I've been spending many hours reading build logs and getting a few insights into designs for moving gantry routers.

Couple of questions for the DIY CNC Hive Mind:

I have seen that people arrange the x axis carriages such that the point of the tool will be between them. Z axis depth means that the y axis beam ends up set back towards the rear x carriages. (One or two designs I've seen don't bother with this, they just have a more massive gantry with more depth...)

Is there an optimal spacing for the x axis carriages - assuming that loss of x travel is acceptable to get a more rigid gantry?

Does having the carriages further apart add much benefit?

I guess this apples to all axis?

So for hiwin 20mm carriages, set with a gap between the seals of say 40mm, the total width of both carriages comes out at something like 200mm, so to have a working x of 1500mm, you need another 200mm of rail and bed.

This dimension really sets a lot of the overall dimensions of the machine...

When sketching up a layout, I found that starting with the carriages and rails makes getting the overall dimensions easier, so I want to be sure I get the carriage spacing in the right ballpark first!

Assuming the machine is for milling aluminium and wants to have a working area of around 1500,750,250 ish!


Second question is about the z axis. I read that rails on the spindle plate is better, but many don't build like this. I assuming that by mounting the rails on the spindle plate, you get a lot more z travel, but the spindle can overhang (or rather over drop?) the carriages. doesn't that lead to possible vibration and flex in the z axis?

I suppose large z axis travel means that a vertical mill configuration is the best approach, seeing as gantry designs are more about 2d cutting...

So is there a rule of thumb for z travel given a pair of 20mm rails with a 2.2kw spindle?

Cheers!