Looking at some large overhead gantry systems, got some questions that maybe you guys can answer. I'm refering to the large picture posted below.

1. legs: some even larger machines use what looks like concrete legs (along the Y axis on the pic). I understand that steel legs can be moved later on, but wont concrete creep a bit over time, affecting accuraty? It would be quite easier to build with concrete...

2. speaking of axis: is there a standard for axis? most machines I have seen from DMS (see http://www.5axiscncrouter.com) got the gantry moving along the Y (and it seems like the Y is always longer). Must be easier to have a shorter gantry right?

3. Rack and pinion: Since you CAN'T have for exemple a ballscrew in the middle of an axis, how to do slave accuratly to make sure both racks and pinions of the same axis always move in tandem?

4. Z Column: here, the YZ axis plate has the rails for the Z axis (while the blocks could be bolted to the plate and rails to the Z axis). I understand that this allows you to have a counterweight for the Z. But, that adds lot of weight. counterweight or not?

5. Motors with rack and pinion: looks like the motors on the Y axis are direct drive. Is it possible?

6. ballscrew over X: is it me or it looks like their is a 10+' ballscrew over the gantry? would accuraty and backslash suffer a lot to go with belt or chain?

thanks again guys