The Home Machinist!

My 9 inch southbend lathe has served me well, for many years, and numerous live steam projects. ...the spindle has a 1 1/2-8 thread, and the chucks just screw right onto it. ...I would like to get a new 3 jaw chuck for this lathe, possibly an adjustable 3 jaw. I've looked online, and thru all the tool catalogs I can think of....but can't find any that just thread onto the spindle ?

You are going to have a hard time finding a new chuck that has integral spindle threads the size you need. You'll probably need a backplate and a plain back chuck. I've seen semi-finished backplates that have that thread.
I don't seem to find any that have the 1 7/8" x 8tpi thread I need - so I get to thread the things too!
1941 13" SB - just got out of the shop, turned a part I needed to +/- .0005 with the old girl. Ya gotta talk nice to her.

Oops, Rudd beat me to it . . . .
The combination of threaded body AND adjustability in a chuck of an appropriate size for your lathe puts you in a very narrow market. You'll probably end up needing to use a separate threaded backplate. One benefit of a threaded body is that tends to minimize overhang on spindles which benefit from minimal overhang, and a backplate tends to increase the amount of chuck overhang, but sometimes that's the only option.

Years ago I picked up a couple 6" Buck adjust tru chucks at a school auction. I put one on the 10" Logan which has 1 1/2" x 8 threads and I ended up modifying a backplate to work with the chuck. I can't recall if they were just unobtainium or just silly expensive but it didn't take long to machine a boss and attach it to the backplate for the adjusting screws to work against. Well worth the effort.

https://www.ebay.com/sch/i.html?_nkw=3+ ... F2+x+8+tpi


What’s old? My Heavy 10 was shipped to Quartermaster US Army Oct. 1940.

Hoppercar...
Looking for a 1-1/2 x 8 backing plate? Check out CDCOtools.com. These people are located in the NW Chicago 'burbs. They carry threaded backing plates from 4" up to 8". The larger sizes run in the $60 - $70 range. You'll have to face the backing plate and cut the OD as close as possible to the chuck inset on the back side. Would suggest "sneaking" up on that OD. Then, transfer the chuck mounting hole locations and you're in business.

Did this for the CDCO 5C chuck I got several years ago. On several occasions, have pulled the 5C collet out of the lathe chuck and installed in a 5C square or hex block to continue machining in the mill. Was nice being able to transfer work like that. Hope that's the direction you want to go.
Carl B.

These two could be made to work together;

SHARS 6" 3 Jaw Self Centering Lathe Chuck Reversible Jaw .003 TIR Certificate L

https://www.ebay.com/itm/301168683534?h ... Swn7Rb5FGE

South Bend SB1388 6" 1-1/2" x 8 TPI Back Plate

https://www.ebay.com/itm/294318895475?h ... Sw~uBhDZl6

for about 240.00 shipped

clif

If you want an adjustable set true type of chuck as you mentioned, then afaik your also limited to using a plain back chuck with a back plate anyway. The way they work is the bolts that attach the back plate to the rear of the chuck body are loosened a bit, the chuck zeroing screws are adjusted to your minimal run out condition with those bolt tips bearing against a step on the back plate that fits the chucks recess, and then those back plate bolts are re-tightened. So those adjustment bolts require something to bear against and the chuck has to be able to move around a bit until it's centered. I can think of no way this could be accomplished with direct mount chucks and the spindle thread already machined into the chuck body.

My 9" South Bend S/N dates it to 1928. It has the 1 1/2" -8 spindle as well, but a much heavier bed like the 10" lathes of the eara.

I have a 1943 9" SB with the same spindle thread. You have to get a backing plate like the others have suggested and finish machine it to accept the new chuck. I prefer the way ccvstmr does it.
You'll thread it on, face it, and then turn the step to tightly fit the new chuck. Then transfer punch, drill, and tap the mounting holes though the chuck.
Mine came with both the original 3-jaw and 4-jaw in decent condition, but I have been looking into a larger diameter setup so I can hopefully grab the outside diameter of 7" wheels and be able to bore the centers for axles and cut the recesses in their faces.

I don't think one can rely on a 3-jaw chuck clamping things with any more accuracy of less than ~.002" indicated runout. Although time consuming, I use my 4-jaw for work that has to spin true. If I am going to be doing any repetitive work(like finish turning wheelsets/axles), I indicate in a hub/collar, and then slide each axle into the hub with a drive dog to turn it.

I was very fortunate . . . my first mentor had only a 4-jaw on his lathe so I learned lathe work using only a 4-jaw. At the time I wondered why he hadn't gotten himself a 3-jaw, he could afford it, and it would've been so much more convenient. He eventually did, but in the meantime I came to appreciate the 4-jaw for all it could do and truing up became 2nd nature. If I could have only one chuck it would HAVE to be a 4-jaw.

gwrdriver wrote: ↑Thu Jan 13, 2022 3:22 pm If I could have only one chuck it would HAVE to be a 4-jaw.

I'd agree with you if I didn't own a three jaw with master jaws. The time it takes to remove and insert a part with soft jaws is just seconds and I can rely on a level of concentricity and perpendicularity that is well within acceptable limits. If you have multiple parts to machine, a four jaw is quite slow, although it can be the epitome of precision, and allows for gripping irregular objects. That can be a challenge (but not impossible) with soft jaws.

Soft jaws---the magic bullet of part holding.

H