7' X 4' Combi

[Jonathan]

You want the lower two bearing blocks on Z as close to the bed as you can get them, to reduce the overhang. This is because the Z-axis can be modelled as a cantilevered beam, since it's only supported at one end. This means that the deflection at the end is proportional to the distance from the tool to the support (lower Z bearings) to the power 3, so even a small reduction in this distance can help a lot to increase the stiffness.

I'd put them above the top rail and below the lower rail for the above reason, and also because this increases the spacing of the blocks which is important to get maximum stiffness and it probably makes assembly easier. If However you can't afford the loss in travel thin entails, it's OK to have one between and the lower one below.

[Ricardoco]

You want the lower two bearing blocks on Z as close to the bed as you can get them, to reduce the overhang. This is because the Z-axis can be modelled as a cantilevered beam, since it's only supported at one end. This means that the deflection at the end is proportional to the distance from the tool to the support (lower Z bearings) to the power 3, so even a small reduction in this distance can help a lot to increase the stiffness.

I'd put them above the top rail and below the lower rail for the above reason, and also because this increases the spacing of the blocks which is important to get maximum stiffness and it probably makes assembly easier. If However you can't afford the loss in travel thin entails, it's OK to have one between and the lower one below.

Well the Z rails i have were going to be 400mm, but since i got hold of some 520mm rails i can afford to loose a little more travel than expected, i will take that advice, and thank you very much for it.

I will be completing the design before the end of the week i suspect, i am going for 30mm alu plate if i can get hold of it.

Regards Rick