Thank you!
The production of the ring groove gave me some headaches.
A small note on the production of the T-slot.
Surely you can work it out with a hook chisel. I went a different way. I first cut the groove on the lathe from the top side. Then I drilled a 19mm clearance hole from the bottom side to the groove.
Now I was able to mill the groove with a Woodruff milling cutter that had approximately the same dimensions as the later groove.
I clamped the cutter with its 6mm shank from below through the 19mm clearance hole and the groove into a collet chuck.
With moderate speed and a lot of air cooling I could make the groove. But it did not fit in its height. The milling cutter only made a slot of 2,5mm and the width is not adjustable.
From China I ordered carbide T-slot cutters with the corresponding dimensions 1/2" x 1/4" x 5mm ; 6mm shank. When these were delivered, I re-milled the slot in the same way and brought it to a height of 5.5mm. The clamping bolts later have a height of 5mm and fit perfectly. These carbide cutters are actually intended for woodworking, but they also work in grey cast iron, but the speed and feed rate has to be adjusted.