hi doug, I never meant to drill an 18mm hole? the meaning was that in engineering specs an 12mm hole drilled in 40x40mm box section should have steel
1 and a1/2timesthe diameter of the 12mm hole ie 18mm surrounding the HOLE IN THE STEEL! so if u drill the 12mm hole in the centre line of 40mm box it will only have 14mm of steel
left either side of the hole?If on the other hand you drill a 10mm hole it will have 15mm of steel either side of it? I am sorry if I did not explain it fully the first time? 15mm of steel left around the 10mm bolt will satisfy the 1 and a 1/2 times the diameter of the hole spec's but you make sure u are far enough from the end as well .Well folk's that's the end of the whole story fo now.