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You can zero in on your vise, and make some cuts, you want to keep things short and close so there is little deflection on the mill.... Now, you have to drill a hole, and the chuck and drill bit is too long so you have to raise the head. Now where are you in relation to zero on your work and vice? Your head can swing side to side, and even if the rack lifting the head is tight, you have lost accuracy of your setup.
This is why people went to the knee mill, to be able to raise and lower and remain true and dialed in. I realize that it is a serious cost and some people can't afford it, so they get a round column mill. You can start out this way, but you will sooner or later want to go to a knee mill as your skill and knowledge expands with what you learned on the round column.