Is the following something any competent machinist could accomplish on a decent lathe with one hand tied behind his back, or would it be difficult to achieve even for a master craftsman given the small size of the workpiece and the desired tolerances?
Remove 40mm of thread from a 50mm stainless steel M6 hex-socket-head machine-screw, leaving a 10mm section of thread near the head, reducing the diameter of the now threadless section to 3.2mm, tolerances on the diameter not to exceed .075mm
Yes. Anybody who is not a beginner with right equipment should be able to do this relatively easily.
Thing is though the bolt will need to be supported in both ends. So it will need a center hole in other end. Possibly a steady rest, or needs to be cut in 2 passes since its longer than 10 times the diameter.
To your tolerace is plenty.
Now bolts, especially plain steel ones, aren't allways the best possible material for machining. Also the concentricity to the head isnt stellar. So be aware that it might be easier to get quality steel as hex sock and make the entire bolt in many cases. So starting off with a bolt isnt necceserily a benefit.
