Custom Threaded Chuck

Just because you can doesn't mean you should.

How much do you have invested in this machine? Did it belong to your grandfather?

Please post some pictures in this thread and explain why this project is important to you.

You will get plenty of great ideas of how to resolve your problem on this forum but ultimately it comes down to what matters to you.

There are plenty of old lathes out there and many of them will have standard spindles that you can get chucks for. More information is needed here before we can give good advice IMHO.

John
 
FYI. Reading that thread may help you in the future.

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Any competent machine (job) shop can thread a back plate for a lathe chuck.

The question in the OP was clear enough, however the root problem isn't clear at all.

WHY is your chuck running out .006"?

Was it crashed and slipped off of center on it's back plate?

Is it .006" out on 1" stock? 2" stock? 3" stock?

Those are just some of the issues that need to be understood.

It's usually best to understand the fundamentals of the problem before deciding on a solution.

Pictures are usually helpful.
 
As a relatively inexperienced machinist (beginner) I was able to machine my own back plate for an ER-40 set true chuck. I had never done anything like it before. It was a little nerve wracking to hit the dimensions, but overall wasn't that hard to do, even the threading. It's a matter of taking your time and getting the order of operations straight in your mind.

Due to the pandemic, I couldn't buy a partially machined backplate, (unavailable) so I machined it from a chunk of cast iron. It was a great learning experience and came out very well. Far better than I ever expected.

The point of all of the above is to encourage you to try. Be methodical and ask questions here if you are unsure what the next step is. There's no shame in asking questions here.
 
Sounds like you need a whole new chuck anyhow - show us what you have and what lathe it is
You'll need to buy a new semi-machined backplate / chuck set and get the backplate threaded for your machine
Then if you can do simple turning you should be able to do the final finish job of mating the two together
I won't even mention the cost $$$$
 
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I believe he has a 12” 1942 Sebastian bench lathe.
If I understand the problem correctly, your only option is to buy a plain backplate, have a machine shop plunk a 2 1/8-8 thread in the center of it, buy a new chuck, and machine your new backplate to fit the brand new chuck.
There is no boring, or threading involved in this plan. You will have to drill and tap holes in the backplate to fit the new chuck.
 
I have been to every shop in town (5 of them). They only want to do commercial work. I talked to a shop yesterday. Not interested.
I don't know why the chuck is out .006. It was like that when I got it. It was made in 1942. The chuck has no name on it, but looks like the originals in the Sebastian brochure. The lathes are no longer made. D.C. Morrison made them last. Have no parts. Have called them twice.
The chuck itself, and the backing plate as well, is out .006. I determined this with a runout indicator. The spindle somehow is not.
Loosening the back plate wont help. Recess is too tight, chuck will not shift.
Lathe came from a dead friend. Been in storage over 30 years. did not pay anything for it but it is on permanent loan from his son.. I have $1100 or so in tooling, parts, phase converter, etc.
I have been all over the web everywhere I could look for a used chuck this spindle setup. Nothing. Even bought one online that was supposed to be 2 1/8 but was not. Nice paperweight. It is a 16 inch Sebastian floor lathe. Best I can tell they are the only company that used that spindle setup.
I understand the concept of truing the current backing plate concentric for a new chuck. No big deal. But the new chuck will need a concentric recess the same size to center the backing plate. The old chuck is not concentric and cant make a concentric part as I see it, and the backing plate will no longer fit the old chuck so I would not be able to use it, even though off center, to machine the new part. If I ruin my current chuck I will have no way to use the lathe at all. I have an 8 inch chuck and would replace with that big or larger. Not sure if a 8 inch backing plate or chuck will fit in my 8 inch chuck.
Again. I have seen ads from companies that will ship a new chuck with the threads I specify. This was several months back, and I cant find these ads now.
 
Why is this lathe important to me? the previous owner was a top notch machinist and looked for a couple of years for a lathe to play with in retirement, but never got it running before he died. His son was my best friend when we lived in another town. When I got it to run I sent the son a video and he said his dad was smiling in Heaven to know it was at last running. I always wanted a lathe but could never afford one. I had a chance last fall to buy a nice Harbor Freight 16 x 36 lathe but did not buy it at an auction. The plug was removed to prevent testing, the building was built around the lathe, and I preferred a US made lathe. I have a bunch of Harbor Freight stuff but I did not get this one. Would have been ahead if I had I guess. I have an obsession with getting broken things running. Usually spend about 3 times the value on old things to see them run. This post sounds too much like a 4-H essay. I don't have a closeup of the chuck but I assure you it is .006 out of concentric and has 2 pinions missing, and I had to make a chuck wrench as well for it. I also had a guy call last night about selling some parts I am making. I know this is not an Asian made, CNC controlled wonder. It is 80 years old. And it weight 2200 lbs.
1693676864385.jpeg
 
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Cool looking machine and great story! That is all helpful information.
Hey, are you indicating on the chuck body or a pin held in the chuck? I would check both for starters.
The existing chuck should have a factory made recess on the back that is concentric with the chuck body. The back plate has a boss that should fit tightly into this. So if you loosen the bolts on the back plate it is normal that you cannot move the chuck around.
The way to fix this is to remove the chuck from the back plate and leave the back plate on the spindle. Then turn off the existing boss (which is apparently .003 off center. TIR=.006) Then turn a new boss that tightly fits into the recess on the back of the chuck. If you machine it correctly you should be dead on.
It would be very helpful for you to separate the chuck and the back plate and post pics of the back of the chuck and the front of the back plate before you do any machining. Also check these areas for damage. Some back plates fit very tightly and it may be difficult to separate these. Have you tried this already?
 
I loosened the screws and the chuck will come off. However, if I get the chuck concentric, it still is missing 2 pinions and won't grip the work correctly. I f I am making a 3 inch part I put a 10 inch blank in the chuck and lose 7 inch es of stock. It is .006 off at the threaded mounting collar and the outside of the chuck
 
@CJ5Dave
Thank you for the excellent replies. We have a much better idea of what's going on now. A few options come to mind.

If you could get the existing chuck to run true enough, would you consider it acceptable?
What I mean is, if it's original to the lathe, it's literally been through WW II, has two pinions missing and may be so worn that it is best replaced.

I can't resolve the construction of your chuck from the picture above. Like rwm, I'd like to see a photo of the chuck side of the back plate (removed) and another of the back side of the chuck body showing the mounting features.
On a side note, You say you don't have a faceplate. Rather than get someone else to thread a back plate, I suggest you buy a faceplate and have it threaded to fit your spindle. With one, you can learn to thread your own back plate(s). I suggest you keep an eye out for a faceplate with enough flange present to open up to 2-1/8-8.
 
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