206RB4 thread snaps across bobbin face

[Dave4]

206RB4 TH

I read the Navy SM Chpt4 and Consew and Seiko manuals, did all the adjustments in the Consew manual, also the feed adjustment from the STH8BLD3 manual, and also set the correct presser bar spring bracket height.  My various timing specs and my latch opener are all good.   

The machine sews well, but the thread snaps abruptly across the bobbin face instead of gliding smoothly over in a lovely poetic sweep, and I'm missing an occasional stitch.  

I have tried lighter and heavier variations of upper and lower tension, with no apparent effect.  

I saw a Uwe reference somewhere that mentioned timing of upper and lower shafts as a possible cause of this problem.  The recommendation was to make sure the shafts are properly timed, but the machine in question was a belt drive.  Mine is gear drive.  

Unless I overlooked something, I can't find any info on timing upper and lower shaft on 206RB gear-drive machine.  Is this something I need to check / adjust?

For a 206, what would be the most common causes of the thread snapping over the bobbin face?

Thanks!

[MADMAX22]

Thought this was just covered but cant remember what thread its in. 

Uwe has a couple of videos showing this issue and how to correct it if I remember correctly. 

EDIT thought it was for 206's but its for Juki under this thread 

 

Edited November 29, 2016 by MADMAX22

[Uwe]

My videos on the thread snapping topic (before and after, belt adjustment ) were related to vertical axis hook machines with a timing belt (the TacSew T1563 and Consew 225 in my case). Gear driven horizontal axis hook designs are different animals. Mixing and matching adjustment rules may have unintended results. I don't have an RB4  machine and I don't have an RB4 adjustment manual, so I can't really investigate.

One quick check you can do is to make sure that the thread take-up lever is in its lowest position exactly when the thread wraps around the very bottom (far point) of the bobbin case, thus providing the most thread slack exactly when it's needed. I believe that's kind of a universal rule regardless of the machine's hook axis orientation. If that's the case and the thread still snaps around the bobbin case, something else is amiss (or a mess). If the thread take-up lever is NOT in the bottom-most position when the thread wraps around the far side of the bobbin, we need to figure out why and how to adjust it.

A close-up video snippet would be great if you have the option.

[Wizcrafts]

If the check spring throw is too long the top thread will be taut going around the bobbin case. The solution is to shorten the throw by means of the sliding bottom stop bracket, or the curved slot if your machine has that type of spring adjuster. Experiment! The check spring only needs to hold the top thread taut until the needle has fully penetrated the material.

[MADMAX22]

Good info Uwe, I forgot about the orientation of the 206's (have not had one yet) and thought it was the other way around. 

[Dave4]

OK, thanks to Wiz's suggestion, I gained a much better understanding of the check spring.  

The combined available info from the 3 manuals referenced above is that the spring should come to rest on the stop only once the needle tip has touched the goods and the spring should "pause" while the needle raises as the thread passes the bobbin case.  Based on that info, I set the stop in a position that turned out to be too low (too much throw) because my main focus was to be sure the spring was holding tension until the eye buried itself in the goods.  

Taking Wiz's suggestion to "Experiment!", I learned to start watching the tiniest details in slow motion.  I learned that the stop can be set to catch the spring just as the needle tip contacts the goods and when so, the subsequent takeup action will keep adequate tension on the thread until the eye is buried, even though the spring hit the stop a few microseconds earlier.  I learned that the upper end of a grabby thread that is snapping across the bobbin face actually kisses the spring several times if the check spring stop is too low, which basically tensions the thread while it is trying to cross the bobbin face, which is bad.  By fine tuning the stop, I achieved some improvement: the thread became a little less snappy across the bobbin face.  

Note: my stop is now advanced almost to the limit, and my check spring throw is now quite short.  I has a total sweeping movement of only 4-5mm, and only takes up tension when the needle clears the goods and pulls the thread up to cinch the lock stitch, and only holds tension until the needle tip comes back down to touch the goods.  Other than the Seiko 0.039-0.078N spec, there is no frame of reference for setting check spring tension.  Lacking the proper scale and linkage, I just played with mine until it seemed to match my overall upper and lower tension settings.  When too loose, my knots were "fat".  When too tight, it made the snapping sound of the thread across the bobbin louder, and it made the top thread lay deeper and tighter in the material, so it seems like too much check spring pre-load tension contributes to overall thread tensions.  

Lastly, the tiny screw at the top of the discs warrants mention.  One - the head on mine broke when overtightened, so don't do that.  Two - loosening this screw allows you to rotate the discs slightly CW or CCW.  The CW adjustment adds to check spring tension and action by moving the "passover finger" between the discs away from the vertical axis of pull of the takeup lever.  This creates a slightly bigger additional corner that the thread must pass around as it routes between the discs and takeup lever, which equates to more spring action.  CCW adjustment can move that finger completely out of the route, equating to less spring action, which is the way I went because my overriding goal was to alleviate the snapping across the bobbin face problem.  All of the above helped, but didn't completely fix the problem, which was vexing.  

Max linked the Uwe thread I had read earlier, where Uwe mentioned the timing relationship between the lowest point of the takeup lever and the thread crossing the bottom of the bobbin face.  For the second time, i took Uwe's advice to confirm that this was happening, and yep, it looked pretty damn close, so I was still vexed, so i did a little more reading and noticed some careful language in Uwe's suggestions:  "exactly" and "very".

He ain't kidding.  

The takeup lever has a slight dwell at it's upper and lower extremes.  My thread was snapping across the bobbin, crossing the lowest extreme of the bobbin during the dwell period of the takeup lever bottoming out.  In slow-motion, I saw that the thread was arriving at the bottom of the bobbin at the end of the dwell period.  The timing looked pretty good, but in fact there was just no margin left in the sequence and it was biased to being too late.  Hope that makes sense.  Focus on the word "exactly", and you may see what i saw on my machine.  The hook was bringing the thread around a hair too late in relation to the takeup lever bottom dwell.  

This brought me back to an original question in my OP: timing between arm shaft and hook drive shaft (as it is described in the Navy manual).  They describe the process for the 225/226 type machine, which has a horizontal bobbin with belt drive.  Obviously, the 206 is vertical bobbin, gear drive.  My Seiko and Consew manuals don't mention "arm shaft to hook shaft timing" in the adjustment sections, so being  the noob, it didn't register in my head until i did some more reading and (finally) realized that "Timing hook to needle" is the same thing.  duh.

A little background: My machine was grossly maintained before i got it.  It was well oiled, but nearly everything was out of spec, the latch opener was disabled, set screws were missing, others were loose, the lower gear case was cracked and near a catastrophic event due to a set screw floating in the grease, the feet lift heights were way different, the needle wasn't centered in the dog hole, the dog timing was off, the dog travel was off, the check spring was disfigured, the thread discs were flipped backwards, etc.  It was a mess.  BUT, one pleasant early discovery was that my needle eye height above the throat plate was exactly to spec (22.3mm) and the hook passed exactly 2.4mm above the eye, so that part was timed perfectly!

So why would i have to adjust that, right?

I guess the answer is because book spec's aren't always the exact settings you need.  So to fix my snapping thread problem, I advanced the hook shaft to move the hook past the scarf, making it too "early".  As follows: 

I sewed 5 stitches in my test piece, left it in place, flipped the machine back, rolled it by hand to find which of the three set screws on the distal side of the clutch would be available when the thread was a 3rd of its way into its snapping journey across the bobbin face, rolled it more to loosen the other two, rolled it more to bring the offending thread into the snappy tight position just shy of the bottom of the bobbin face, then loosened the 3rd adjustment screw on the shaft and advanced the hook slightly by hand to bring the offending thread just past the bottom of the bobbin case (a mere 8° twist of the shaft), and locked everything back up.  

Then I lowered the machine, removed the throat plate, and made the final hook-needle timing adjustment by raising the needlebar slightly to compensate for the earlier arrival of the hook.  My eye height spec is now 23mm, my thread no longer snaps across the bobbin face, and the machine sews nicely.  

Caution: if you advance the hook shaft too far, you move into a timing realm where the needlebar rock frame will no longer travel upwards enough from bottom to create a loop for the incoming hook, no matter how much you play with the needlebar height.  

There are some things I still need to do:

1 - I need to go back and check the timing of the latch opener.  It's driven by a cam on the hook shaft, and i advanced the hook shaft, so i will probably need to reset the fork on the opener shaft to restore the proper timing of the opener event.  

2 - re-examine the CW / CCW adjustment of the thread discs, because i can now afford to have more check spring action.  

3 - I need to get some tips on how to set presser pressure.  I know how to turn the adjustment screw that pushes down on the bar spring, and i have just wheeled away on that thing with no real observable effect, so my problem is that I don't have any frame of reference for identifying when I have too much or too little pressure.  Where do you guys start, and how do you know when you're "there".  

Lastly, thanks to everyone who contributes to this board.  In my younger days, I modified and raced jap bikes.  1 Honda, 1 Yamaha, 2 gsxr's and a gsx1300r.  I service my own autos.  I build small custom circuits.  My hobbies led to my taking for granted the availability of comprehensive (even exhaustive) service manuals for technology.  Then I got some sewing machines.  It didn't take long to realize that comprehensive service manuals are not common companions of sewing machines.  That makes this board a goldmine.  Of late, I have been especially grateful for the contributions here from Uwe, Eric, Bob, Gregg, the Wiz, the Constable, and Art.  I'm sure there are a few others, but having read half of this board over the last few months, and being over 50, I can't remember what day it is, much less everyone I read, so thanks to all!

Hopefully, my documenting my own learning curve in (too) long posts will be of some help to future readers.  Pay attention to the E-words: Experiment, and EXACTLY.  

[dikman]

Sounds like you're having fun! . One thing I slowly learned, as you have, is that once you get into "old" sewing machines you can't treat them like modern machinery etc as the information can be difficult to find - if not impossible (as in the Singer SV machines, lots of models but virtually NO handbooks/manuals anywhere!).

I also learned to treat the adjustments/settings in the manuals as merely a starting point. It takes some time to understand how the machine works and how the various bits interact, and I have tweaked all of my machines in various ways. Just because the manual specifies a particular setting (hook above needle eye, for example) doesn't mean that your machine must be set that way. A particular thread/needle combination may require a different setting to work.

I consider it very important to have an understanding of the machines I have, not only from a maintenance viewpoint but so that I know I should be able to fix the inevitable problems that will arise (after all, these ARE old machines - bit like me, I guess ).

[Dave4]

I was working on my 206 again today, and found so more info that may help others with snapping thread problem.  

Read This.

 

[Wynk]

Bless this thread and much thanks for Owe, Wizcrafts (who consistently come up in my Google searches when I'm troubleshooting) and to Dave4. I was having the same problems with my Chikon (aka Seiko aka Consew) STH-8BL and Dave4, yes, it was the all mighty timing that I was afraid to touch after playing with tension, check spring, feeders, etc.,  but which, following your directions, has improved my machine (that I just got off Craigslist for $250) from hair pulling to at least acceptable. Just joined the forum to say thanks (and I'll probably be back with more questions which, of course, I will search the hell out of this forum for first). 

[Gregg From Keystone Sewing]

Also, check the bobbin case, often when or if there is a problem the latch is not secured or damaged.  Among other things.