Cath79, on 18 January 2012 - 09:24 PM, said:
I am having mega problems getting proper tension my cobra. It's been months of hours, trying and giving up. Going back to hand sewing and my old crappy machine. I am worn out, and confused! I finally got it tensioned for one type of sewing I do, took DETAILED notes of the tension settings, then went back to try and tension the other way I sew, and it took hours and hours, blah bah blah. Then I need to go back to the first way I sew, and my notes don't work. I can't get it tensioned back. Am I missing something? I am also powering through my thread and using up leather to test samples like crazy. It's wasting so much of my time, and a lot of materials. I've asked Steve, but he hasn't been able to help. I am hoping someone else with a Cobra can offer me some tips?
The first type of sewing I do is one layer of veg tanned about 6-7oz to one layer of chrome tan 3.5 oz. I use a 25 needle, with appropriate thread.
The second type of sewing I do is min 2 layers of chrome tanned, to max about 4-5 layers. I use a 21 or 22 needle, with appropriate thread.
I have got the machine to flawlessly sew both, eventually. But every time I try and tension for the other type of sewing, it takes me hours, days. Or longer. Or I give up (and thank God my old crappy machine still works well enough!)
In these photo examples, you can see how it varies between top tension is too strong, and then too loose. I CANNOT find the balance. (and yes, the top & bottom thread are the same thickness & brand). It seems I cannot make a small enough adjustment, so it goes between too little and too much tension immediately. Sometimes I can get it to sew normal for 5-6 stitches, but then it always reverts to one way or the other (top is too loose or too tight)
First of all, although you haven't said what model your Cobra is, I want to reassure you that the Cobra 441 clones are fully capable of sewing balanced stitches in leather thicknesses from under 6 ounces to over 3/4 inch. I regularly sew with a Cobra Class 4, so I will try to help you with some suggestions.
The 441 clones are not light duty sewing machines. They need a certain amount of top and bottom tension and pressure to operate properly. An overly tight bobbin is as counterproductive as one that has no tension at all. Set a modicum of tension on the bobbin spring for the thread you are using. Not too tight, not too loose. It should pull out smoothly, without any hiccups. Hiccups can be caused by leaving a piece of the tied down starting thread looped around the hole it goes through and the top of that side of the bobbin. Cut off any thread that protrudes from the holes in the end disks of the bobbins.
Thread the bobbins to feed backwards in the case. With the bobbin case open, rotated backwards so the opening is facing back towards the front, hold the wound bobbin so that the thread feeds off the top towards the back of the machine. Place the bobbin in the case and feed the thread through the slot and snap it securely into the tension spring. Make sure the bobbin thread feeds under its spring and out the mouth end, then snap the bobbin case shut. Pull on the thread that protrudes (4 to 6 inches) and make sure the tension has not changed.
Consult the instruction DVD to ensure you are threading the top thread as per Steve's instructions. Before you feed the thread through the needle's eye, pull on it to make sure it is under tension from the top disks. Adjust the spring tension as needed to get a strong amount of resistance, while still being able to pull the thread a bit. Lift the presser foot release lever with the foot pedal and see if the thread pulls easily. Let go of the foot lifter. Now lift the hand presser lifter on the back and see if the top tension has been released at all. I find that the hand lifter only releases a little top tension, whereas the foot lifter lets it go almost completely.
The hand or foot lifters, when lifted, should force the top disks apart (with a little lever that moves from the bottom of the disks) to release thread tension. Then, when you release the lifter, the lever that splits the disks must drop down to allow the disks to close and pressure to be applied, as set by the finger adjuster nut. If this lever is sticking in the disks, the tension may not be properly applied to the thread.
If the check spring has too much tension, it can cause additional drag on the top thread. The spring has two possible adjustments. Once is the movable stopper plate under it, which sets how long the top thread is kept taut during the down stroke.This is set by the single flat head screw that sits inside the slot in the stopper plate. This is a trial and error adjustment. You don't want it to stop short, nor all the way down, in most cases. set it somewhere in between. If the spring has a LOT of tension, reduce it by loosening the nut on the back of the face plate where the bottom disk shaft is mounted. With the nut loose, insert a small flat bade screw driver into the split shaft and turn it whichever way loosens the spring action. Loosen the spring a little then tighten down the locking nut.
Make sure you thread through the needle from left to right, with the cutout scarf facing the dead-right side.
If you are using a #25 needle, it works best with #V207 or T210 thread, top and bottom. It will feed #277 on top, but 207 on the bottom is easier to pull up into the leather. If the leather is soft temper, you may be able to get away with a #25 needle and #277 thread, top and bottom.
For #138 thread, I use #23 leather point needles. Yes, a 21 or 22 works, but the #23 seems to sew more reliably. Use system 794 Schmetz needles if possible.
Set the top pressure spring adjuster so it sticks out the top between 1 and 1.5 inches.
Before you start sewing, hold the top and bottom threads back hard. Sew in reverse a couple stitches, then throw the lever to forward and cross over the starting threads. Let go of the threads and sew slowly for a few inches. Lift the foot lifter and remove the leather. Check the position of the knots. If you see the top thread knots on the bottom, increase the top tension adjuster. If the knots are on top, reduce the top tension.
If the stitches look decent, sew a longer run of stitches, using your edge guide. As your machine is sewing, look up at the top thread and see if it is winding around anything and causing knots to form. Also, see if it is trying to twist out of the top tension disks.
I have actually seen a knot forming as the top thread twists after going through the post on top of the machine. When that happens, try changing how you feed it through that post. Twisted thread causes tension problems. It can even twist right out of the top tension disks!
Some thread is just plain shitty. If you have really springy thread, remove it and try another brand, or another spool. Thread can vary from run to run.
If the thread is feeding properly, without kinking or knotting, and without popping out of the center of the top tension or bottom tensions disks, and the bobbin thread is feeding evenly, with a bit of resistance, using the proper needle size for the top and bottom thread, your stitches should all fall into the same vertical position in the leather. Failure to do so indicates that there is either a problem with the thread or the machine needs adjusted. Sometimes a screw may shake loose and affect the timing or repeatability of good stitching. Please rule out everything else before you assume that the machine needs to be adjusted beyond the tension settings I mentioned.
I will watch for your replies tomorrow morning, or night. I'll be out all afternoon.
If you get a chance, photograph how you are threading the top thread, from the spools, all the way to the needle.