I have a tool and cutter grinder which is, in theory, capable of grinding spheres. On my particular one (early Alexander) there is no easy way to disable the 90 degree stop.
The lathe speed is synchronised to the current working diameter, but the VFD control is slightly laggy so might get left behind on small diameters / short passes.
The speed changes are more obvious at the start and end of each pass, but it should be running at constant surface speed.
The VFD is controlled by Modbus through a USB interface. I would like to bring that into the realtime layer, but that needs new drivers for the Mesa interface cards. This is not an unsurmountable problem, as I am the guy who writes the LinuxCNC drivers for the Mesa cards.