Actually, the "rule" applies to any linear bearing arrangement, but I was mainly refering to the X bearing trucks. If the distance between the rails (across the table) is, say, 1 meter then you would be served well to spread the X axis bearings at least 330MM apart from eachother (along the X axis) on the same rail to minimize or possibly eliminate the potential for skew. In stark contrast, I am looking at a commercial router that has a beam of 2 meters and the bearings are only 300MM apart on the X rails, but I have also seen this machine skew pretty easily. I would spread them out (if you can afford to w/o losing too much of your envelope) to the 1/3 rule.
On the Y axis - placing one on the top and one on the front would really help with rigidity but it would also make them much more difficult to align. Placing them on opposite sides would probably work OK but it would be a real pain to construct a structure stout enough to realize the benefit from the slightly unusual arrangement. I would suggest placing them on the front surface as they are shown, but spreading the rails out as far as you can. Same for the trucks on those rails - distance is your best friend for rigid guiding.
Scott
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