CB4500 reverse stitch bottom loops

[Wizcrafts]

I don't usually have top thread loops on the bottom layer on my CB4500 when sewing in reverse. But, the bottom tension is sometimes reduced and I occasionally get skipped stitches going backwards. These are timing and check spring problems that I adjust for when they bother me enough. One should first set the timing to the default position. This is done with the stitch lever in the zero length position. When the needle passes BDC and has risen about 3/16", the hook should meet the center of the needle about 1/8 to 3/16 inch about the top of the eye. This is not a precise measurement. I find that different thread colors and sizes change the size of the loop. But, if you have a nice size loop at the zero stitch position, the forward and backward stitches should be good in both directions.

What could cause the hook to eye of needle timing to change?

Sometimes, despite having two hex head adjuster set screws on the needle bar clamp, the force of impacting tough leather can cause the bar to move up inside the clamp on the crank shaft that moves the needle bar up and down. This raising of the needle throws off the timing. When this happens you can simply zero the stitch lever, lower the needle until it passes BDC and moves up a few silly millimeters (3 to 4 mm for my South African and European friends) and the hook is in the middle of the needle. If the hook is too close to the eye, loosen the needle bar clamp screws and manually lower the needle until the hook is properly centered inside the thread loop, or about 3 to 4 mm above the eye. When you re-tighten those screws, make sure the actual needle clamp has not rotated out of position. Note, or photograph its original position as a reference.

The idea is to position the hook in the cutout (scarf) above the needle's eye where it has the best amount of top thread loop to pick off. There is a happy range of needle positions to do this. But, if the tip of the needle is too low, it will hit the bobbin case: = very bad. Too high and the hook will tend to miss the loop. Most dealers recommend lowering just the needle first to test for this vertical position problem. If lowering the needle improves the situation, then lower the needle bar. If not, you've saved the turning effort on a couple of screws that are usually set very tightly at the factory.

Advanced information!

The actual hook timing is adjusted with a large hex wrench inside the round access port cover on the right front of the machine body. There are probably YouTube videos showing how this is done. When I need to re-time my machine, I loosen the big bolt inside the access port, leaving it just tight enough that it turns with the hand wheel, but is turnable by finger pressure. I adjust the position of the hook by hand to meet the middle of the needle on the upstroke, centered in the thread loop, where it is giving me problems, then tighten the bolt down. This adjustment may only need to be tweaked a couple of degrees to make the machine stop snapping the thread or skipping stitches. Do this at your own risk!

Enter the check spring

The check spring must hold the top thread taut at least until the needle pierces the top layer. If it lets go too soon, the tip of the needle might split the thread before it penetrates the top layer, or on the way through the leather. There is a bottom stop that is movable to stop the spring's travel higher or lower.  It must stop moving down after the needle passes though the bottom layer, or there won't be enough loop for the hook to pick off. In between the extremes is a small amount of happy adjustment range that can cause the stitches to be looser or tighter.

Often overlooked is the tension of the check spring. There should just be enough tension to let the spring all the way move up on the upstroke and all the way down to the movable bottom stop on the downstroke - of the take-up lever.

A jumpy check spring may indicate poor or marginal timing, or pinching of the top thread as it goes around the hook. If there is insufficient clearance for the top thread to clear the exit side of the hook, it will bind and cause loops on the bottom, split thread, or other problems.

[Uwe]

I decided to get to the bottom of my machine's issues in reverse at max stitch length. I now know how it snags the thread and why it occasionally skips a stitch in reverse. Exactly how to fix the issue, I'm still not sure about. 

I made a needle feed throat plate from clear acrylic so I see could what was going on below. I made a video for the occasion and it might be of interest to folks with a Juki TSC-441 class machine, even if they are not experiencing any issues.

On my machine the thread snag is caused by the top thread loop getting pushed into the race channel by the hook. Using thicker thread may actually avoid this issue because the thick thread is less likely to squeeze into the tight gap between the shuttle and race channel. I'm using size 138 thread in the video. I may need to tighten up the race channel a little (not sure how/if that is even possible.)

The hook-to-needle timing on my machine looks spot-on to me. Alas the hook does not go back quite far enough to make reliable stitches in reverse at max stitch length. Perhaps retarding the hook a little bit may improve clearance between the needle/thread and the tip of the hook in reverse at max stitch length.  Sewing at higher speed may also help, actually, as the hook gets tossed back and forth and is more likely to rest against the shuttle drivers push point. My slow-motion hand turning in the video does not create much momentum to toss the shuttle to the limit stops.

 Here's the video for your entertainment:

 

Edited May 27, 2017 by Uwe

[Constabulary]

Acrylic needle plate - EXCELLENT IDEA - not only for Cowboy 4500

[LumpenDoodle2]

Agree, what a great diagnostic idea. 

[gigi]

9 hours ago, Uwe said:

Here's the video for your entertainment:

Thank you for releasing another great video for the community!  We've learned a lot about timing from your videos.

[RockyAussie]

This is just a thought that comes to mind as I have known of some sewing machine mechanics to tighten the shuttle race screws fully in tight.The 2 I am referring to are the ones with springs behind them.I believe if they are tight they can impede the thread from going around without being caught up.This following link I think should be checked around the 9 minute in mark to perhaps better understand.
https://www.youtube.com/watch?v=TvHgMDlfQo0

I initially received mine with them fully tightened and had some problems.I have not noticed any problems since I have loosened them and timed the machine following the Juki 441 engineers manual instructions with regards to height measurements. Thanks for the Video share Uwe and Wiz for your as usual superb break down of useful tips.

 

[Zandeosaddles]

On 05/26/2017 at 5:19 PM, Zandeosaddles said:

Good day all. I am having a problem with my machine. It stitches fine forward, but in reverse it makes loops underneath. I have run through all the instructional videos on you tube. All the basic checks are done. When in reverse, the machine sounds as if the tension of the upper thread is too tight as opposed to sewing forward, yet it is the top thread that loops underneath in reverse. I would greatly appreciate it if someone would take up the challenge of running through all the settings with me esp the uncommon ones. My machine was never properly setup by the agents in south africa. I got a technician out to set it up. I recently re did the timing, feet balance threading, oiling race screw tension from videos, just to be sure. The thread is wound correctly on bobbin.  my email address is zandeo@gmail.com. thanks very much.

 

[Zandeosaddles]

Hi Rocky Aussie. Thanks forthe advice, I have the screws on the race one full turm loose. The check spring theory makes sense butI would like to ask if a poorly functioning check spring should affect forward stitch as well?

GiGi, my machine is 3 years old and is not nearly worn. Could there maybe be a manufacturing glitch on one of the parts?

[RockyAussie]

Just a couple of things I've noticed by going back over Uwe's video. My hook goes back at least another 3 to 4mm than does Uwe's. It appears that the thread in Uwe's snags in the race and logically that could impede the bobbin/hook from falling back away when it should. I would guess as Constabulary suggested that the shuttle race packer needs to be a bit thicker in that case. I believe a coke can thickness could work if that helps.Equally if the race is not freely moving without thread in it the packer may be damaged or too tight for that size thread. Best of luck Brian

[Wizcrafts]

10 hours ago, Zandeosaddles said:

Could there maybe be a manufacturing glitch on one of the parts?

You betcha it can! I thought I would upgrade my CB4500 to a Juki hook, which cost about $250. It sewed perfectly in forward and virtually not at all in reverse! The top thread got pinched between the shuttle and the race every time I tried sewing in reverse. I backed out the screws to no avail. I retimed it to no avail. I finally cut my losses and went back to the OEM Cowboy hook. The lesson? Juki 441 hooks belong in Juki built machines.

Seriously though, your hook may have a manufacturing defect. Take the machine to a qualified Cowboy dealer, or ship the hook to one for inspection and possible replacement.