In post #32 the Z bearings are trapped between the fixed distance between the angle iron sections. This means they cannot sit naturally, or have any pre-load control, and you can't adjust it either.

I'm guessing you welded it together with the angle iron bolted to the bearings? If you release the bolts into the Z bearings do they slide out easily from under the Z axis (which is what you want), or does the Z axis 'spring away' or 'close up'? Ideally this should be a nice gentle fit onto the bearings , or possibly have a slight bit of pre-load.