In particular, are they regular hexagons in shape (all sides the same length) and is the cell size measured from one flat side to the opposing flat side, rather than corder to corner or some other measurement?

I am especially interested in the machining side of things though, so it's great you've mentioned that.  I too will be looking to machine down to 3mm and 5mm - could be a useful comparison with the Nomex.  Do you have any tips or guidance as to how you found the machining to work best?