Once this lot was done, a test assembly of the parts and think about the stud on top for screwing the mount on.
A rather nice trip to visit Rob (slowcoach) resulted in some brass bar coming into my possession, cheers Rob. I had been thinking about making this part in steel, but worried about it rusting, so the brass was ideal. Unusually I didn't do any CAD for this part it was all on the fly and done this afternoon. I opted to use a taper than the flange I had originally thought about. Makes life easy as you can set the top slide and do both components at this setting for a guaranteed match. The stud was profiled and parted off. I then remounted the stud in the 3 jaw chuck using my play in the scroll to get it accurately mounted. Yes that is 0.00025" runout, Rob might confirm this technique works just like setting up a 4 jaw, but quicker as you are down a jaw, but only on round stock.
Next get the stud ready for threading 3/8 UNC, since I had a die I thought I might as well use it. I made a bit of a balls up doing the threading, I had either left too much material on the stud, or my die isn't very sharp. It was hard work and despite the jaws being tight, they slipped and marked the part. No big issue as this part is out of sight, but annoying just the same.
Time to machine the taper in the top plate of the extension and get the stud to fit just below the surface. I had to swap to outside jaws and re zero the part, all went well with no issues.