A quick update. and a short tale of woe that may help some one else avoid the same problem.
I decided to make a dummy spindle nose to be used when making things like backplates for my old red lathe.
So I took the measurements, chucked up a piece of leaded steel in the Denford lathe and proceeded to make something as near exact as I cold get it.
Got the register spot on. ground up a tool to cut the 8 tpi thread that the chuck, faceplate etc screw onto, 55 degrees. cut a short parallel diameter at the end to indicate the depth of cut. And away I went.
I have no thread counter on this machine, so it was a case of forward/ reverse. no disengaging the lead screw.
Got down to the indicating diameter. tried the red lathe's faceplate. Wont fit. Cut a bit deeper. Still won't fit.
measure everything in sight. Everything looks ok. Why won't it screw on ?
After a couple of hours, when the stove had gone out and it was starting to get cold in the shed, I decided to call it a do for the day/ But I first took the cutting tool from the Denford and tried it in the red lathe nose thread. And the problem was revealed (well, at least I think it will be). My tool has a sharp point. the bottom of the lathe's thread has a rounded form. And the calculations didn't allow for this.
So, when the point of the tool is down to depth, it really needs to go a bit more or the thread will be (is) too tight.
All I need do is stone a nice radius on the tool tip and then fiddle about re-engaging the tool with the thread, and the job should be a good one.
Moral; don't forget the tip radius when machining things like spindle threads.