Keep reading the build logs and looking closely at the pictures, you will pick it up quickly.

You shouldn't underestimate the importance of rigidity This isn't just a case of using strong materials, but bracing too. There is no point making a strong gantry if the bed is plywood for example; if the work moves (bed resonates) it is as bad as the cutting head wobbling about. Making a machine to cut wood and plastics is one thing, making it cut aluminium is another so if you don't really need it to cut metal you can save money and design/build time.