Limiting unsupported extension on Zaxis.

[Rogue]

As you've drawn it in 2. if the gantry moves to the right it knocks the red block over...oops. You would need the spindle further down, but then the Z-plate is longer adding to the flex.

Yes, I know that the red block would get knocked over if the table moves. The work area is very, very narrow there ( the little lines/arrows underneath the picture reflect the amount of movement available) but I was thinking of setting up some kind of limiting system for when I was working in this "narrow" mode. In that situation I don't really need much Y travel, just the Z travel.

The better way to do it is to mount the rails to the plate the spindle fixes to, and the blocks to the gantry. Rails move up and down, blocks fixed. You'll find it mentioned in more detail in quite a few places on this forum.

I've noticed a few ideas that intrigued me, including one that i think might have been yours where the linear system was parallel to the gantry - that is, it was mounted in a U section of some kind with the blocks on the side rather than on the back wall closest the gantry? I liked the idea because it brought the whole spindle assemply closer to the gantry which I can only imagine is a good thing in most cases.

You're in the same situation as me with regards to only occasionally needing more than 30mm of Z travel. Remember to add a bit for tool changing if you have not already.
Since my Z-axis has 400mm travel continuously operating it at maximum travel to do thin parts is just asking for trouble due to the large moment it applies to the gantry. I countered this by eliminating the sides of the gantry so that the Z-axis works below the level of the X-axis rails, and having the height of the bed adjustable so I can optimise it for different height parts.

An intriguing approach. The adjustable bed height might not work in this situation because of other constraints I have.

Aside from (i) the limit in size of workpiece and (ii) the danger of the workpiece being knocked over when working with something taller (as in diagram 2), can you see any significant mechanical problems or disadvantages with running the Z axis in the way illustrated?