Picture of what you want would help to see if can be done on machines I have.
Sorry by the “book”Brooke?
Or insert coins in the clamp gap and (carefully) screw in the bolts from the other (threaded) side until it opens up a bit. With a wedge/screwdriver I always managed to damage the stem. Also think no machining is needed if it´s not a 1 1/8" steerer an a 1" stemIt's more likely that the steerer tube is slightly flared wider at the end, dressing with a file should sort that out. The stem can be opened by carefully inserting a wedge into the clamp slot, a flat blade screwdriver will suffice.
A member on hear called “The reach” he’s a first class engineer and will be able to do this for youit’s literally 1mm
By the Brooke the stem should fit the steerer but it doesn’t!