Thanks for the reply Jazz,

As the title suggests this is my first go :-)

My original plan was to make a 2' x 4' bed with the gantry running across the 4' length. That way when the time comes i could just extend the 2' sides to 8' without remaking the hole frame. As you pointed out, this can work in theory but comes with issues to consider.

When you say high sides do you mean on the x axis?

In terms of frame materials, is the 80x80 aluminum profile i used sufficient? I originally considers 80x120.

Should i be using rack and pinion with my plans to extend the frame in the future?

Let me know your thoughts and I'll go back to the drawing board.:-)