corner stitches.   When sewing a straight line of stitches the tension balance is fine, however when turning the corner stich knot pulls up like the upper tension is too much - resulting in the upper thread slightly cutting across to the next stitch.  I stop and turn when the needle has started to rise and the hook has captured the upper thread.  I have never had this problem before, sewing the exact same material, same thread, same needle.  It leads me to believe something may have gotten knocked out of spec?    I can sew straight for inches and inches with the tension remaining consistent, but as soon as I make a corner stitch the bobbin thread comes to the top.

Here's what I have tried, all without luck:

• Retarding the timing back to where it was before I set it to factory specs
• Reducing upper thread tension (It lessens the effect of the problem, but the problem still exists and then the balance of tension on the straight stitches is off.) 
• Using more and less bobbin tension with compensated upper tension  
• Turning the material at various different points during the needle stroke
• New needle
• Smaller needle

It doesn't consistently do this on every corner, I would say 50% of the time when turning material clockwise and 40% of the time when turning the material counter-clockwise.  I can't wrap my head around what may be causing the tension balance to shift to the upper side when turning the material?

Do any of you gurus have any insight you can lend me?  What else can I check?  Thank you in advance for taking the time to read this!!

 

[CowboyBob]

A smaller needle can cause the thread to hang up in the eye,when sewing webbing you can use a size or 2 larger than recommended for your thread size because the hole closes up after sewing @ you'll end up breaking less needles too.I have customers using #138 & #24 needle in webbing.You might have either too much check spring travel or it could be set too tight .But tyr a larger needle first.Then move on to loosening the pressure on the check spring & or shorten the travel,BUT just one thing @ a time.

[showtime]

Hi Bob,

Thanks so much for your reply.  I should have mentioned that I am and have been using #92 thread and a #22 needle, which in the past has worked perfectly. 

Based on your reply I went ahead and tried a #23 needle but unfortunately still have the issue.  I also tried changing the travel on the check spring (shortened and lengthened) as well as reducing the check spring pressure, all without a change to the problem corner stitching.  

Tried everything one thing at a time, I'm at a loss on this one *scratches head*.

[CowboyBob]

You might have some thread or lint under the bobbin tension spring or the spring might have gotten bent.You could try to take the bobbincase out,take the spring off & bend it in alittle & it might not hurt to get a couple of new spare ones just for times like this.Do you have a backlash spring in the bobbincase?

[R8R]

Maybe check the clearance and centering on the needle to the hole in the center foot? Be sure the needle is centered correctly in the hole. Perhaps thread is getting hung up there on turns, or in the needle plate.

Does this happen when making turns in motion while sewing, or while making turns while stopped with the needle buried and the presser foot lifted?

[showtime]

Thank you CowboyBob and R8R for your responses.

 

CowboyBob:

Upon further investigation It does appear that part of the bobbin spring is slightly bent.  When looking at the edge of the spring where the thread exits, the top portion of the spring seems to be bent outwards slightly.  I suspect the thread is being pulled up towards the higher end of the spring when turning and thus receiving less tension.

I am working on getting a replacement spring on hand and will give it a try, I will update this thread with the results.

R8R:

Great suggestions, in this case the needle is centered in the foot and throat plate.  However, your comment may help someone searching for this same issue in the future.

 

Thanks again everyone, I'll be back with an update...

[DDahl]

My new 1508NH had irregular tension problems.  In my case it turned out to be the tension disks.  I removed them and polished them to a mirror finish with Jeweler's rouge. Problem solved. Juki probably got the parts from China.

Dave

[showtime]

Dave, thanks for the lead, I will take the tension disks out and take a look / polish them up.   I'm not sure that would cause the issue only on corners but anything is a possibility.

[mikesc]

It is highly likely that most of ( if not the entire ) Juki machine ( if it is a "new" one ) was made in China.

Edited December 5, 2017 by mikesc

[NCArcher]

If this is occurring only when you lift the presser foot to turn the corner it may be a problem with the tension relief associated with the presser foot. I don't know if your machine has this but the tension should be relaxed as you lift the presser foot. If that is not happening it may be over-tensioning the top thread on the corners.