Do you need to water cool erhm a water cooled spindle?!!

[Jonathan]

Re soldering an earth cable onto the spare pin ...is this towards connecting an earth to the spindle's casing?)

Yes, so you'd want to unscrew the endcap, drill and tap a blind hole in it inside and use that to secure the cable.

[HankMcSpank]

Just wondering what the 'win is' by connecting an earth in the way you suggest?

The (aluminium) spindle mount conducts & the spindle chassis itself is conductive ....... the spindle mount is secured to the aluminium Z axis backplate...& I actually ground the backplate (for auto Z tool tip zeroing purposes)...therefore my spindle's casing is essentially grounded (albeit in a somewhat round about way!)

[Jonathan]

It's just generally a bit neater than having a separate wire poking out to the spindle mount.

As you've already grounded the Z-axis, be careful regarding ground loops...