Thanks for the replys
Andy, your choice of course, but IMHO 20mm would be an expnsive overkill unless you have something else in mind, like mounting a spindle, in which case you need to be thinking larger
So you think I should use 15mm, like I said it only has to support the Z axis and plasma torch.
There is no contact with the torch and sheet, except when it lowers to reference the height for the THC
I am going to post a Build Log soon showing my proposed modifications to my plasma machine

Andy