Juki 341 Clone timing

[HardenGoods]

I've got a new Juki 341 clone and after printing out both the instruction manual and engineers guide, I fine tuned the entire machine. This machine was purchased from a dealer for a great price because it (the head) was sealed in the box straight from China. Yes, I could have purchased from Ali or another giant overseas but I wanted easy access to parts and accessories without the wait. I also went into this purchase knowing very well how much fun I would have learning every possible aspect of the machine and it has been incredible so far.

The only snag I've run into without being able to fully understand is the top to bottom shaft timing. When the top shaft brings the take-up lever to its highest position, the markings on the bottom shaft (line marking pointed at centre of shaft collar fastener) are where they should be and the belt is securely in place. The machine works well but I'm getting some snapping (not breaking) of thread across the face of the bobbin case. I'm fully aware that this is due to the fact that my take-up lever is ascending just a smidge too early. I'm also aware that I can change the check spring slide rest position to compensate (but this leaves the check spring travel very low which minimizes its use and tells me that the check spring isn't the root of the problem). I can also advance the timing of the hook so the thread wraps around the bobbin case before the take-up lever begins its ascent (and adjust the needle bar to suit) but this is also not getting to the root of the problem. Lastly, I can move the top shaft gear back one tooth on the belt to slightly set the point at which the take-up levers begins its upward travel, but it still doesn't seem like I should have to do this on a new machine.

I guess I'm wondering how this could be on a machine that is new with everything fine tuned and adjusted as per the manual. Oh, I've also confirmed that all adjustments and measurements that I've made are exactly the same in the manual that came with the machine as they are in the official Juki paperwork.

The machine sews fine when I make any of the three adjustments above but just trying to gain some more insight into the machine as I'm really having a great time learning what I can about it.

Thanks in advance for the help!

[Wizcrafts]

Adjust the timing belt for the best result. Ignore the marks they inscribed onto the bottom shaft. It could be inaccurate.

[Uwe]

The timing belt position and synchronization of top/bottom shaft mainly affects the timing of the feed movement, which is important, but really has nothing to do with how the thread wraps around the hook. You can adjust the hook position to time properly with the needle movement no matter where the timing belt is set (especially if feed motion is set to "0".) It's possible to have perfect hook-to-needle timing and very wrong feed movement timing.

My Juki LS-341 also has two thread holes in the take-up lever (intended for double needle versions.) If your take-up lever has two holes, use the lower of the two holes. It may provide an extra millimeter of thread slack when it counts.

For moving beyond the science of adjusting by the book to the art of buttery smoothness, I'd recommend ignoring the needle rise figure for hook timing. I personally stopped using needle rise figures for hook timing a while ago. It's a pain to measure and often not very precise. I now use hook position at needle bottom dead center instead to time the hook. Make a note of the hook tip position when the needle is a bottom dead center (after adjusting by the book.) It very likely will point at 7 o'clock (needle is at 9 o'clock.) Advance the hook tip to be at 7:30 o'clock on the clock face at needle BDC. Adjust needle bar if needed. Experiment with subtle hook position adjustments. With a little luck, this may be all that's needed to avoid thread snapping around the hook.

Ideally, the check spring alone would provide a small amount of tension as the thread wraps around the hook. Later designs have a separate adjustment to control the thread-round-the-hook slack and allow the check spring to determine thread tension somewhat independently of hook timing. We had a topic relating to these newer adjustment devices recently. Your 341 class machine lacks that device, but the topic will help you understand why they eventually invented it - because they were struggling to avoid thread snapping around the hook!

Here's a picture I made a while ago to illustrate the imaginary clock face concept.

 

[HardenGoods]

16 hours ago, Uwe said:

16 hours ago, Uwe said:

The timing belt position and synchronization of top/bottom shaft mainly affects the timing of the feed movement, which is important, but really has nothing to do with how the thread wraps around the hook. 

This does make good sense. My thought was from a little bit of a different angle. I was thinking that if the hook, needle bar and shaft were timed as per the book, my only option was to move the top shaft gear back a tooth to set the take-up lever behind a pace or two. This however, will likely have an effect on feed (maybe negligible, but even small adjustments seem to matter a great deal on machines).

16 hours ago, Uwe said:

My Juki LS-341 also has two thread holes in the take-up lever (intended for double needle versions.) If your take-up lever has two holes, use the lower of the two holes. It may provide an extra millimeter of thread slack when it counts.

Thats the first thing I looked for as I know some double needle machines use the same take-up lever as single needle. I just have one hole. I also completely agree on the BDC rise figures not being 100% accurate. Its starting to make more sense that the book is more of a starting point really. Buttery smoothness as you say is exactly where I want to be. Also, the progression to an independent thread-around-hook mechanism seems very logical. Thanks for providing the diagrams, much appreciated.

16 hours ago, Uwe said:

16 hours ago, Uwe said:

Ideally, the check spring alone would provide a small amount of tension as the thread wraps around the hook.

 

Is the little bit of tension while the thread passes around the hook supposed to cause a slight jerk in the check spring before it rises? No snags on bobbin case opener or under throat plate at all. Maybe the slight bit of tension across the bobbin case I'm worried about is perfectly normal (just that the check spring jerking didn't seem right).

[HardenGoods]

Here is a quick video of the machine with the hook advanced to 7:30 (and the needle height adjusted to suit). The check spring still jerks slightly which just doesn't seem normal. I could be wrong though, I just thought the check spring was supposed to come into play after the thread made its way around the bobbin case and then rested again just as the needle was entering the material. I can adjust the range of motion of the check spring to barely move but it still takes a load before it is supposed to.

I could advance the hook even more but then the needle won't rise enough to create the thread loop it is supposed to.

 

[Uwe]

As slow as the machine is sewing, it's still hard for me to see the critical moments during the cycle. A very slow hand-turned stitch may be helpful.

I think the still frames below tells the story best, at least for me. In the first still image, the thread is wrapping around the farthest point of the hook. The check spring is engaged and applying tension to the thread, but the check spring is not pulled up to the limit (note the thread still has a slight bend where the check spring touches it - the thread is not a straight line.) This tells me that the spring is applying the tension at that moment and that no additional thread is being pulled from the spool. I have a strong feeling this is exactly as it should be.

The spring may jump slightly at this point as the thread makes its way around the bobbin case finger, which changes the length of the thread path ever so slightly.

 

Later, when the take-up lever reaches is highest point and the knot is pulled tight, the check spring is pulled up to the max (the thread is straight line, completely overpowering the check spring) and more thread is pulled from the spool. This is the still from the moment when that happens:

 

Edited October 2, 2018 by Uwe

[HardenGoods]

I will post an ultra slow hand turned stitch on my machine tomorrow. I'm starting to think this is how it should be too. The check spring jerks in Al Bane's Cobra 26 video (341 clone) as well.

 

[shoepatcher]

That spring is not working quite correctly in the video.  The spring works twice when it is only supposed to work once.  Watch the take up spring.

glenn

[HardenGoods]

On 2018-10-05 at 11:33 PM, shoepatcher said:

That spring is not working quite correctly in the video.  The spring works twice when it is only supposed to work once.  Watch the take up spring.

glenn

Thats what I thought!! Something didn't seem right with mine. Some adjustments to the tension unit and all is well.

[Uwe]

11 hours ago, HardenGoods said:

Some adjustments to the tension unit and all is well.

Can you elaborate on that? Your check spring does not move at all when the thread wraps around the hook now? What adjustments did you make to the tension unit?

[brmax]

I would have to study the video again, as I thought I seen a shadow of the spring so initialy thought this was the double blip. 

Its good to hear your adjustment made a change for the good. 

 

Have a good day

Floyd

[HardenGoods]

On 2018-10-08 at 8:12 AM, Uwe said:

Can you elaborate on that? Your check spring does not move at all when the thread wraps around the hook now? What adjustments did you make to the tension unit?

Yea for sure. I experimented with various tension combinations between the upper tension disks/spring along with the bobbin case spring making sure that with each combination my knots were still in he middle of the material. I'm not exactly sure why this alleviated the jerk of the check spring to be honest but it did go away at the time of experimentation. I'm starting to wonder if the thickness of material in relation to the needle size could have something to do with it too - perhaps less needle hole space for the knot to be pulled in and inside of the material (requiring more upper tension) would cause a jerk before the knot finally gets pulled through the small needle hole and seats properly?

(side note: bought this aftermarket table made by Phillocraft which Is super solid. Guess I like it a lot because I learned on a pedestal machine.)

Edited October 9, 2018 by HardenGoods