Thread trimmer woes.

[hamradio]

Howdy. I've got a couple machines with auto thread trimmers that I'm currently ripping my hair out over, and have poured way too many hours into them. I've never felt completely competent working on thread trimmers...any time I've ever had to fix them in tackers, etc., I've mostly just tried to set everything up by the service manual and hope it works. This has been an OK approach, but I still don't have a great understanding of the fundamentals behind it (what needs to be timed where/when during operation). 

Issue 1: Juki dln5410n-7. I've got another one of these that runs perfect, but it's an H designation. This one is not. This came to me with an inop trimmer...someone must have jammed it hard and broke the thread guide below the feed dog. I put a new guide in and sharpened the fixed knife. It does a beautiful job trimming threads, but always loses the needle thread if I don't start sewing with it underneath the foot. Adjusted the thread tail length excessively long, no change. Thread trimmer cam timing is spot-on via the manual. Hook timing is also book-perfect, and set identically to the other 5410. However, it's hard to use the working 5410 to cheat off of, since the H uses a little different trimmer configuration. My working theory is that the H has a spring to retain the bobbin thread, whereas the normal (non-H) one doesn't, and it's not picking up the bobbin thread easily? The H has no problem starting even with a fairly short thread tail.

Issue 2: Brother LT2-b842-905. Kind of a long shot here, since there aren't a ton around... I'm just using this one to sew labels and have the left needle removed. This was another one that I got with thread trimmer missing parts, and I pieced it back together. Trimmer cam timing was way off when I got this one. I fixed that, sharpened the knives, and adjusted the tension release. It can consistently make a clean cut, but always leaves an extremely short thread tail (~.5") and comes unthreaded on the next cycle. I've tried everything I can think of...moving the timing belt around a tooth or two in each direction to change trimmer cam timing, playing with thread controller tension and timing, the pre-tension discs before the main tension, etc. Nothing seems to have any noticeable effect. The manual sucks for this one, and offers a bit of contradictory information to itself on trimmer timing and adjustment. The cam on the lower shaft is also non-adjustable, so changing timing on this one involves popping the timing belt off and moving the whole shaft. It's not a great time.

Material and thread aren't anything too demanding...1000d cordura and T70 thread. 

Thanks much in advance to anyone that spends the time to look over this and offer suggestions!

[keithski122]

Not for your machine but mine is similar so this may help.

Thread tail length should be adjusted by a button on the upper thread guide on mine.

https://www.brother-usa.com/virdata/content/en-us/ipd/consumer/instructionmanual/im_ls2_b891_b892_enes_2243.pdf

Edited January 21, 2020 by keithski122

[Gregg From Keystone Sewing]

Under bed trimmers on sewing machines have always been difficult to service.

[hamradio]

20 hours ago, keithski122 said:

Not for your machine but mine is similar so this may help.

Thread tail length should be adjusted by a button on the upper thread guide on mine.

https://www.brother-usa.com/virdata/content/en-us/ipd/consumer/instructionmanual/im_ls2_b891_b892_enes_2243.pdf

Thanks for the heads-up on this one! The trimmer is a bit different, but it does have a nice troubleshooting table at the end. Interesting to see some design changes...this one looks far more user-friendly to adjust.

I ended up solving the thread trimming issue on the Brother. Still struggling on the 5410. If anyone stumbles upon this thread in the future, here's how I sorted out the brother LT2-B842-905:

The service manual offers two different, contradictory ways to set the resting position of the moving knife. The first is a measurement between the tip of the knife and boss in the bed casting of 26.6mm (on the right side). The second is a specific alignment with the bobbin thread retention spring. The problem is, these two lead to different moving knife positions. This might be due to the fact that my knife isn't OEM, but every other sewing machine and overlocker knife I've bought from this mfg (Eagle/Strong H) has worked as expected. Anyway, if one sets the knife resting position according to the bobbin thread retention spring drawing, it'll end up too far forward, and the tip of the knife will protrude a couple mm into the notch in the throat plate that the bobbin case tab interfaces with. The top thread snags a little bit on it. Setting it to the 26.6mm spec solved the problem completely, and the tip of the moving knife is just out of the way of the thread when resting. Machine is doing a beautiful job of trimming.