Hi Stefan,
from what you describe, you are talking about square steel tubing.
probably pretty thick wall.
this is common for HEAVY duty router tables, depending of the size of
course. I was looking at 2 inch x 2 inch square with a 0.187" wall
for a 4 foot x 8 foot (1.2 meter x 2.4 meter (just guessing)) table.
to check accuracy, this can be complicated. and getting really high
accuracy will take lots and lots of time.
If you are only looking for a PCB table, considder making a frame
with aluminum as extruded aluminum is usually pretty darn straight.
or, attaching a rail to the top (or bottom) and one side. all you
need to do is make the rail accurate and you can bolt that in the
middle pretty easily. your bearings would ride on the rail.
If you want to true up the steel box, get a machinests level, and a
small grinder. level the table at best you can taking measurements
so you know all the lowest points are at the same level, check them a
fet times to be sure.
then mark the height of the high spots with a feeler gauge and
chaulk. think topographical map here.
then grind off the high spots. one note is that grinder wheels are
cheaper in the 10 pack.
additional notes,
wear hearing protection
wear eye protection
wear breathing mask of some sort
get wifie to rub your poor aching back.
Once you get the thing close, start checking side to side. then mark
it all over again.
lotta work, but a really great result.
Dave
above is synopsys of a proceedure written by Les Watts
http://home.alltel.net/leswatts/straightedge.html