Issue With Consistent Stitch On Cobra Class 18. Help!

[Boa]

Hi,

Posted on here a while back but couldn't retrieve my login details so here I am with a new account.

Hoping I can get some expert opinion on what is going wrong with my Cobra Class 18.

Basically we are having tension issues every few seconds while using it. We will be stitching and there will be a small 'thump' from the machine which will result in the tension going slightly off and showing an inconsistent stitch. The stitching is for a product that we are trying to sell and we can have these issues constantly keep being a problem. We've fought with this issue since we got it and after having a technician come and try and address things on two occasions, and helping temporarily, the same issue keeps coming back. The tech that came said it could be the servo motor and would recommend removal as a fix............

I've made a video (apologies for my drab presenting style!) to show the problem: https://www.youtube.com/watch?v=IjTBUKUask4 and would welcome anybody's opinion on what might be going wrong (what I might be doing wrong) and how I can permanently fix it. All I need is a 100% consistent stitch. Nothing fancy.

The video is deliberately non public on YouTube and only accessible via the link in this post. I'm conscious that Cobra is run by good people and I don't want to be detrimental to them, but I still need a solution. I'm not out to adversely impact anyone.

This is super frustrating and is holding us back.

Any help, no matter how small, would be greatly appreciated.

Thanks in advance from the UK.

Grant

[Constabulary]

I once had a similar issue with my vintage Singer 111 and figured it was the thread stand that did not feed the thread properly.

Check if your thread travels the right path and if the thread comes off the cone correctly w/o hanging and causing a higher top thread tension.

You know what I mean?

I guess your machine is quite new, if not check the the tension discs if they are probably worn (but I doubt they are worn).

Or it probably the thread it self is bad. Sometimes cheap thread is not evenly twisted. Try a different brand and look if it works better with your machine.

Just my 2 cents...

Edited May 3, 2014 by Constabulary

[Boa]

Thanks. I hadn't thought about it being the thread.

I doubt that is the issue because the thread I am using is the thread that Cobra supplied with the machine. The thread seems to travel ok from the thread stand. The tech that came put a little bit of oiled paper in the bobbin case which seemed to help for a little while, but I need a permanent fix.

Machine is essentially brand new in terms of the tension discs being worn. I will discount that theory because of the fact that we have had good stitches out of it for a couple of days with the oiled paper in the bobbin case. If it was worn tension discs then that fix would not have worked.

I probably should have mentioned the oiled paper fix to help people understand what is going on!

Thanks for the reply.

[Constabulary]

Does it make the "thump" when you run the machine completely w/o thread (top + bottom)?

I´d try it once with the bobbin and once without the bobbin in place just to see if it makes a difference.

Edited May 3, 2014 by northmount

[dirtclod]

I've been sewing for a lot of years and i don't think i ever heard a machine make a thumping noise like that when sewing. Do you have any ideal whats causeing that ?

One thing you might check is make sure that who ever is sewing isn't hitting the knee lift while their sewing. I have one machine that when i was sewing i would hit the knee lift and not even notice it and it messed the stitching up. I finally figured that out and moved the knee lift a little and i moved my stool over some to make sure i didn't hit the knee lift when sewing.

Nice straps !

[amuckart]

Is it the top thread pulling the bottom thread up, or the bottom thread pulling the top thread down?

If your thread path is Ok, and your tension on the top is working Ok, it might be the bobbin winder or bottom tension. If your bobbin thread isn't wound absolutely consistently you could be having issues where the tension is 'spiking' due to uneven wind.

[ponypome]

have you tried hand cranking it without the motor on to see if it still happening

[Trox]

Thats not a motor problem. Here is several things to check out:

Its most likely the twisting (twink) of the thread that does this. Thread top thread true all guide holes, several times around the top head thread pin. And center the guide above the thread spool to be exact in the middle of the spool. This will take the "twink" out of the thread, that make uneven tension. Otherwise, check the thread tension release pin, that push the tension disk open when you lift the presser foot (check for faults). Increase foot pressure will sometimes help on such errors. Check for burr on hook tip. Wrong size or damage needle.

Good Luck and keep the motor.

Tor

[CowboyBob]

You have some good susgestions here already but,I'd like to add one more,watch the thread as your sewing between the thread stand & machine to see if it ever looks like it tightens ,once in a while a spool is defective & as it comes off the spool esp on the bottom it gets pinched & causes this problem.

[Wizcrafts]

Something might be wrong with the bobbin case or basket, or the bobbins themselves. Make sure that the shuttle mechanism is fully tightened and not able to slip as you sew. Also, after winding your bobbins, cut off the beginning thread stud flush with the bobbin. An exposed stub can catch and cause intermittent tension problems.

Try to watch the bobbin as you sew. See if the thread is twisting as it feeds out of the slot and under the bobbin tension spring. This action would cause sudden changes in bottom tension and cause the knots to appear near the top. If this is happening, troubleshoot the bobbin area and your bobbin thread winding technique.

I usually recommend inserting your wound bobbins so that they feed against the direction of the loading slot, making a sharp turn backwards to the spring. Going the other way sometimes results in changes in tension as you sew.

You might try altering both the top and bottom tensions to see if there is a happier place for both of them to get along and give a consistent knot placement.

Edited May 4, 2014 by Wizcrafts

[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.

[Constabulary]

Never heard of such issue before but I learn something new every day. Glad this is solved.

[Boa]

Hi all, sorry about the time taken to get back to you on how this all worked out.

We thought we had it solved but alas it seems we don't. This machine is a total headache.

It all worked well to begin with............

After ordering the new v belt and deciding that was more than likely the sole problem I received after a week or two it and it didn't fit! I was so impatient to get the thing working however I took off the problem belt I got with the machine from Cobra and went to work shaving the problem area down with a super sharp knife, only taking off a dusting of fibres with each moment. I tried it on and off the machine until I was happy there was no change in resistance or thumping as the machine was brought right up to speed. The machine and servo ran without a hitch. We put our 3mm (0.12 inch, sorry I'm a metric kid!) substance leather in and it sewed without a problem and with immaculate, consistent stitching for the whole 20 straps we had to produce. We were delighted.

The problem came last week. The curse of the inconsistent stitch reared it's ugly head again. We are trying to product a new type of guitar strap using a really nice quality full grain Italian leather. We had the prototype all set of the final process of stitching and yet again we got the same random stitching where about 15-20% of stitches either had the top or bottom thread pulled out of the opposite side of the strap. Stitch length is fine and normal but this nightmare with the tension never being consistent persists.

The difference between the Italian and the first (American, most of you will be pleased to know) leathers is the consistency. The American hide is a softer 'doughy' waxy drag leather and the Italian is a firmer veg tan. The Italian is a perfectly normal veg tan, nothing out of the ordinary. American: http://www.aacrack.co.uk/catalogue.asp?product_id=50 and the Italian: http://www.aacrack.co.uk/catalogue.asp?product_id=87 I thought I would link the leathers just to help in case anyone in the UK is looking for leather suppliers (they ship worldwide too). AA Crack are fantastic. Good people with a good warehouse which I have personally visited. You can order single hides or in the 1000's of sqft. Highly recommended...................Anyway.........that's off topic. The leathers have similar substance and are well within the range for the Class 18 but it is just messing up on the veg tan and I have no idea why.

I'm happy that the servo issue and 'thump' is fixed now. A faulty v-belt was the culprit and I'm past that. Unfortunately I now have no idea why we are still having issues. The sad thing is we got this machine solely to start a business and we've had a really good response from our product so far and we are really encouraged, but until I get this problem sorted we can't put out second strap type out. We could have bought a cheaper machine here in the UK but went with Cobra on their reputation for more money and 6-8 weeks longer shipping because we are trying to produce the best work we can and we are making a high end product. I'm just pulling my hair out now. It costs us £15+ in materials every time we have a stitching malfunction on a strap. I want to product a Horween strap but frankly at the cost of that leather I'd be terrified to attempt it on this machine. Thoroughly fed up. Sorry to sound so negative, but it is the one sticking point in an otherwise amazingly enjoyable start up business.

Any suggestions? You all got to the bottom of the problem with the v-belt, so I am hoping we can solve this one too.

[Constabulary]

I´d probably try a different thread brand and maybe a thicker needle. What needle brand are you using? SCHMETZ is highly recommended.

Do you pull the thread over the little hook in the tension assembly? If not - make sure you do it!

Have you tried to give the thread take up spring a bit more tension?

[Northmount]

Did you make any changes to thread (including colour) between the successful run and the new run? Was the machine re-threaded? Part of you trouble shooting procedure should be to look at all the changes that occurred between the 2 runs, not just that the leather was changed.

"15-20% of stitches either had the top or bottom thread pulled out of the opposite side of the strap." This tends to say that both the top and bottom tensions need to be increased. And as the previous post mentioned, needle size affects the tension behaviour too.

Tom

[catskin]

My guess is that you are missing one place in the threading either going the wrong direction through something or missing something that it needs to go over through or around since you seem to never have had it working consistantly well since you got it. The reason I am making this suggestion is I was threading one of my machines wrong for years and it was doing much the same thing. It would some times sew perfectly for miles and then do what yours is doing and then seemingly fix it self. I had no threading diagram. So check the threading as though you are threading it for the first time ever.