Couple of questions about carriage positioning on X axis and z axis rails

[Lee Roberts]

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...

Don't think there is an optimal, like you note, you want them as far apart as possible with the least amount of compromise on axis travel, if you get down into the science, it's just not possible to mount the bearings at distances apart that the numbers suggest.

I think an absolute minimum distance the bearings should be spaced apart, is whatever the distance is between the rails, lets say 250mm.

Designing this type of cnc machine is a constant search for equilibrium, great fun though a!

[Neale]

As Lee says, engineering is the art of compromise. FWIW, my own thinking was along the lines of...

I want quarter-sheet travel plus a bit of spare. Say, 1400mm. Next bigger off-the-shelf ballscrew available in UK was 1700mm. Use hiwin rails, 1750mm, then 250mm carriage length gives 1500mm travel. 1800mm box section lets me overhang rails off end of machine for end cutting while still keeping adequate bed length for panel work. Just about fits workshop floor area available. And that was more-or-less the design process. Took a few months of cogitating to get there, mind you, but in the grand scheme of things, 50mm extra bearing spacing costs almost nothing, adds little to size, and gives that slight feeling of doing the right thing. Y carriage width looks about right at 200mm for 750mm travel on 950mm gantry (inside dimensions).

In the end, the decision often comes down to some simple key facts and you just have to work out what the real decision factors are. Like stepper motor sizing - for this size machine, you can analyse and calculate as much as you like, but the answer will be 3 or 4Nm Nema 23 motors with a 68V psu. You don't use bigger motors because they are heavier and slower, and smaller motors aren't that much cheaper.

Sometimes the engineer in me overcomes the need to analyse to death and at least you can then start building!