Gymnast

I think you can make it sew quite much, if you are prepared to do some modifications to the machine, such as a speed reducer and other things. A serious issue with this machine will be marks from the feed dogs in the leather surface. So it do also depend on how nice you like results to be. As DoninReno points out, it can be a good machine to start learning on. A big roller foot will help, as Wizcraft points out. In my experience the main problem is, that thread tensions work against the move from the feed dog. With the roller foot the thread gets another direction to the fabric, so it do not pulls back the fabric that much. The same can be done by a modified food with a notch to the back, and it holds the fabric better than a roller foot, when the needle is down. This is a video about this kind of foot, and you will be able to find some tips for domestic machines on the same channel: https://youtu.be/rBIulDuhDDs

If you look at the linked video about thread twisting, you will find, that it is important how you make the pass of the thread through the steel plate with holes in it. You go clockwise in forward direction. I would chose to go anticlockwise to reverse the twisting from this steel plate.

Two years ago we had a debate on thread twisting and tension. If the pre tension for a tensioner becomes two low in relation to the "output" tension, small kinks can formed or the thread can get a different and unstable pass through the tensioner causing tension variations. Perhaps you may be interested in the findings and videos there:

In my experience, thread tension variations can be caused by a lot of different issues. And for some machines tensioners you cannot get rid of too much annoying tension variation. I disagree with Doninrino about getting such instruments, and I can get them for about $1.5 each. I have seen the same issue about tension variation due to direction of thread from bobbin case. You save time, if you have got an instrument to help you instead of doing test sewing to get results all the time, when you diagnose the machine. The issue of tension gauges have been discussed some times in this forum. This is one of them:

I think the Towa bobbin gauge is too expensive and over complicated. Its simpler and cheaper to use spring dynamometers, and they can measure the thread tension in more ways and be used to measure needle thread tension as well. Last time I looked for these dynamometers you could get them for about $1.5 each via Aliexpress from China. I would choose to get three types, 0-1 N, 0-5 N and 0-25 N. I prefer them over digital types, because you can get a better idea of the tension variation. This a short video that shows how these gauges look like:

I just read a few specifications for a Dürkopp Adler 525i-811. As far as I can see, it is a fast stitching zig zag industrial machine. A user wanted it second hand to be used for DIY sailmaking and some marine canvas work. It is not a walking foot machine. https://www.duerkopp-adler.com/en/main/products/shoes_FB_ZZ/525i-811.html The gearing will normally be 1:1 to the motor, but it is possible to change it. When I read the parameter settings for the controller here, you can set the minimum speed of the motor down to 10 RPM. It is page 15 here: https://www.duerkopp-adler.com/commons/download/public/525i/0791_523901.pdf So it is much lower speed for this kind of motor compared to what I have seen before.

Your garage table saw may be with an induction motor. I argue that most smaller table saws at about 1800 W for home use or craftsmen got universal motors. This is three examples of that: This is a link to a small Metabo 1700 W table saw. The small size of motor and gearing to motor clearly indicate, that it is a universal motor. The gear ratio is above 3 and no load saw speed is 4200 RPM, so the no load motor speed is about 12000 RPM. https://youtu.be/2kVjOW-VMrk This is a 2000 W Dewalt DWE7491-QS. I can hear on the noise, that this is a high RPM motor. https://youtu.be/I_uLAz7ferE?t=334 This is a Bosch GTS 10J table saw. In this video you can see the sparks for the brushes of the universal motor. An induction motor got no brushes. And you can hear the high speed of motor. https://youtu.be/RDMBiUF80Gc?t=142 Saws in this size becomes cheaper and lighter, when you use universal motors. I guess that bigger and more industrial table saws may use induction motors. Perhaps older table saws for craftsmen got induction motors and you may have an example of that.

I think most of the smaller table saws are made with a brushed universal motor. Universal motors are used for most of the vintage household sewing machines and in a power input range of 50-250 W. A lot of Universal motors are used in tools, vacuum cleaners and other household equipment. They are good small motors with a high power, but with a limited operating time due to wear on brushes. They typically can run at high speed, so they will make more noise. A vacuum cleaner typically runs 25,000 RPM and can have an input power of 2000 W. Induction motors are very silent in operation, because their speed will be below 3,600 RPM and they got no brushes. Both universal motors and induction motors got a higher inrush current, which can be reduced by a soft starter.

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: