Assortment of questions on design

[routercnc]

My current machine uses 15mm rails on Z and cannot recommend it. Lots of reason including stiffness , packaging ball screw
Go 20mm

If you want to do steel convert a mill else you really need to go to town on a router

[Rainbows]

The beginer moving gantry pinned thread mentioned that you if desperate could skimp out if desperate on unsupported rail for the Z axis, now realise though thats probably because it has a short span compared to the other axis rather than it being under low load.

[Clive S]

The beginer moving gantry pinned thread mentioned that you if desperate could skimp out if desperate on unsupported rail for the Z axis, now realise though thats probably because it has a short span compared to the other axis rather than it being under low load.

The Z axis gets the most load although 15mm rails will do the job there are good reasons to use 20mm and they are not much different in price. You will find it difficult to get the ball nut in using 15mm as you have read most people that have used 15mm say they would not do so again.

[EddyCurrent]

Also if you did use unsupported rails of substantial proportions then the moving Z plate might end up some distance from Z back plate thus creating a cantilever and increased turning moments.