Do it properly, make a new one. Whether by CNC or manual is irrelevant

we’ve been here before
https://www.model-engineer.co.uk/forums/topic/lathe-chuck-thread-size/#:~:text=jim1956,form%20as%20Whitworth)%20but%2011tpi.

32/4 BSw x 9 sounds like an early Myford ML1, 2, 3, or 4.

Could you bore out the “new” backplate and bush, before boring and tapping to 3/4 BSW?

The tapping will be hard work, but not for too long.

One tapped, fit the backplate to the spindle,and skim the face and then the OD to be a snug fit in the chuck.

Hopefully, the holes in the backplate will line up with tappings in the chuck.

Let us know how you get on!

Howard

If you have the tap, buy a slice of CI and make one.

Hi, yes a slice would do it, I just mentioned the one from Chronos, as it has the normal shape, and therefore less machining and less waste of material.

I’ve used a slice of steel at times, but I’ve been lucky enough to get pieces of the right diameter from work. I agree with the way JasonB has said to thread it.

Regards Nick.

I heartily agree with Jason’s advice to screwcut the thread, and maybe use your tap to clean up the last  couple of thou. This is more likely to give you a thread that is straight, square and concentric. (Plus a lot less sweat involved !)

These days screwcutting boring bars and inserts are so  cheap and do such a good job it really is a doddle. And cast iron is nice material to screwcut.

I was suggesting that you don’t use the tap. Either screwcut on the lathe or threadmill on the CNC

No need to grind both sides as the chuck side gets finished when it is on the lathe. I’d also tend to mill the face that goes against the spindle register and do the thread in one setting ensuring that surface is true to the bore/thread. So mount on the CNC in the orientation you have it in your first image. Level the top, adaptive and finish cuts to form the spigot with a corner radius cutter to leave a fillet in the internal corner. Use the mill to bore the 5 holes and then threadmill, knock off edges with a chamfer mill. Mount on lathe and turn larger OD, face and any chuck locating spigot.

Edit. Looking at the cost of thread mills of that size (£70+) I’d screwcut it on the lathe. Though I suppose you could make a single point threadmill which would not be as costly as a carbide thread mill.

Put packing pieces between the job and the faceplate to create clearance for your boring bar to run into. Pieces of square HSS tool bit or key steel work well for this.

Machine the thread, the register recess and the back face all in one setting so they are all true to each other.

Wear on your lathe leadscrew will not be a factor unless you have a phenomenally clapped out lathe. It’s a short thread length so error will be negligible in practical terms. That’s why the halfnuts are brass or Mazak , so they wear but not the steel leadscrew. I have screwcut backplates and similar on lathes made before World War 2 with no problem. And your tap used to clean the last 2 or 3 thou out will correct any error as you say.