I am currently designing a massive mod to really improve the shapeoko 2 3d printer. Its a complete overhaul of the machine. The hot end will move only in the xy direction and the bed will move in the z direction. I have a massive piece of borosilicate glass i have found in a scrapheap. Its about 6mm thick and a area of 480mm x 300mm approximately. It will be raised up and down with two threaded rods with their own stepper motors driving each one. I have not yet designed this part as i am concentrating on the xy movement first. Fine tuning is a large part of the design. Alignment of the belt drives and other components is vital to decent operating 3d printer. I keep my blog updated on the design progress as i go along.