I found that the base had some bowing in which attaching the fully support rails levelled out for me. I had to place additional cross pieces on the underside of the base to eliminate additional bowing across the width. This also give me more depth so I could sink screws deeper into the material to hold certain components such as the bearings and motor mount. This seems to be a common thing with home-made CNC builds made of MDF. Moisture and gravity can take its toll on the material causing it to sag and twist. This is coming with the cutting table or waste-board where everything has to be true. Often metal rails are screwed along the edge straightening out the board. Something to note for the future when I get to this stage. Which is a long way off!