Adapting a Sebastian to use L0 or L1

Shotgun

Shotgun

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I've got this 80yr old Sebastion with a mostly worn out 12" 4-jaw. The Sebastian has an odd-size thread spindle. . . 2 1/8" x 8tpi. I've been thinking hard about my options. Everything from making backplates from blanks to locking a new chuck in the old one.

This morning's brain shaker is, "What if I could thread the backend of an L0 or L1 spindle nose, spin it on and lock it in place with a taper pin?"

This would require that I obtain a spindle nose, maybe cut off of a damaged spindle. All I need it maybe a 1/4" behind the retaining collar forward. I'd mount the largest round I could find in my current chuck, face it and turn it true. Mount the spindle nose on a chuck, and "grab" the round with it, making sure it is fully seated by forcing it toward the headstock with the tailstock. With both chucks locked in, I can face off, bore and thread the new nose for my current spindle. A taper pin would make the new nose semi-permanent.

Sitting in front of my computer, this doesn't sound like a bad plan. Does anyone have a bad spindle they wouldn't mind parting with?
 
Would you be able to re-cut the taper on the spindle nose to make it run true?
 
MrWhoopee said:
Would you be able to re-cut the taper on the spindle nose to make it run true?
Click to expand...
I could, but would I have too?
I mount the biggest round I can find and turn it true to the lathe, regardless of what the old chuck is registering. Then I lock on the new chuck to what I know is running true. I guess I would need to indicate the chuck and then the shoulder of the new nose that I have mounted in it? But, assuming it is running true, then I'll be boring and threading a pocket that should be true.

Or am I just kidding myself? I'm feeling like it would be no more difficult or error prone that machining a new backplate.
 
Shotgun said:
I could, but would I have too?
I mount the biggest round I can find and turn it true to the lathe, regardless of what the old chuck is registering. Then I lock on the new chuck to what I know is running true. I guess I would need to indicate the chuck and then the shoulder of the new nose that I have mounted in it? But, assuming it is running true, then I'll be boring and threading a pocket that should be true.

Or am I just kidding myself? I'm feeling like it would be no more difficult or error prone that machining a new backplate.
Click to expand...
When you thread a new backplate, the next thing you do is put it on the spindle and true it up.
 
A threaded-on A-6 adapter might be the easiest to machine.
It has a very short taper and register on the face and whatever A-6 chucks you find simply bolt to the front of the adapter.
It's the same dimensions as D-series, but without the camlock pins.
 
That's an interesting take, @Choiliefan . What I have is a Skinner 12" plain back. Maybe just swap it out for another plain back? I can still add a taper pin if I want to run it in reverse. Though, to be honest, I haven't been able to get the backplate off, so that may not even be necessary.
 
Are you sure it is 2 1/8 - 8 and not 2 1/4 - 8? The latter is quite common.

Likely a dumb question but figured I'd ask
 
It's a good question, but do a search on Sebastian lathes. Lots of guys groaning about the odd thread size. :-(
 
So I would avoid going L1. It is large enough that you're stuck with 8"+ chucks, and backing plates are expensive, and stuff isn't particularly available. PLUS, it is going to increase your stickout a bunch.

The A spindle adapter looks super easy to do, its a short enough taper you could cut or grind it on the machine, and the bolts aren't used to index on. Since you have a 4 jaw for your current setup, you might be able to find a broken lathe with that spindle, and just 'fill' the pass thru with some steel insert, and drill/cut it to the 2-1/8 8 thread pitch.
 
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