kgg

I would also do a test in combination with the kevlar lining and thread that you are plan on using. Personally I think the domestic sewing machine is going to struggle if it will do the job at all. Remember the thicker the leather the better the protection. Yes the thicker the leather the heavier weight wise the jacket will be but the last time I dumped a bike in a bend the pavement burned through the chaps, jeans, medium duty leather jacket and the 4mm thick belt I was wearing. Road rash is a nasty thing. kgg

What machine are you planning on using? kgg

I'm going to assume your "Heavy Leather" measurement is in inches so 0.175" is 4.45mm which is about 11.5 oz thick leather. The needle size for V138 (T135) thread should be good for thick/tough leather. However the bobbin thread being 75 which I'm going to assume is T70 which is V69 is the smallest size really for industrial sewing machines. I think some of your problem is your bobbin thread is two sizes smaller and flimsier then the top thread. This I feel is causing / related to your stitching problems. Why are you using a smaller bobbin thread? Normally the bobbin and top thread should be the same size and type of thread (bonded nylon or bonded polyester). The left twist thread is the standard twist direction sold today also known as Z-Twist, or Standard Twist. Mixing thread colors and types of thread can also pose problems as darker threads like black are stiffer then lighter colors like white and bonded nylon is also stiffer then bonded polyester. kgg

Measuring any thread is difficult at the best of times let alone cotton which can absorb moisture and swell. That doesn't even account for what changes it has be exposed too for the last 30 (??) years. kgg

A true flat felled seam not the mock flat felled seam sometimes referred to as a military seam. An example can be see in this video as the first seam done with jeans materials (www.youtube.com/watch?v=E1vJzVW2lLk) kgg

My understanding is that for cotton thread it is converted to sort of Tex measurement by dividing 1000 by the size of the thread. So in this case it would be 1000/24 = Tex 41.7 or somewhere around V40. Braking strength for cotton is less then bonded nylon or polyester. kgg

For a start the 160 (#23) needle is only good for V138 thread in thinner materials. If you want to use V207 you need to go to at least a 180 (#24) needle and maybe a 200 (#25) in thicker / tougher / sticky materials. If you use an underside needle like the V207 thread with a 160 needle the hole the needle punches isn't big enough to allow the top thread to form a proper loop on it's upstroke so the hook can catch it. You may be able to use one size of needle up from the machines rating but two sizes or more probably not. My take on thread is V69 is the largest thread for domestic sewing machines and the smallest thread size for industrial sewing machines. The V138 thread is generally the max rating for most upholstery class machines while the V207 thread seems to be the cross over point from upholstery class machines to the heavy stitchers of the 441 Class machines. kgg

I agree with @Constabulary that this motor is not safe and should be replaced with what in North America is generally referred to as a Universal sewing machine motor or a servo motor. My experience is when a motor has the plug cut off like what is shown in your first photo it means the motor has a problem not worth fixing for one reason or another. kgg

Under the section Hints in the manual " In operating the machine always turn the wheel from you." From that statement I would assume the hand-wheel turns in the clockwise direction. I have attached the manual. kgg 46k15.pdf

The Juki LS-1341 or similar clones should fit what you want to make. When the time is right it and you feel you can use a machine, manual or motorized, at least now you have a starting point of information. kgg

Have you check your manual for your machines sub class. The information I have the max size needle thus the max thread depends on the sub class. The max needle the machine is rated for is 180 (#24) but mostly 140 (#23) or under. The 180 needle will be able to handle V207 in thinner thickness but you may have to go up to a 200 (#25) in thick or tough stuff. A good cross reference for needle size to thread size can be had at: https://www.tolindsewmach.com/thread-chart.html The machine information I referenced can be seen in the attached manual pages 2 & 3. kgg 268_instructions_for_operating.pdf

Yes those machines are all manual machines. Singer no longer and hasn't for many years made industrial sewing machines that I'm aware of. This narrows the field down for sewing machines. If you only want one machine a sort of all rounder machine would be a cylinder arm machine with a flatbed attachment. You can do items that are best done with a flatbed sewing machine (example: wallets) as well as items done with a cylinder arm that are circular. Then there is the dollar factor. Depending on how thick your leather will be including the seams an excellent machine would be a Juki LS -1341 or a clone machine. The Juki LS-1341 new will cost about $5200 USD while a clone of the older Juki LS-341 will cost about $2500 USD. Juki is the typical machines that the clones copy. So accessories like binding attachments and parts are readily available at reasonable prices. Then there is the used market for both brand name and clone machines which can be had from either a dealer / repair shop or the owner directly. I would suggest getting one from a dealer that has been gone over unless you or you know someone that can properly evaluate a used machine. The only thing is the footwear part of what you want to make as this has a very broad range from slippers to workboots and everything in between ????? kgg

I would definitely investigate getting a machine particularly since hand stitching will probably aggravate your arthritis. My mother use to love to sew all sorts of garments, hand knitted everything from socks to heavy real wool sweaters and use sign language for her day job. When she turned about 50 her arthritis got so bad she could hardly sign anymore let allow sew or knit. My advice is put the needles down and get a sewing machine to reduce the stress on your hands. Determine what items you would like to sew and what items you eventually would like to sew and purchase a motorized sewing machine accordingly. I think the one armed bandits like the Tippmann Boss ($1000 - $1575 USD) , Cowboy Outlaw ($1400 USD) and the Master Tool Cub by Weaver ($2000 USD) will also aggravate your arthritis but not as bad as hand stitching will eventually. I look at the bottom rating reviews for items, the ones giving the bottom end reviews. I think those probably give a more accurate view of what to expect. kgg

If you enjoy hand stitching you still can do that it for your own or special projects but when doing large quantity projects mechanizing the work process is the way to go. To me it's like hand sawing versus using a circular saw, one piece of 2x4 no big deal but a 100 I'll use the circular saw. Doing those sheaths all by hand will I bet quickly lose the enjoyment of doing that many similar items. How mechanized you want or can afford will be the question. Just adding a proper sewing machine will as @Dwight mentioned from however long it takes to hand stitch to about five minutes for sewing as will as reducing the hand and mental fatigue. kgg

I would figure out i) an average cost for materials like leather, dye, thread, etc including wastage ii) an average time to complete a knife sheath from cutting the leather to the finished sheath iii) a fair hourly rate you want to charge iv) a profit margin Just as an example would be $10 for materials plus $15 hourly labor rate (say 1 hour to just to keep it simple for the math) plus say 40 percent for profit for a total: $25 for labor / materials plus $25 x0.40 = $35 per sheath. The only two areas you can really save on the cost per sheath is the labor cost which could be reduced using equipment and the profit margin. kgg

Have a look at this video of the Consew 226 being converted to the larger bobbin it should answer you questions and the parts you will need. kgg

If I had to choose I would go with the possibly a Juki DNU - 1541S for thickness and material you noted. The Juki LU-1508-NH and the Dürkopp Adler maybe to heavy a duty machine for the task. i) The Juki and the Dürkopp Adler are brand name machines which when you decide to sell will command a better resale price. ii) Dürkopp Adler parts and accessories are generally expensive. iii) Juki both parts / accessories are fairly inexpensive and realty available. If I'm not mistaken 2 layers of 18 oz vinyl would be 1.01 mm thick and 2 layers of 18 oz canvas would be 2.13mm for a total of 3.014 mm so maybe a Juki DNU-1541S would be an appropriate sewing machine as the thickness is well within it's sewing capacity and a lot cheaper then either the Juki LU-1508 NH or Dürkopp Adler. That said I would take a sample of what you are planning on sewing and visit a dealer(s) to try out various machines so you can make sure the machine you select is the right machine for your needs. kgg

If you decide to re-drill and re-tap the hole be aware that Singer was notorious for using their own thread pattern on screws. Be prepared to drill and tap for a standard thread pattern and use a readily available screw with either metric or imperial threads in the correct length or as @nylonRigging use a nut and bolt. kgg

Looking at the photo of the thumb nut it appears to me that the body is split as the result of someone at some point screwed the thumb nut in to far. My suggestion would be to remove the center screw and the part the center screw screws into. With those items removed you should be able to unscrew the thumb nut from the threaded rod. Those threads inside the thumb nut are probably striped or cross threaded. You are more then likely going to have to replace the thumb nut with a new one. The installation of a new thumb nut may solve your stitch problems. kgg

I think the repair shop installed a 190 needle system needle which I think is what is recommended for the H4 sub class and the 134-35 are for the H3 subclass. The 190 needles are about 1/4" longer so the timing will not be correct. The needle needs to be installed so the two groves on the needle are on the left and you thread the needle from left to right completely opposite to a domestic sewing machine. The stitch length is changed by turning the thumb nut on the lever. Here is a manual for the PFAFF 545 the manual covers models from 141 to 545. kgg 545.pdf

Any industrial sewing machine repair shop probably can do this but the general conversion project will require: i) the timing belt needs to be removed ii) the lower shaft needs to be removed so the hook saddle can removed iii) the hook saddle will need to be machine milled out to accept the larger hook iv) the bobbin case opener needs to be changed v) the take-up lever has also has to be changed so the head assembly needs to be striped. vi) then the sewing machine needs to be reassembled and re-timed. vii) since the machine is basically striped to the bare bones you probably should repair the timining belt The real question is the conversion really worth it on an older machine? Doing it you self, if you can mill the hook saddle, for probably $150 US or so. Getting a sewing machine repair shop to do the work you are probably looking at $600- 800 US. So is it really worth $750 to $950 to upgrade the machine? I would suggest either: i) leave the machine as is and put up with the more often bobbin changes. ii) sell the machine and purchase a machine with the larger bobbin like the Consew 226RB-5. A new complete setup machine , table and servo motor would cost about $1500. t If you are going to get someone to do the work for you what are you willing to pay . kgg

Here is two manuals which may help. kgg 45k53.pdf 45Kmanual-2.pdf

Here are two manuals which may be of some help. kgg Pfaff_141,_143,_145,_151,_153;_142,_144,_146_Instruction_Manual.pdf pfaff_145-manual-EN.PDF

A good place to start for info would be the "A1 Repairing or the "Shoe Makers Friend" at http://www.sewmuse.co.uk/bradbury/a1 repairing.htm Parts maybe as scarce as hens teeth particularly with the machine not being produced for the last 100 years. The closest copy of the machine I can think of would be the Chinese Shoe Patcher fondly known as the "Tinkers Delight". I would love to see some photo's of your machine. kgg

Very nice setup for the right handed user. kgg