I am re-visiting some earlier rough work on my telescope in an effort to improve things. I originally cut a 3mm keyway in a 12mm shaft that skewed slightly so I want to recut it straight. Also making new tight fitting keys. What would be the better method? Using the original 3mm end mill and make two cuts in the old keyway widening it by .5mm each side producing a 4mm keyway or using a 4mm end mill and cutting over the original keyway cutting both sides at the same time also producing a 4mm keyway. Unfortunately without a mill I cant use a woodruff cutter.