SteveG's Van Norman #12 mill
Great to see chips Steve.
The method you used to mount the mandrel, using the collet itself should work but for some reason doesn't. I did the same thing resurfacing the mount for a collet chuck. I turned a heavy piece down to give a 1 inch spigot, then grabbed that with the collet chuck. You would think this should remove all runout, but when I ground the taper for the lathe spindle mount I still had 1 thou runout. No idea where it crept in.
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After thinking about it a bit overnight, I've realised that at least part of the issue is that I was very slightly undersize on the plain portion of the shank. At the time I didn't think it was critical and that the taper would locate it all OK, but reality is the taper is only short and can therefore allow movement at the top of the arbour unless its a good fit up there (even if the arbor taper is perfectly aligned with the collet taper)
Taking the collect section into account there's a bit of length below the taper, so that will magnify any runout.
To check the theory, I loosened the drawbar and repositioned the arbour a few times and each time the runout was slightly different. Unfortunately not different enough to make it go away!!

Ultimately I just need to make another one properly, but in the meantime I might try taking a bit more off the diameter of near the top then fitting a bronze bush or similar to get a better fit in the spindle.

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