I am still in the planning stages of my retrofit (still waiting for the machine to arrive) and know I will need to either get a phase converter or VFD. I will most likely go the VFD route because I can also use it to increase the spindle speed. Everything I have read says I can push the VFD to double the frequency, or 120hz, which will effectively double my spindle speed. My spindles current max RPM is 4200 RPM so I could theoretically push it to 8400 RPM. But this is an older large (4500lb) knee mill and I am concerned how much speed the actual spindle can handle. At what point (RPM) do I worry that the bearings can't handle the speed? When do I need to add bearing coolers? What about spindle balance, tool issues or anything I am not thinking about? Am I being too ambitious in trying to push a spindle designed around 4200 RPM to double that?