Hi Guys
Ideally the place which to apply the force to move the gantry/table is in the same plane as the linear guides. Plenty of people don't do it that way and have the screw in a less than ideal position and still achieve good speed and accuracy. In the case of Joes 2006 machine with the 4 pipes the ideal place to drive would be at a plane centrally located between the two levels of pipe. As Joe has pointed out in his diagram above you don't need to have the screw located centrally in the mechanism. By using cables to apply the force in the desired plane you can put the screw anywhere you like along the driven component of the gantry/table and it wont 'rack' since the forces on each side are the same because of the cables. The position it is in in the diagram above is ideal since it is as close as is practical to the cables line of action of force.
I would like to point out another consideration that can be made during a machine design is the position of the centre of gravity of the gantry/table being driven. During acceleration and deceleration a moment is generated about the linear guide due to the position of the centre of gravity (COG) relative to the linear guide. You can minimise this force by keeping the COG of the driven member as close as is practical to the linear guide. You could even add weight (he didn't say add did he, he's MAD!!!) to shift the centre of gravity to be in the same plane as the linear guides. If your system could cope with the additional torque and weight (deflection) it would allow much higher acceleration and deceleration of that axis. Of course the COG changes as other axis shift position but you can still design out the majority of the moment applied to the linear guides during accel and decel due to a COG not in the same plane with the linear guides.
Cheers
derekj308  |