A rather common problem with 6200 series bearings is having them get axial preload . It's usually having either the wrong length spacer, spacer being too short, or no spacer at all. They get a notchy feeling ! If there has to be a preload, it has to be radial. I would guess that the clown before you jammed the nut into the thread end. A new pin wrench and some heat would be how I would attack it. The over tightening is a killer on 6200s. While 6200 series bearing do take some thrust loading, they have a deep race, but they are a basically a radial bearing . On one machine, I solved that by giving the operator a much shorter wrench. On your application the top bearing should be floating, whether on the shaft or housing. Thermal expansion has to have some place to go. When guys say don't mill on a drill press, I laugh. On my drill press the bearings are 6200s. You should only mill, not drill ! My guess your mill/drill is just a beefed up drill press.