Cheers everyone, what about the ends of the X supports. Could there be an issue of strength where i have cut into the box section, I am hoping it wont matter so much at the end of the travel as the carriages will still actually be on full box section, only by about 25mm though. I could glue a 5mm steel plate under those areas.
One last thing for now, will 4mm thick steel be strong enough to tap through for bolting the rails?
Cheers, Charlie