Lathe chuck back plate rethreading

[BCNU6OF1]

I have a Seneca Falls lathe #30, 1-9/16*10 tpi for the backing plate threads. I need a backing plate for either the 6 inch 4 jaw I have, or one along with a larger 4 jaw. I'd prefer cast iron.
So my question is - shars lists some with 1-1/2*8 tpi thread. Would there be enough material to take it out to the size I need? Or should I just start with a chunk from McMaster-Carr? Hope this makes sense, long week. Thanks in advance!

 

[markba633csi]

1-1/2" is too close to 1-9/16". I would start with a chunk.
I have heard of a barbell weight being used to make one, but you run the risk of the iron being a very impure grade with hard spots in it.

 

[BCNU6OF1]

Thanks @markba633csi ! I just found a used 1-1/4*10 tpi, I think it might be an option. Thoughts?

 

[Jake M]

I just dug up a chart. For a 1-1/2 8 thread, you wanna make the starting hole somewhere between 1.378 and 1.390 inches. Maybe adjusted after the fact for final fitting. Some lathe makers took some liberties with the clearances. But most didn't. At 1-1/4 you've got room.

 

[benmychree]

You MAY have enough room IF there is adequate wall thickness after boring it out. I have made many backplates over the years and I prefer to buy a rough casting or make a pattern and have one cast and machine it myself, I measure the spindle threads with thread wires or thread mike and make a thread gage to that exact size and about the same length of thread with a knurled handle like commercial thread plug gages to check the thread fit, it should thread into the backplate freely but not loosely, the part that fits the unthreaded portion of the spindle should not fit too tightly to guard against galling; it is (to my experience) the threads that center up the chuck, not the unthreaded part of the spindle and it's fit in the backplate.

 

[OTmachine]

Could you bore out the backplate oversized to say 1.875 diameter, do a combination press fit and green loctite rough bushing install, then thread that?

 

[benmychree]

To my thinking, there is but one true path, and bushings are not on that path ---