Gymnast

I have seen, that the servo motors have a slow start function, that you can set by a software parameter. It controls how fast the motor should increase speed to the lowest constant speed, that it can make (typically 100 RPM or 200 RPM).

Sorry, but you cannot do that to a servo motor. If you do it on the input side to the servo motor electronics, the electronics will not get the right voltage to operate. If you do it between the servo electronics and the motor, it will create all sorts of problems. The voltage here got a lot of higher frequencies, that the slow start electronics cannot handle.

As Uwe points out, there is a lot of specialized machines for shoemaking and boots. This is another kind of machine I find fascinating:

Amann thread may also be possible. It is mostly used in Europe, and it is considered a very good quality brand. They got a dealer in Canada: https://www.amann.com/company/location-contacts/ Bonded polyester with good UV-properties is named Serabond: https://www.amann.com/products/product/serabond/ For tents or something to be more water resistant, you may choose Serafil WR.

I have studied two manufacturers of gymnastics mats (or other foam shapes) with covers, that use this type of vinyl for with basic straight stitch seam. So it is mostly not for topstitching, and therefore appearance do not matter. I noticed the thread size and it is mostly Tex 90 or perhaps Tex 70 - so you are right. However I have also seen such mats after 10 to 20 years of use. Then the typical problem with the seems are that a thread is broken, so a seam starts to unravel. The other problem is, that the thread of the seam starts to cut through the vinyl. In both cases I should guess, that a thicker thread should increase lifetime.

I have measured the thickness using a caliper, and the two layers are 1.1 mm in all. The thread size used here is Tex 135 polyester with a 140/22 needle. And yes, I agree, that it is easier to balance the stitch with more layers of this fabric. Perhaps the variation in hight of the lock is the same, but it is better hidden in the thicker material. Thanks for the adwise on thread size. You can enlarge the photo to see more details. Would you think that the variation you see here got acceptable quality?

The vinyl used here got a glass grid embeded between two PVC foils. It is 600 g/m2.

I have tried to do some test sewing on two layers of vinyl with a Tex 135 polyester thread. I do have some variation in the balance. Some locks are near the top surface and some near the lower surface. I suspect, that you always will have some variation. I noticed, that sewing in the vinyl I got is quite critical regarding thread tension. An increase in thread tension of 10 % can change the seam from having the locks predominantly at the lower surface to be predominantly on the upper surface. But what can you expect from a good sewing machine? The picture below have red thread on the bobbin and black needle thread. Stitch length is about 4.3 mm. Would you think the thread tension is too low? Do the stitch got resonable quality? It is a close up photo, so details and errors should show up quite easily.

If you trust Amazon, then it seems that Organ do deliver 135x17 titanium needles. But I am not sure this should be the right supplier to use. https://www.amazon.com/TITANIUM-135X17-Industrial-Sewing-Machine/dp/B01FGDJNJE?th=1

I would keep away from a Chinese brand named "Flying Man". It do not mean, that all of their needles can't be used, but some are not useable. This is a close up photo from the same package of ball point needles from them. None of them were useable. One without a groove and the other almost no groove. I think needles from Organ, Groz-Beckert and Schmetz is considered good quality needles. I got a few needles from a brand called AMF, and they seems in order too. The Titanium needles is the same body of steel needle, but they got a kind of cheramic coating so they should last about 3 times longer with normal wear for normal woven fabrics. I am not sure about wear from leather.

Even though this is an 2½ year old thread, I think it will be in order to revive it, because I know most people above giving information is still active in this forum. I have read the good references given above. I like to add, that in my impression that, nylon can be harder to color, and the amount of colors to buy are more limited with polyester thread. I guess, that the main advantage of nylon thread is, that it stretch more with the same tensile load. Nylon have got a lower youngs module than Polyester. If you get a concentrated tensile load on a seam, then nylon will be able to spread the load to more stitches than polyester. If you get one thread broken one place in a seam, the seam will slowly start to unravel from this position in the further use of the sewn product. Nylon do not seem to have a significantly higher tensile strength, than polyester got. I have noticed, that the thread brand, Amann recommends the use of nylon thread in high strength webbing applications for lifting purposes, that require high safety issues as well. However I discovered yesterday a video from Dr. Bob from Superior Threads. He mention, that nylon got a problem of being brittle over time, and got a low heat tolerance. I know, that the melting point of nylon and polyester got about the same melting temperature, but it do not mean, that a lower temperature may affect the thread. I know that Superior Threads do also sell nylon threads for leather. His audience in this video is garment home sewers - it may also be an issue here. But do Dr. Bob got a point here?

I agree with Wiz. For the brushed DC motor type, the motor is controlled from zero. However the drive types with a brushed DC-motor do not have a steep moment curve as a brushless motor with more drive electronics. If you run a brushed DC motor drive for sewing machines very slow, and it gets some high torque load, the motor can be stopped. A brushless motor drive running slow will resist being stopped with all its torque available. In this way these servo motors are different and some may like one more than the other. And they may each be better for different jobs. This video show how this response to torque works on the two motor types: https://youtu.be/SOyQtt3eDLw The Jack servo motors are of the brushless type. I tried to look for their specification regarding lowest constant speed, but I could not find it.

I just looked at motors from some suppliers in Europe, and I agree that it is hard to find the brushed DC motors. Almost all servo motors sold in Europe seems to be of the brushless type. I found one supplier in the UK via ebay with a brushed DC motor: https://www.ebay.co.uk/itm/Industrial-Sewing-Machine-Servo-Motor-Mains-240v-with-On-Off-switch-Fittings-/263472630087 I guess it will work like the Family motor Wizcrafts suggested. If you go for a brushless servo motor, as it seems most users do in Europe, then look for a motor with a minimum speed at 100 RPM. Most motors got a minimum speed at 200 RPM or even 500 RPM. Do not trust a specifications that say 0 RPM to 4500 RPM. It do have a minimum speed higher than zero. This is one brushless motor sold in Europe, with a minimum speed of 100 RPM: https://www.sewtex.de/Motore--Gestell--Tischplatte--Zubehoer-fuer-Naehmaschinen--Stuehle--Leuchten--Transportwagen--Industriestuhl--Schere/ZOJE-TZ-ZJ550W-S-Servomotor--Naehmaschinenmotor.html I guess you can find a supplier for that in UK as well. I never used a Zoje servo motor, but I have seen a few good reviews.

I got no daubt that this is a good motor to use. But I am a bit puzzled by the specifications in the link: 110 volts AC, 550 Watts 3/4 HP output 3/4 HP output is the same as the same as 552 W shaft power. It is actually a bit more than the specificed input electrical power to the drive. I know something about this size of motors, and my estimate would be an effiency of about 60 % at max power output. So something is wrong with these figures. Perhaps the the specified shaft power is less or the specified input power is higher. If the motor provide 552 W shaft power at 60 % motor and drive efficiency, the input power should be 920 W.

A facebook private group exsist with about 4000 members, and it is quite active. It is called China Leather Shoe Patcher. A lot of owners got problems with the machine in this group and others try to help. I think you need some mechanical handy man skills to be able to fix the problems with of it. Yes, it uses the normal needles for domestic sewing machines. But some do a modification to the needle bar on them, so they can use 135x17 needles.

On this link from Groz-Beckert I find both 16x2 and 16x257 needles. The sizes are from 55/7 up to 110/18. https://my.groz-beckert.com/irj/portal/sewing#navurl://883c12ee6117b6739051dc523b91b68e?0

This is a video, that in my opinion shows good speed control of a leather sewing machine and with an available servo motor. This is a video of speed control, with a DIY solution on a domestic sewing machine. I think it have got exceptional good speed control. It have got a very fast responsive foot pedal, and it have got a factor 70 between lowest speed and highest speed. If I was a manufacturer of a servo motor for leatherworkers, the performance should look like this:

Uwe, when I look at you link to Keystone, I guess the motor got a minimum constant speed of 100 RPM. And I have not seen brushless servo motors with a lower speed than that. Have you seen lower speed than 100 RPM? For some years ago a problem with some servo motors with this issue were debated. It may be this problem you see: https://leatherworker.net/forum/topic/41916-having-problems-with-servo-motor-speed-control/

I think that canvas do not have sufficient stretch as the rest of this fabric. If I use a patch of canvas here, the forces locally to the seams can be significantly higher. The patch and the seams needs to be able to stretch. I found some limitations on what you can get of domestic sewing machine needles. When I look on websides for Organ and Schmetz, the max size jersey needle is 100/16, and the max size stretch needle is 90/14. The stretch needles should have a different scarf, so some sewing machines can catch the thread better. Some vintage machines may not be able to take advantage of a stretch needle. However there seems to be some jersey needles around of size 110/18, but this is the only supplier I found (Sharp Sewing Supplies in US): https://www.amazon.com/Titanium-Sewing-Machine-Needles-Multiple/dp/B07NQNWVP2/ref=sr_1_22?dchild=1&keywords=Organ%2Bneedles%2Bball%2BHAX1%2B110&qid=1601984198&sr=8-22&th=1 Another option may be to use thinner thread. I agree, that the top tension is less than normal. SUK needles - is it stretch needles?

I did succeed making some patches of the cover. This is photos of the inside and outside of the cover, and the patch size is about 2½ x 3 inches. The stitch width is about 1/8 inch. Of couse I like your comment on this repair. So it is possible to sew such a difficult stretch fabric with a vintage zig zag machine. I got some comments elsewhere, that I should use thinner thread and a stretch needle for it, and it might be better. This top cover is about 7 x 10 feet in all.

Thanks for the advise, @Wizcrafts I have tried to vary this check spring stroke, but it did not change anything. But what did a big change was more obvious - needle size. I have normally used size 14/90 or 16/100 with this thread. But I now tried size 18/110 and 19/120. And 18/110 worked best. Perhaps I should not have asked with a solution like that I think the theory here is, that you need to have low local thread tension at the needle eye when the needle is at its lowest position. The thread tension here will pull down the stretch fabric from previous stitch. When the needle rise the stretched fabric will pull up the thread again, so no loop or a smaller loop is created. With lower local thread tension at needle eye, the deflection of the fabric will be less, and then a bigger loop can be made. A bigger size needle got a larger grove for the thread, so the friction against on the thread from the fabric will be less when the needles advance to lowest position. When I look at the thread tension above the fabric it is zero regardless of the check spring adjustment when the needle is at lowest position. But I acknowledge, that for some other machine this might not be the case, and then this adjustment of check spring stroke would be important. I have tried sewing the same with a Bernina 910. This machine needed a size 19/120 needle to be able to sew at the same conditions. With the 18/110 needle the machine occationally made a birds nest of upper thread below the fabric. I suppose the hook stabbed the thread. This machine got a special check spring, that you cannot adjust. It also struggles more with feed tracktion on this fabric. However the stitch got more balanced in both sides of zig zag. The singer 237 produced stitches with the lock at the top fabric surface to the left and near lower fabric surface to the right.

The problem is skipped stitches. I got a job to sew some patches on a heavy stretch knit fabric used for cover on some gymnastics mats. I currently use a domestic Singer 237 and zig zag stitching with a V69 polyester thread and a 100/16 universal point needle. Zig zag is used to allow stretch of the seam. It seems like the stretch fabric couse a smaller loop to be catched by the hook. I have tried to adjust the distance between hook and needle to a minimum, and it did help, but I do like to have a little distance here for normal use. Can you adwise some change for me, to make it easier for the machine to sew this kind of fabric? Is another machine better for this?

When I you see max speed in the video, is it with the pedal pressed to max speed? Is the max speed limited by a setting? It seems to me, that the lowest speed is about 1 stitch per second and max speed is about 3 stitch per second.

I have tried to look in more servo motor specifications, and I think it is hard to find what the lowest constant speed is. Normally you cannot find it or it is hidden very well. I guess it is because leatherworkers and other users seldom ask for this specification. Sometimes the machine got a ramp up function from start, but it do not help much. Perhaps most servo motors are designed for garment high speed machines, that do not need a bigger variation in speed. Normally the display shows the speed as Costabulary write. But I should think that 41 should mean 4100 RPM and 16 should mean 1600 RPM. But in general you do not know how these numbers translate to RPM. If the lowest speed is 1600 RPM, it is a very high low speed, and I newer heard of that before. Something could be wrong as reported in this video from 2012: https://youtu.be/X6CCxv3i4No This problem was discussed in more threads of this forum back 2012. This is one of them: I use my domestic sewing machine for many different jobs, and it have got a DIY speed control with a factor of about 68 between highest and lowest speed. Furthermore the construction of the pedal make a fast and easy control of speed: https://youtu.be/uTB8DnyYAlA I think it would be no problem for a servo motor manufacturer to make a similar speed control with very limited extra costs, if they got the demand for it.

In another forum, a guy using this black bonded nylon thread ended at a similar solution to have the thread roll off the spool and for more sewing machines. But according to him, he got problems with hook catching the tread, if he let the thread come off the end of the spool. I have never heard thread twisting should be able to cause such problems. Another guy reported kinks on needle thread in the shuttle area. Up to now I mainly have read about problems with kinks in the thread pass before main tensioner. What kind of problems have you seen, that should be due to thread twisting?