Z-axis design - help sizing materials please!

[Jonathan]

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.

Yes! I was thinking that just hadn't got round to posting it...I'll allmost certainly do that. Maybe have 75mm or 400mm travel. Thinking about it I may as well have the lower bearing with several positions - it's only a few more holes after all.

With regards to supporting the rail at the centre I could do this when the Z range is limited, as above. It would make it take slightly longer to change between the two set-ups though.

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. ...

For such an overhang, you might want to choose RHS for the part which attaches to the router/spindle... 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.

Just to clarify I'm not using actual I beam, just 3 aluminium bits slotted and bolted together to make an I. I can't find RHS in the right dimensions so I guess I'd have to use a similar approach, just now with two 6mm aluminium plates instead of one. I'm concious that using RHS is going to further increase the horizontal distance from the router to the Y axis. Would it be ok to cut the front out of the RHS (to make a C shape) just where the spindle mounts. It's either that or get big enough RHS to fit the spindle inside?

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.

Unfortunately that's not the case otherwise yes, I would just use blocks. One project I have in mind which needs a long Z axis is making wind turbine blades. That would, or course, be wood I should be OK.

Thanks for the help. I'll have another look for what steel RHS I can get hold of and modify the drawing accordingly.