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.

M6x1 is standard, so it should be relatively easy to improvise something out of a common screw.

I have no idea about what it should be, but as 1.25 is the standard pitch on M8 screws I find it unlikely to be used on M5 (that usually has 0.8 pitch, and the thread doesn't look out of the ordinary in the picture).

The main casting on most sewing machines is probably good for it. And all the rest is just parts you can replace, as required.

The more technology you put in a machine, the more things in it can (and will) fail. Manufacturers will only supply the parts for so long (remember, they make a living from selling new machines) so a complicated machine is bound to die as parts become unavailable or it breaks down too often so the machine becomes unusable in a production setting. There are >100 year old sewing machines out there still going strong, and many parts in such a machine can be refurbished or replaced if you just can work metal. The moment you add computers and such specialised electronics to a machine you limit it's life span to how long you build the electronics to last, and intend to supply it as spare parts (which may be not at all, for some chinese manufacturers). Ten, perhaps twenty years? There are servo motors with needle positioning, adjustable max speed and so on, so you can get some technology even on "dumb" stone age machines. It will also break eventually, but the thing about sewing machine motors is that you can get a new universal one to replace it tomorrow. There are of course machines that have to have computers, such as embroidery machines. A friend has (or had) an older one, I think it uses floppy discs to transfer the design from computer to sewing machine. The machine itself is probably easy to keep running, but the day the electronics fail it's most likely going to the dumpster.

This is the cause of my interest. If I ever get my hands on a 29 it'll probably be an old worn one, and research has said that these parts are often in more or less bad shape and new replacements are not available - as you have confirmed here. However, gears with straight cut teeth (pinions) and straight racks are relatively simple items to make. You need (preferably) a mill, a device to rotate the gear blank and lock it in the precise positions for each cut, and a cutter with the right shape to cut teeth with the profile you want. The rack should be even simpler, as it's a straight motion between each cut, not rotation, but you need the correct shape cutter for that too. For "modern" involute gear profiles, at least for 12 tooth and up you can easily buy the cutting tools assuming it's a standard size. In the pictures it looks like the 29K51 and higher has this style of teeth. The earlier ones however seem to use a different profile, and that wouldn't be a surprise as it's a rather old mechanical device, people were still trying to figure out what was the best type of gear etc. The downside to this is you need to figure out what shape the teeth were before 100 years of wear if you want to make fresh copies (or you need to redesign it and make a kit of all new parts that only will work with each other). As for buying the right cutters you can probably forget about it, but you can grind HSS to the proper shape. But to get there you either have to figure out what system the gears were created with and "do the math" to come up with the shape to aim for at the grinder, or find a NOS set of parts to copy (and hope that the production tolerances on that day were good, so the NOS gear actually is close to perfect). Might as well ask: are the pinions and racks hardened, or are they made from softer mild steel?

Very interesting. And I notice that the tooth profile appears quite different from the 29K4 and other early machines, the teeth on the racks ( but not gears) have straight edges here (like a V) while the older machines have teeth curved sides on the racks as well as the gears. Perhaps a involute profile here and a cycloidal gear profile in the old machines. (The kind of things you start paying attention to when you think about how to make new gears to machines where parts aren't available.)

If it's sold at a "really good price" it sounds like you should be able to get the money back and possibly even some profit on top, if you find it can't do what you need and want to upgrade.

Have you checked the manual about how to adjust the position for using a different needle spacing? That may tell you how to move what you need to move.

The Cub does look interesting, but at the same time it looks a bit rough and unfinished. The thread guides look like they come straight from the hardware store, which is fine, but it gives you the impression that your $1500 machine is built of cheap parts you can get at your local hardware store. A bigger issue I see is the check spring, being long and poking out from the machine. I can easily see something getting tangled on it and destroying it, either when in normal use or especially when being moved. As I said, it looks interesting, but it would benefit from some minor tweaks.

Or if it's made of cast iron. Or if it's set in a table. Or if it vaguely looks like some sort of sewing machine. They've found the word "industrial" bumps the price up, so they use it, no matter if it's right or not.