Why not use unsupported rails?

[Jonathan]

This question arises in almost every build log on the forum and every time someone explains why they are unsuitable for most CNC Routers. Here are some simulations I have run to prove the point...

All rails are 20mm diameter with 150mm between bearing block outer faces. The bearings are attached to a 75x160x20mm plate to which the forces are applied.

Assumptions:
Rail / box section ends are perfectly rigidly supported - in reality wont be so deflection will be greater than calculated.
All joints are perfect, modelled as single entity - in reality will get some deflection between joints.

The following diagrams each have different scales to make the deflection visible throughout.

Unsupported rail 200N in Z direction to simulate weight of Z-axis:



Supported rail 200N in Z direction to simulate weight of Z-axis:



Unsupported rail 200N in Z direction to simulate weight of Z-axisand 50N in parallel to X for cutting force:



Supported rail 200N in Z direction to simulate weight of Z-axis and 50N in parallel to X for cutting force:



Max deflection for supported 0.0074mm vs 0.34mm for unsupported... draw your own conclusion.