https://www.shars.com/5-1-1-2-8-threaded-back-plates-1 I can't seem to find a 1-7/8" back plate on this site but I found one that is 1-1/2" What if I got one and re-threaded it to 1-7/8"? The price is good.
It looks to me like there's more than enough extra material to handle the re-bore: they must use the same casting for a range of sizes, possibly D series backplates.
The depth of thread is about .080" for an 8 TPI thread (depending on which table you look at) so two times that is .160". If you machine out all the existing thread you will have a hole that is about 1.660". Therefore you could buy the 1-1/2 and machine out the threads and still have meat in there to machine new threads. Machining internal threads is not the easiest project. If you attempt it we can help. One major question: if your 3 jaw is not working right, how are you going to machine the threads? I would offer to machine this for you but it would be hard to get the fit correct. I would have to make the threads a little loose to guarantee a fit. That would not be ideal. Ideally you would have the spindle right there to check the fit. You can do this on the same lathe as long as you keep the work in the chuck at all times. To test fit it you take off the chuck and test the back plate while it is still gripped in the chuck. Yes it will be heavy! Robert
South Bend Lathe 13" has 6 TPI OD of spinlde ether 2.25 or 2.375 I do not know if SB made any other sizes Dave
https://www.ebay.com/itm/southbend-...033142?hash=item3d77177d36:g:ztUAAOSwoBlaiJ3p This 1-7/8" backing plate is $35 with no bids, and $7 shipping. Courtesy Pete716.
Oh wow my internet just came back (was down for most of the weekend, been running on my phone hotspot). The last one seems like a good buy. I like the 6" since it will give me some room to drill other chucks in. I went ahead and bought it. Hopefully it fits my spindle!! Thank you for looking for me. I've been searching "1-7/8" Face plate" or "1-7/8" back plate" and the pickings have been very slim. For 60 something bucks shipped that plate is a steal if it fits. All I'll need to do is just face the plate and fit it. Cheaper than buying stock metal and failing several times. Now I can learn on cheaper smaller materials. I'll let you all know how it goes.
I'm sure the thread will fit. You may have to make some minor modifications. Some tips on mounting: Take the existing back plate off your 4 jaw and get an accurate caliper measurement of the diameter of the recess. Firmly attach your new faceplate to the lathe and then face it off. You will then need to turn down a boss that fits the inside diameter of the chuck recess. It does not have to be deep. 1/8" will do it. DO NOT OVERSHOOT on the diameter. Ideally you want the boss to be .002 oversize to the chuck recess. File a slight chamfer on the edge of the boss. When you are ready, bake the chuck in an oven at about 300 deg to get it up to temp. The parts should then fit together. When the temp equilibrates you will not be able to easily get them apart. Remember it is the back plate recess that determines alignment not the screws that hold them together. In truth, this process is not critical for a 4 jaw chuck but you may as well do it right since you are learning to use the lathe. This process would be critical for a 3 jaw. Keep us posted. Robert
I've been out in the woods (no internets) all weekend. zaps. The 1-7/8" spindles are the older southbend style these lathes have Cast Iron spindle bushings. The newer larger 2-1/4" spindles have bronze bushings. Switching one style of spindle to the other machine is not a simple or recommended task. About the only way to make it work is replace the whole headstock (which isn't cheap if you can find someone parting out a unit the head stock is normally as clapped out as the bed...but sometimes you can find one). the choice of Cast Iron for the backing plate is multi-fold, it's ridged, it helps dampen vibration, it also is less likely to tear up the spindle threads if you get it started crooked or over tighten it. It also machines easily. in a crash it's most likely going to crack before the spindle shaft... think of it as a mechanical fuse. Something like 4140 can be harder than the spindle (the old ones were not hardened) it can also be a bit more challenging to machine. and it's going to stay put or deform well past the breaking point of the spindle in a crash. Picking up a low buck 1-1/5" cast Iron backer is a good choice. You can also pickup a few inches of 5" cast iron for not alot of $$ (https://www.speedymetals.com/pc-4291-8388-5-rd-gray-cast-iron-class-40.aspx) but your going to need a chuck to turn the id and thread the ID of the bore. Might want to wait on this till you've practiced on the cheaper ones. Maybe for the next chuck you bring home. You can pick up a cheap internal threading tool on places like banggood (think $20 with inserts). Mind you it's not going to be Sandvik or Kennametal quality but you should be able to get the job done with it. Between your struggles with this and some other projects I've been putting off. My old Southbend is in the front of the line...in fact the apron and Quick change gearbox are on the healing bench now. In like 500 pieces getting cleaned, oiled, painted and refelted. Hopefully in few more weeks it will be up making chips again. (the first time in over a decade).
The back plate arrived in CT, my mother put it on the lathe for me (backwards ahaha) but the fit is good. Should be easy to face and tap for my 4 jaw.
Your mom did very well. My mom would not have tried. And two months ago you might have done the same. Looks like you have found the practical solution.
Looks perfect. I'm sure you will get it. Post a pic of the back of your chuck when you get the back plate off. Robert
That's a Great Start!! Getting it turned down to fit your Chuck should be easier than learning to turn internal threads! When you get close to size slow down and measure often. You can sneak in the last couple thou with a file if need be. You want it to register nice with no slop (slip fit with no side to side wiggle). If you get it to fit like that you can fudge the screw holes a little over-sized in case you don't get one 100% dead on. CBB
The screw holes should always be oversized. The alignment is made by the tight fit of the back plate in the recess as you indicated. On my factory made Bison the screw holes are quite a bit larger than the screws. Robert