Gunnarsson

That machine family (class 19/20/21) may be some of the best domestic machines you can get, but that's all they are. They're intended for fabric and have bottom feed only. I have a few and mostly use them for jeans repair/alterations, if I had some thin(!) leather work to do I wouldn't worry about it being a problem for those machines, but that's because I already have the machines and have spent quite some time working on, fixing and adjusting them, if I was to f**k something up I can fix it again myself. I would NOT buy this or any other domestic machine with the intention of using it for leather. Unfortunately the machines suitable for the job are harder to find and far more expensive - which is why so many try to use unsuitable machines.

Assuming the exact height of the surface is not important, how about filing/grinding it down until the groove is gone/mostly gone? If I remember the construction it's just a spring holding the thread down against that surface to give some tension.

I expect they use machines that are relatively complicated and time consuming to adjust for making a specific needle, but once it's good to go it will keep spitting them out 24/7 with very little work. This means that 500 needles could be done very quickly once the machine is set up for it, perhaps in minutes, and those few needles have to be priced to compensate for the production loss due to spending perhaps hours on adjusting the machine. Those needles would have to be VERY expensive. If I remember it right the SD28 uses a curved needle, complicating matters even more. Most sewing machine needles are straight, and logically most machines to make them will only be able to make straight needles - there's no point in building machines with features they'll never need. The machines able to make curved needles may not be possible to adjust for another needle radius or such changes may require more or less extensive modifications, including making new machine parts. So, if it's even possible to find someone who can make the needles they would have to be very expensive, otherwise it would not make any sense to spend the time required to make them.

I'm thinking it could be done, but you'd want to use pattern seams/settings that won't put a lot of stitches close together. Stitches close together becomes a perforation it can tear along.

About the current, electric motors (depending on type, controlling electronics and so on) can draw a very high inrush current initially, but that settles down to a far lower current once rpm and load stabilizes. The picture is from a relative compression test of (probably) a small car engine, where the starter current is measured to see if all cylinder compression events puts an equal load on the starter = they all draw the same current. As you can see it settles at around 120A, but the initial load of getting a bunch of heavy engine parts moving gives a peak current around 500A. Don't know the exact construction of the servo motor, but 160A isn't impossible. Not very interesting though, as the inrush current lasts such a short time, what it settles down to is more relevant.

It converts to inferior, sorry, IMPERIAL, easily enough. I'd say that when taking and saving a bunch of measurements it's better that the person taking them works with the system he/she is used to, and if anyone needs the numbers in another system they can convert it themselves. Because if the person taking them in the first place makes a mistake in the original measurement/conversion the original saved data is corrupt and useless, while the risk of such mistakes is far smaller when working with familliar systems, and the original saved data is more reliable. Anyone who get their hands on the original data can then convert it to whatever system they like, and if they make a mistake that will only affect themselves, not everyone who try to use the original data.

This agrees with kgg, if I remember the content right. https://www.sailrite.com/How-to-Sew-Webbing-Loops

Also, it can't handle very thick thread. The one thing it is unbeatable at is access, it can get in almost anywhere.

Now, THAT'S how you get repeat business!

I am sure the technology to make good needles in small batches is available, but the production time lost to resetting & testing the machine(s) for different sizes and types would probably make the needles very expensive, and the demand would be very small. Modifying machines for available needles is most likely a better option. It's a cost once, and then you can get all the needles you want for it cheap.

I'm sure there is a service manual for those machines too, but I made a quick search and was unable to find one. I have come across several manuals for the two main Husqvarna generations that came after your machines (the automatic & 2000 series, I use those), so you can probably find one for the class 12 too, if you just look in the right place - asking in the right Facebook groups may be the best bet. If you don't find one the good news is that your machine should be rather simple by comparison, the newer zig zag & pattern seam machines have A LOT of extra adjustments to get right in those systems. https://www.facebook.com/groups/687487491317989/

Basically, you can put any motor you want on them. But as you already have one machine with motor you can use that, just make a speed reducer with different size pulley/pulleys to slow the machine down.

It's an old domestic machine. It can probably do some light leather work like many other old domestic machines, but it isn't the right machine for the job.

Many lubricants will work well enough in many places. For some of us experimenting is interesting, finding our own solutions, while others just want their machines - whatever kind - to just work. For the people who just want things to work it's a safe bet to use the lubricant the machine manufacturer recommends. It's not like you go through gallons of it every week, in most cases.

Also, manufacturers can use their own special thread to make sure you buy parts from them and not some one else, or use standard screws from the hardware store. It is still possible for others to make copies, but non standard threads makes that a more expensive project, many won't bother. And then there's the old companies that started making things before there were thread standards to follow (or the existing ones were unsuitable), if there was no standard they had to figure out their own.