Hi Jonathan,

The drawing helped, thanks. Wow, alot of overhang.

You could design a lower 'floating bearing'. This is where the lowest bearing block has two or more alternative mounting positions on the Z axis part which holds the router. For long reach work you bolt it where it is now, and accept the deflection. For shorter reach work you bolt it further down, spacing the bearings out so that the loads are taken towards the ends of the unsupported beam, rather than towards the centre. You need a moveable limit switch with this idea.

Another thought, although it doesn't really work with your requirements, is to support the rail at the centre, as well as the ends. The bearing blocks then run in the 'upper part' above this centre support, and the 'lower part' below this centre support. Your rail deflection is instantly halved, but there's a compromise with Z range.

It looks like you are using an 'I' beam for the router mounting plate. You're only considering the bending in one or possible two directions, which will give you good results on the deflection for these load cases. But since the router or spindle will be mounted off this, and the cutting forces will be offset, there will be a twisting force (Y direction cutting) and this section choice will give you poor results, particular with that overhang. Overall it might be worse than the original plate, all loads considered.

For such an overhang, you might want to choose RHS for the part which attaches to the router/spindle. This is good for all the load cases you are requiring. Ideally any bolting into this section should go all way through with spacers inside to get the loads into the section, rather than locally deflecting the wall. The calcs all assume good load transfer.

As Ross has already suggested, I suspect you don't need to mill down 400mm into things, but rather you want to be able to machine to a depth of perhaps 30mm into things which are up to 400mm deep. If this is the case you could limit your Z travel to say 100mm. You then block up the workpiece with spacers to suit. This would be fairly stiff for all cuts, but give you options in terms of workpiece sizes.