Fitting a used chuck and backplate

I have a good Swedish (SCA) 6" four jaw independent chuck that I want to fit to my SB 10K. (The lathe isn't here yet. Its sitting in an unheated shop (15 degrees today) 40 miles away, and I will need to disassemble it to get it to my basement. Hoping for warmer weather next week!) The chuck has a 1 1/2 8 tpi threaded backplate already mounted, but I don't know what lathe it used to ride on. Am I correct in thinking that it is unlikely to run true on my lathe without refitting? If I need to refit, can I expand the old boss by peening, or should I reduce it and turn a new one underneath? Finally, the backplate is about 1/4 larger than the chuck in radius. Is there any good reason beyond esthetics to machine this down to match the chuck?

Thanks in advance for all replies,

Jon

jdgoguen said:

...I don't know what lathe it used to ride on. Am I correct in thinking that it is unlikely to run true on my lathe without refitting?

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Very much so!

jdgoguen said:

If I need to refit, can I expand the old boss by peening, or should I reduce it and turn a new one underneath?

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I would just turn a new one. If you just peen it over, I don't think you'll have a very snug fit.

jdgoguen said:

Finally, the backplate is about 1/4 larger than the chuck in radius. Is there any good reason beyond esthetics to machine this down to match the chuck?

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I have one like that and I've just never gotten around to trimming it down. I was going to do it after I finished fitting it, but once I bolted it down, I never bothered.

Here's a video:
https://www.youtube.com/watch?v=K7sg940CYgg

I know the video is done on a banned lathe, but it's the process and not the machine we are talking about here.

Also, be sure when you counterbore the hole, you do it from the correct side. Get out a sharpie and write "counterbore this side" on the back, and "counterbore other side" on the face.

Don't ask me how I know, but hex heads sticking out don't look as good as recessed socket head cap screws.

Steve

one good reason for reducing the backplate would be weight, enertia ....
on the other hand some extra weight might be beneficial sometimes.....