Mystery Lathe Chuck Backing Plate

[AlxJ64]

Long story short, I put a deposit on a nice Colchester lathe that uses the L1 spindle mount. When I put the deposit down (didn't have a way or time to pick the machine up yet) I gathered some parts and pieces and took some chucks. Not looking closely until tonight I noticed one of the chucks had fine internal threads. It is a 6" diameter 3 jaw chuck with what appears to be something like a 1.97" x 18 tpi threaded backing plate. 1.97 is darn close to 50mm but is an 18tpi going to be a 1.5 pitch? 17 tpi would be the closest but I don't have a 17 tpi thread gauge.

What out there has a 50mm x 1.5 spindle thread? I found a topic on practical machinist for a Yuasa rotary table where a guy was looking for lock nuts, but thats about it. The old guy who sold me the lathe doesn't remember anything about it and said he would just hold the small chuck in the big 4 jaw and indicate out the parts before turning anything.

Thanks in advance!

 

[4GSR]

That is an odd thread. People over the years put what ever thread they needed to get a job done on the fixture. Probably not a lathe spindle, but who knows. Threaded spindle could have been on a index head, dividing head, special fixture, just about anything else. I've used smaller chucks like this in larger lathe chucks in past years. I still have a 4" Pratt Burnerd 3-jaw chuck that has jaw marks on the OD where it was chucked in a much larger chuck. BTW- I have a couple of back plates laying around here with odd balled threads in them where they were used on older machinery with odd threaded spindles.