K8400 Z-axis improvement

The Z-axis of the K8400 has no rigid reference.
The entire platform rests upon a flex coupling.
This is unfortunate from out of more resons than I am willing to bore you with.

This modification transform the Z-axis from being referenced by random and pure luck, to being rock steady.

You can see the result in the below Youtube video:
https://www.youtube.com/watch?v=wrLdwmv8V64
(Ads are enabled only beacuse Youtube tend to show such search results first.)

I have used this modification