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alpha2

CB4500 stitching nightmare solved!

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Skipped stitches, shredded thread, complete re-timing of machine, every imaginable tension adjustments. Numerous needle changes to no effect. Same machine, needles, thread, leather that have always worked before. (You MUST see a pattern there!) After reading through all the threads on this forum, watching videos, talking to Cowboy Bob, I decided to just strip the thread off the bobbin and reload with fresh thread. Voila! The bobbin thread had developed a curly-q nature. What gave me the idea that might be the issue, was a video on missing stitches in reverse. (Wiz, Uwe? Sorry, don't remember. But if those clear plastic needle plates are for sale...call me! Best troubleshooting device...ever!) Not my issue, but watching that little loop formed when the needle started back up, so the hook could catch it, and just how important that loop was, suggested that any coiling of the thread from that bobbin, (only a third full of thread) that had been sitting on it for SOME TIME, got me thinking. I also found that I had put the conical spring in the bobbin carrier upside down. I reversed, (corrected) it, but it didn't do any good. I could tell that when it was in backwards, there was no bounce back when you pushed on the bobbin, like you usually see.

Another thing, I remember seeing somewhere to tighten the bobbin carrier race screws, then back off one and a half turns. I tried to find that again, in case that was an issue, but couldn't. I did find two locations where it showed, but didn't explain, tightening them and leaving it at that. If anyone has any thoughts on that, feel free to post something. I had always backed off 1.5 turns, but now have them tightened. My thought is that if you have a screw with a spring, the spring is there to assure the appropriate tension when the screw is tightened. The very idea of tightening a screw, then back off an ambiguous amount, to achieve the proper tension, ignores the concept of a proper tension spring.  

When testing all the available avenues of repair, I used a considerable amount of the same weight and type of leather. It would sew just fine for from 3 inches to as much as 12 or more before the skipped stitches started and the thread shredded. You can imagine how I felt when I put the actual work piece in and started stitching. Cold sweat, pounding heart, promises to higher powers. I did sew three feet of successful stitching before I attempted it for the money.

Finally, when I figured it out, I put in a call to Bob in Toledo with what I found. If we call these distributors with problems and questions, if we discover the issue, we really need to let them know for the next person with the issue. It's hard to believe, but they haven't seen everything. When I was working, I thought for sure that I'd finally seen everything, then reality bit me on the...arm.

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3 hours ago, alpha2 said:

we really need to let them know for the next person with the issue. It's hard to believe, but they haven't seen everything.

True, and thanks for sharing here as well.  Has me thinking about those bobbins with odd thread on them sitting for long stretches at a time...

YinTx

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Thanks for posting this information. There are so many different things that can cause skipped stitches. I've had trouble with coily top thread, but never suspected the bobbin thread before reading your analysis. It makes sense though that is one winds their bobbins from the same spool, or thread batch that is coily on the top, it will also be coily in the bobbin. I have found that Weaver's lubricated black bonded thread tends to be really coily/springy. I have to route the thread counterclockwise through the top post to counteract that coiliness.

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3 hours ago, alpha2 said:

Another thing, I remember seeing somewhere to tighten the bobbin carrier race screws, then back off one and a half turns. I tried to find that again, in case that was an issue, but couldn't. I did find two locations where it showed, but didn't explain, tightening them and leaving it at that. If anyone has any thoughts on that, feel free to post something. I had always backed off 1.5 turns, but now have them tightened. My thought is that if you have a screw with a spring, the spring is there to assure the appropriate tension when the screw is tightened. The very idea of tightening a screw, then back off an ambiguous amount, to achieve the proper tension, ignores the concept of a proper tension spring.

Bob Kovar advised me to back off the shuttle screws so that the coil springs let the shuttle assembly move in and out with thick thread. Tightening the screws all the way fully compresses my springs and the shuttle pinches thick thread as it goes around the shuttle. This throws off the tension settings. Backing off at least 1/2 to 1 turn lets the assembly float with the thread, as necessary. I don't back off 1.5 turns, just 1.

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On 2/19/2021 at 7:39 PM, Wizcrafts said:

Thanks for posting this information. There are so many different things that can cause skipped stitches. I've had trouble with coily top thread, but never suspected the bobbin thread before reading your analysis. It makes sense though that is one winds their bobbins from the same spool, or thread batch that is coily on the top, it will also be coily in the bobbin. I have found that Weaver's lubricated black bonded thread tends to be really coily/springy. I have to route the thread counterclockwise through the top post to counteract that coiliness.

Interesting point about the top post. Thanks for that.

On 2/19/2021 at 7:46 PM, Wizcrafts said:

Bob Kovar advised me to back off the shuttle screws so that the coil springs let the shuttle assembly move in and out with thick thread. Tightening the screws all the way fully compresses my springs and the shuttle pinches thick thread as it goes around the shuttle. This throws off the tension settings. Backing off at least 1/2 to 1 turn lets the assembly float with the thread, as necessary. I don't back off 1.5 turns, just 1.

I'll back it off a bit then. I had always done that previous to this issue, and it always worked fine.

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