Rough estimate of the force applyed by spindle on the gantry

[Kitwn]

But remember these become moments when applied to the gantry so are a force at the tool cutting edge x the distance to the gantry bearings. Yes this will try and bend the gantry, but it will apply a much bigger twisting force and this will dominate the total tool deflection (in terms of how the gantry reacts).

This has always been one of the reasons for having two ballscrews on the long (X on my machine) axis. Cutting forces cannot twist the gantry horizontally even when using ralis/bearings with a little give in them like the continuously supported rails I use. They can still bend it, twist it vertically and make it lean if the structure is not rigid enough but the worst effect (or probably largest amplitude effect if you prefer) is taken care of.