Roderick Stewart wrote:

Drilling holes after anodising will only ensure that the insides of
the holes are clean, and there is no guarantee of contact here. You
need a small area of the flat surface around the hole to be clear, so
that washers, nuts and screwheads make contact.

Ah, I assumed the holes were being tapped.


There's probably a
special rebate drill that can do this neatly,

A milling cutter called a slot drill will produce a flat-bottomed hole. If
well centred, it could put a tidy shallow rebate around a drilled hole.


otherwise it might be
enough to use those spiky washers that bite through the surface
oxide.

I'd scrape/sand off the anodising from a small area inside, and fit an earth
point with a serrated washer and tag.


Don't forget to file away the oxide around the edges of the
top plate, and the parts of the chassis with which it makes contact.

Not so keen on that - the filed edges could tarnish and acquire resistance.


--
Wally
www.wally.myby.co.uk
http://iott.melodolic.com