Hi Ahtopa - I used a single bearing on a machine like that for a timber machine and it quickly became apparent that it wobbled too much, added the extra bearing and it solved the wobble. All machines have two bearings, some have 3 each side on machining centers.... For aluminium at that size I suggest you use ballscrews or start with lead screws. You need a lot of stiffness, no backlash and a very constant feed. These are provided by screws not racks. Racks tend to jump out at high speeds, see earlier post... Peter