In principle it should be possible. I've done similar sort of thing to round the bottom of a model i.c. engine, albeit in aluminium rather than steel and using my dividing head as a rotary table. I cut most of the material away by setting the milling cutter over the centre of rotation and milling in the X direction at about 15 degree intervals to end up with a 24 sided polygon. (I also need to mill the slope for the upper part of the crankcase but that's not really relevant but you can see the set up)

In order to finish the curve I then rotated the job under the cutter. I had to move the cutter away from the centre line so that it was not trying to cut with the centre of the milling cutter, even though it was a centre cutting 2 flute slot drill

A milling cutter will rarely give you as nice a finish as the lathe but it should be dimensionally accurate even if you can see a lot of machining marks. Worth a try anyway.

HTH

Rod

I have done something similar when machining a large diameter that would not fit in the lathe. I did add a motor to the rotary table via a toothed belt to save me winding the handle round but I did have a fair bit of metal to remove from a 300mm work piece taking about 3 x 3mm cuts and a final finishing cut.

Bob

I think this is unwise. You should not underestimate the precision needed to ensure a good fit in the block and true concentricity. Any error will result in vibration which will show up on the finish on your wood. You would be better off getting a machine shop to do it and take along a block to make sure they got a perfect fit WHEN COLD. Better still have them make a duplicate spindle so you can switch back.