Quote Originally Posted by GEOFFREY View Post
Why is it not possible just to drill a bigger hole in the bottom face of the tube and nut up to the underside of the top face? G.
It is possible Geoffrey but to be honest it's not needed. Mostly I use 4 or 5mm without any problems but in the past I've fitted rails to 3mm box with no issues. The fine pitch gives enough threads engaged and because of the number of bolts you don't need to swing on them to tighten down.
I just use spring washers with loctite on threads and never have any trouble. The rails on my machine are into 3mm box and have been on for over 6yrs without any bolts coming loose so it works well enough.