Tangled bobbin thread on start

[unionattic]

I have a sixty year-old Pfaff 471 postbed machine.  I'm using Serafil 20 and Serafil 30 thread mostly.  Am sewing ~1.0-1.5mm leather.   I'm using a powerful servo motor with needle positioner which makes using the machine very nice.  I've got one problem I'm unable to debug, however,  Intermittently when I start a fresh stitching row (generally sewing leather) the machine stumbles for a few stitches and then starts sewing totally normally.  However, on the underside, a tremendous amount of bobbin thread (I presume it's the bobbin thread) is tangled up.   The remainder of the stitch, top and bottom, is beautiful.

I cannot figure out why this is happening. It doesn't always happen, although it happens in spurts.   Pulling on the top thread and bobbin thread when I start the stitch makes no difference.  It occurs regardless of whether the bobbin is full of thread or nearing empty.  I'm not messing w/ stitch length or tension or sewing speed, so those variables aren't contributing.  It's really annoying because it's intermittent and ugly when it happens. 

 

Attaching a couple test pieces I used when I was trying to diagnose.  As you'll see, I only got the starting tangle once in several tries.  Sometimes it will happen far more frequently.

 

 

 

 

[unionattic]

Does anyone have any hypotheses on what's causing this? It's really annoying and messing up some shoe-work I'm trying.

 

 

[Pintodeluxe]

Timing slightly off and worn tension discs are likely culprits.  

[unionattic]

On 2/4/2021 at 2:06 PM, Pintodeluxe said:

Timing slightly off and worn tension discs are likely culprits.  

what tension discs?  The discs under the white button tensioning the top thread?  It seems to be a bobbin thread phenomenon, not top thread.  

[Pintodeluxe]

Yes the discs under the white knob.  If they are old and have grooves worn in them, the upper tension will be erratic.  This can cause intermittent loops on the under side of the fabric.

[unionattic]

I worked out the issue. Operator error. The machine was fine.  In short, when I place the needle, there was slack top thread, so it would get sucked down into the bobbin-side of the workpiece.   I made a dull video to demonstrate what was happening.

Avoiding tangled thread on start of a stitch

 

[Wizcrafts]

Excellent video explanation showing what was causing the top thread to get drawn down under the work. We learn to hold back the threads at the beginning, or else! This video actually shows the slack top thread above the last thread guide and it is that slack thread that gets pulled under.

[sandmanred]

I've also noticed that when my Pfaff with a rotary hook was a little out of time it was much more prone to this problem.  It is much less prone to it now that I re-timed it.  The hook was a little more advanced that it was supposed to be and would just snag the loop from the top as it was being pulled in the revolution that it's not supposed to catch anything.

[DonInReno]

Great video - thanks for showing us what it turned out to be.  So often we never get to hear what actually resolved the problem.