It's a Juki Disastro

NOTE: Scroll down for updates & fixes.

Well, not really a disastro ... I've just been listening to CocoRosie lately.
Though, my brand-new Juki DNU-1541S is still not working and that's close to a disaster.

After hours of contemplation and watching the hook spin round and round I've figured out that the hook assembly is most likely defective, or was damaged somehow at the factory. Either way, there's a rough spot in the rotation of the hook that causes the assembly to seize up when there's enough lateral pressure applied to it.

Hook assembly pictured above. The inside part with the post is called the basket. It rocks back 'n' forth slightly while the hook rotates around it and is held in place by the bobbin case holder.

The first time I was sewing with this machine, I was going slowly over some number-eight cotton duck, everything's going well, beautiful stitches ... sewing over a few more layers of duck on top of that, and then boom! the machine jams up.
I thought it must've been a bad needle strike, or something like that, and the safety clutch went off. But it wasn't that at all. It took me a while to figure it out -- days -- but here's what's going on:

When there's force on the basket, such as when the machine is handling heavy material, the basket and hook lock up. This makes the basket pop outta place and rotate with the hook. (Note the top red arrow above: This is the li'l nubbin that keeps the basket in place [the finger on the bobbin case holder]. With the second arrow, above, you can see the needle coming down through the slot in the basket.)
Here the basket has popped out of place and is rotating with the hook. The needle is continuing to descend...
...and will collide with the basket ... now! Your needle will now be missing its tip and your case opener (that lever on the bottom of the photo) will now be pointing to the right, knocked outta place.

The great tragedy of this is that Juki sewing machines have such a good reputation that no one believes me when I tell them that I'm having trouble with my hook assembly. They usually try to blame the problem on me, or tell me to call someone else ... in circular fashion.
And since this only happens with heavier material, it wasn't caught at the factory during the final sew off.
Fortunately, Juki America has been fairly helpful, and I've sent my hook in for inspection.

Note: I've received my replacement hook and I've just installed it. So far so good. Thanks, Juki! [Keep reading for more updates.]

Though, I swear this machine must be cursed. First I received a defective table leg, then the motor belt was too small to match the drilled holes, now a crummy hook assembly.
Sheesh. Give me a break.
Lucky I still have my Tacsew 111. Almost half the price and sewing strong.

So here's me taking the hook assembly off (don't do it like this -- the whole thing comes off w/o taking it apart):
I was hoping to just move the feed base up a bit and slide the whole assembly out, but the thing's so complicated on this machine, I just took the assembly apart and got it out that way.
[Note: I figured out you can just loosen the bobbin finger/holder (that piece with the nubbin on it) and swing it out of the way and that leaves just enough room for the hook assembly to slide out. That's when the feed dog is at its highest point.]

Above and below is the hook with no basket.
Made a li'l mark on the machine so I can roughly line it back up ... though, there's a groove in the hook shaft...
[Note: Lucky there's a groove 'cause I forgot what position the needle was at. You need another reference point, otherwise this is useless. Ha.]
Needle plate removed. The hook just about to come back up. This is where the hook would snatch the thread loop.
No more hook assembly.

Update (5/10/15)
Probably should've updated this years ago, but I only use this machine occasionally now, mostly just for thick foam applications ... but I've recently received some emails from people with this same problem and seen this problem come up on some forums, so here it is:

After I installed my new hook assembly back in oh-10 I still experienced the dreaded hook jam, though not as much as with the original hook assembly. I've thus concluded that these hooks are bad -- bad to the bone. [If I can't get other hooks in other sewing machines to jam the same way by pushing on the bobbin, and I can with the 1541S, there's definitely something wrong with this hook, but maybe that's just me.] Though, I've figured out a way to work around this issue. First, let's look at the threading diagrams for the 1541 and 1541H (heavy duty):

Above is the threading diagram for the 1541 and 1541S (safety clutch). Note the sliding thread guide (red star). This is used to adjust how much thread goes around the bobbin -- or something like that. I can't find it addressed specifically in the manual.

 

Now look at the threading diagram for the heavy duty class. No sliding thread guide! This is key. Here's what I think is going on:

These machines are set up to sew through a couple layers of upholstery material, and when you go to sew through anything heavier, there isn't enough thread to go around the hook assembly smoothly, putting inward force on the bad bobbin basket, which then binds.

To avoid locking up the bobbin, I've been threading the machine the H way (or similar) with good results. This is how I was threading it the last time I used it, sewing 1/2-inch closed-cell foam and Cordura. I've skipped the sliding thread guide and the square thread guide, leaving plenty of thread to travel around the bobbin.

Notice the sliding thread guide is oriented the wrong way. I'm pretty sure threading it like this will actually take up more thread. I was playing around with different guide configurations a while back. I should set it back the way it was...

Here is another look at the hook assembly and related parts. Juki has a real nice database. You just need to set up a user name and password.

If this happens to you, the hook assembly will catch the needle and the machine will jam. If you're sewing at speed, your needle will shear off. Clean out your ruined needle and thread and turn the handwheel backwards till the hook assembly pops back into place on the finger and then look for issues (you will most likely have to reset the case opener lever (on an S class). You may have to adjust your threading, as shown above. Or it's possible thread has been pulled into the hook raceway and caused the jam.
This might be caused by another issue, brought up by a sewing tech on Upholster.com (2/7/11):

Re: JUKI 1541-S shuttle/hook assembly out of whack, help me fix it!

I know exactly what is wrong, and I can explain how I know; because I check every Juki DNU-1541S before it leaves my shop for this known (by us) issue with this model, out of the box.
I can't tell you how to fix it, as I do not care to go head to head with another sewing machine mechanic. They may have a different resolution to this that I do, and I don't want to get into a tit for tat, he said she said brouhaha.

But, I can tell you what I know for absolute sure; the hook supporting plate/bobbin case finger that holds the bobbin case/inside hook from spinning is hanging on by a finger nail, and does not go in far enough. Unless this is taken care of, this will be an issue that will continue to plague this machine.

I've reset my hook placement before, and tested it by hand to make sure it was solidly engaging with the finger, and still had it pop off. There is quite a lot of force behind that hook when it jams. The finger-placement problem explains why it comes freely on certain 1541s, though -- and is maybe further proof of a larger quality issue with these machines.

And here is a thread on Leatherwork.net where the hook assembly popped free.

I'm hoping, as I use this machine more, the hook breaks in and never does this again. I oil my hook manually now, and I'm pretty cautious when I sew heavier stuff. Hopefully one day I can relax while I use this machine. :)

Update (5/25/16). I've been informed by a sewing machine mechanic that there is a break-in period for hooks -- even high-speed Juki hooks. He recommends turning the hook oiler all the way up.


Update (6/30/20, Year of the Covid). I've had a request on how to adjust the bobbin case positioning finger (that thing with the nubbin that keeps the bobbin basket in place).

The idea here is that you want the nubbin to go into the bobbin-basket notch far enough to keep the basket in place if there's some friction in the raceway, but not too far that there's no room for the thread to pass around the nubbin. You'll probably have to do this through trial and error. It's possible that if the basket has popped out a lot, your nubbin may have been damaged and you'll need to secure a new finger.

Here's what that finger looks like outside of the machine:

And here's where the screw is that allows you to adjust the finger. Again, don't take your hook apart to get it out ... just remove the finger.

Good luck!