ok i'm going to re-phrase a little of what i said... yes MDF can be made strong.. but when the cost is not to far away from each other then imo using steel box section looks far easier than that setup that gerry above is going for.. and for what it's worth a simple DIY guy is not going to have access to the tools that gerry does.. you can see from his second and last drawing that there is a hell of alot of work for him to create just his gantry beam and that he by the looks of it is using another CNC machine to make the cuts for his new machine

i am making my machine on a budget and had originally thought about using mdf but i honestly believe that using steel box section is not much difference in price and looks a hell of alot easier to get strong