Hook: Off the Shoulder

[friquant]

When a hook has a shoulder, does the thread need to ride in it?

The hook on my JiangLong 341 (clone of Juki 341N) has a shoulder to it. When stitching forward, the thread rides in this shoulder. (See the first two photos)

When doing long stitches in reverse, the shoulder did not catch the thread so the thread is lagging behind the shoulder. (See last two photos)

The reason I am investigating this is to understand how the machine is so noisy when stitching long stitches in reverse. I have set the hook-to-needle timing and the hook-to-needle distance according to the juki manual. I have tried different positions of the latch opener. The latch opens the gate, but the thread does not go through it while it has the chance. Later, the thread gets yanked past the gate....yeek!

[Uwe]

The thread should definitely be sitting in that “shoulder”, as in the second picture. 

If the thread misses that shoulder in long stitch reverse, then something is not aligned quite right. This may also happen if you stop in precisely the wrong position and the hook has a little backlash in its movement as you stop.

A slow motion hand turned stitch with a close-up video snippet of that area would be of great help in debugging. (See sample video below)

It may be that your needle/feed dog are adjusted too far back in the throat plate opening. Ideally, the feed dog movement should be nicely centered in that opening front-to-back.  

The video below shows how that basket opener and the thread pickup works on my machine (before I upgraded it to the 341N hook with a cap.) Perhaps compare timing with your machine. 

 

[friquant]

 

23 hours ago, Uwe said:

The thread should definitely be sitting in that “shoulder”, as in the second picture.

Appreciate you chiming in to confirm. 😀
The edge of this shoulder is sharp, so it makes that we want the thread nestled inside the shoulder groove  instead of dragging across its sharp edge.

 

23 hours ago, Uwe said:

It may be that your needle/feed dog are adjusted too far back in the throat plate opening. Ideally, the feed dog movement should be nicely centered in that opening front-to-back.  

Originally (when I took the photos for the beginning of this thread), the feed dog movement was nicely centered in the opening front to back.  Shifting it to the rear (away from the user) seems to make it so the thread almost always rides inside the shoulder groove. It's still very clunky though stitching in reverse at 4mm or longer stitch lengths.

 

23 hours ago, Uwe said:

A slow motion hand turned stitch with a close-up video snippet of that area would be of great help in debugging.

I'll do my best.. (Hats off to the videographers of the world that make it look easy!) Hopefully this will help to debug. I do love the sound and feel of a machine when it sews smoothly, and it seems I'll never get back to stitching until I unravel this mystery.

 

[friquant]

Repositioned the throat plate so that it aligns better with the overall shape of the cylinder bed, and adjusted the feed dog and needle positions to match. The thread now enters the latch cleanly about 60% of the time (Reverse, max stitch length), but the thread is no longer riding in the shoulder. (So many compromises!)

The camera angle is different this time. I tried to get a directly top-down view (as close to orthographic as possible) to show the relative angles of things. The bobbin case is not square with the throat plate. I'm not sure if this is how it should be. I'm thinking on how I can test with different throat plate geometry to see if I can get the best of both worlds (thread riding in shoulder, with clean thread entry into latch)

[friquant]

Here is the geometry of the throat plate.

[friquant]

I've decided to give up on this quest for now. Putting the machine back together and time to get to building something. 🚶‍♀️

[friquant]

Found a solution for the slack thread not jumping through the latch. That is, set the thread check spring to begin adding tension at that point in the stitch cycle.

Still do not have a fix for the thread being off the shoulder.