the dial indicator seen in the video measures the distance to the fixed jaw of the vise, and how it changes when i move the X axis of the mill table. i've adjusted the vise (with a little hammer) until that distance stopped changing (you can still see it move by about 0.02 mm in the video — you can get closer if you want/need to, but this is enough for what i'm doing)
that means the fixed jaw is now very parallel to the X axis of the mill, and therefore very square to the Y axis of the mill. this means i can put a workpiece in the vise, up against that fixed jaw, and any cuts i make (by moving the table) will be nice and parallel or square to the side of the workpiece. this makes it easier to make precise parts.
basically, machining is just high school geometry that has the ability to remove your fingers
