Issue With Consistent Stitch On Cobra Class 18. Help!

[Boa]

Hi all,

Thanks for all the replies. Some great logical responses to get to the bottom of this issue. My apologies for not being more active over the weekend here. It’s a bank holiday weekend in the UK and I've been away from the computer and, importantly, I don’t live on the same premises as the leather workshop.

I've just gotten back to the workshop today and I wrote a list down of all the points mentioned and sat down to go through them one by one to try and isolate the problem………….this process had unexpected results.

The plot thickens…………….

1. First test I thought we would try was to run the machine without thread top or bottom, just the needle and foot feeding the leather. I was very surprised to see that the exact same problem persisted without any thread involved. We tried this both with the bobbin case removed and with the bobbin present, but with no thread. Here’s the video showing this step: http://youtu.be/UE3GOa6XaPE

2. I noticed that the servo motor was showing varying readings while the thumping noise was happening (with and without thread). I took this video: http://youtu.be/HFb5wMSnpfM You can clearly hear the thump noise and see the corresponding change in the servo readings.

3. I decided not to tell the operator of the machine that there was a possibility that she might be catching the knee pad while sewing so I could check for this myself. I watched without her knowing and at no point did she touch the knee pad. The thumping noise happened throughout.

4. At this point we decided to try and replicate the temporary fix than the local technician did for us recently. A disc of paper oiled with WD40 was placed inside the bobbin case. We took out the old paper he put in and placed in a new oiled piece in its place (not sure how to put a photo off my hard drive on here of the paper discs). With this in place we managed to run the machine for far longer without the thump occurring however it did still occur every 1-2ft of stitching. Still not the consistency we need for a commercial product. This video shows us stitching with oiled disc of paper: http://youtu.be/kataEoPiRng

I've basically just confused myself more with these tests. I think that there is an underlying cause and that the paper disc just helps mask it some how. I think these tests would rule out inconsistencies with the quality of the thread and also the feed of the top thread off the thread stand. I would also believe it rules out improper winding technique.

I’d love to know if any of you have any more ideas at this point? I'm stumped.

Thanks again for all the help.

[Wizcrafts]

Boa;

Your first YouTube video is marked Private and cannot be seen by anybody but you.

[Wizcrafts]

Try running the machine with the top belt off the machine's pulley. See if the variation in speed and thumping exists with just the motor (and speed reducer, if any). In the unlikely event that the thumping continues with the machine out of the equation, make sure that the motor mounts and pulley are fully tightened.

If you have a speed reducer between the motor and machine, unbelt it and run just the motor. A mis-adjusted reducer could cause this type of problem. You might have to dink with the bolt securing the reducer, or replace it if it wobbles.

If you don't have a reducer between the motor and machine, try running the motor with the belt off. I have thrown away v-belts that were poorly melded out of alignment where they are joined. In fact, it happens more and more all the time. I now only buy top of the line v-belts. It is possible that your belt or belts are not up to snuff and could cause this problem.

If everything on the motor is tight and it thumps on its own, with no belt at all, replace the motor.

Hand wheel the machine and listen or feel for occasional binding. If you encounter binding, or any unusual resistance, mark/note the position and direction of the needlebar and presser feet and we can investigate further.

[Northmount]

2. I noticed that the servo motor was showing varying readings while the thumping noise was happening (with and without thread). I took this video: You can clearly hear the thump noise and see the corresponding change in the servo readings.

I see an error code 3 show up. For some servos, that is a motor overload. Have you tried hand wheeling the machine with no thread, no belt so you can feel whatever is catching? It could be a fault in the servo, but I really doubt it. To me it sounds like you have a mechanical problem.

Tom

Edit: Follow Wiz's advice to determine where the binding is happening.

[Constabulary]

Maybe it's just the lack of a stupid drip of oil here and there. Put some oil in all holes and visible moving parts and where ever you think it could use oil.

Sometimes it is just that easy.

BTW - has this machine a safety clutch?

[Wizcrafts]

Maybe there is a piece of broken thread, or foreign material stuck in the teeth of the worm gears driving the shuttle, or between the shuttle and bobbin basket?

[Constabulary]

Maybe there is a piece of broken thread, or foreign material stuck in the teeth of the worm gears driving the shuttle, or between the shuttle and bobbin basket?

... or somewhere else in the system. There are a lot of moving parts. Open the top plate and faceplate and flip back the machine and look from below to see if there is something that don´t belong in there.

[Boa]

Well the prize goes to.............Wiz!..........I think. (Somehow I though that would be the case before I even posted the topic)

I'm 80% sure that the M-37 v-belt on the speed reducer is the entire issue (hoping) and 100% sure it has at least a part to play in the problem.

Step 1: I disconnected the M-37 belt from the servo and ran the servo on it's own. I went through the full speed range multiple times and over a few minutes just to be sure. The servo ran as smooth as butter for the entire test and this issue has nothing to do with the servo. The servo is perfect.

Step 2: I hooked the M-37 back onto the servo and slowly hand cranked the large speed reducer disc to see if I could feel any inconsistencies in resistance. Sure enough I could feel a little more resistance when the joint in the M-37 rolled over the small gear on the servo. I unhooked it again and measured that section of the v-belt with callipers and there was about 1.5mm increase in both depth and width at the join on the belt.

Step 3: I marked the join on the v-belt with marker pen and sat on the floor and while running the machine with my hand on the foot pedal. I tried to see if I could hear any noises as the section marked with pen made contact with the servo gear. Sure enough there was a small thud each time the joint ran over the servo gear.

I did notice during step 3 however that the original 'thump' fault we experienced while sewing only happened at servo speeds above 15. We tried to sew our leather at servo speeds under 10 and while the stitching wasn't perfect it was vastly improved and the 'thump' never occurred. The imperfections in the stitch under speeds of 10 also seemed to have more of a consistent pattern of about every 1.5". I think what is happening is that at faster speeds and increased force the v-belt is jumping and slipping as it is hitting the servo gear causing the thump noises while stitching and the erratic tension. This might also match up to the servo showing the 'overload' error. At lower speeds there are still tension issues but the belt isn't jumping as it hits the servo gear at force so we are seeing the stitch errors with a little consistency.

So, given this I am throwing my weight behind a faulty M-37 belt causing this 10 month issue! Is there a possibility that there is more at play than just the v-belt? I think these is a small chance that there is a secondary mechanical problem because the tension seems to very sensitive to small changes in servo speed even at speeds under 8 on the servo. Who knows? I do know I need a new M-37 v-belt!

Next question........... where can I find the Rolls Royce of M-37 v-belts? In the UK I've pretty much got Amazon and eBay. I've bought 3 M-37's this morning at a total cost of about £10, just cheapo v-belts hoping that I get at least one that is from a good batch and they will arrive before Friday. I simply can't find what look to be expensive quality branded v-belts. Can anyone recommend any suppliers of brilliant belts?

Thanks again to all that got me this far with this problem. I'll try the new belt on the machine and keep you all updated with the results.

[ponypome]

yah glad you got things sorted

[Wizcrafts]

10 months of side-shock from a badly melded belt may have warped the bearing on the speed reducer, or even caused some slop to develop in the flywheel pulley on the machine. De-belt the reducer and spin it by hand, checking for side wobble. If there is an appreciable wobble, remove the wheel and try tightening up the flange on the inside of the reducer wheel, if this is possible. The reducer should revolve smoothly, but not wobble sideways.

Lineups

I go to great lengths to line up my motors > reducers and machine pulleys. The straighter the pull, the less stress there is on the three sets of bearings. Motors can be shifted laterally by loosening the three nuts under the motor mounts, while clamping the cap bots with a vise grip or suitable pliers. Reducers are usually attached to a movable mounting block that can be moved sideways to line the large pulley up with the motor pulley. It may or may not not be as easy to move the machine pulley to line it up with the reducer.

I would say to seek out "Gates" brand belts and see if they are properly welded inline. The first thing I do when a new belt arrives it to unfold it and lay it on a flat table. If it lays flat, or almost flat, after a few minutes, it goes into the "for use" hook. If it sits high in one area, I check it out to see if I can flatten it out. I also press it down at the weld joint and check to misalignment. One can shave off a small amount of material sticking out one side or the other, or just throw it away.

I always throw away new belts that don't lay flat AND have misaligned weld joints.