Just to add to what Andy said, the specified length of ballscrews is the total length including the machining at each end for bearings and pulleys. For 800mm travel you will be looking at screws around 1000mm long. If you go for direct drive your frame will neede to acomodare the length of the screw and the motor. As an example my machine base is 1300mm long, the ballscrews are 1100mm, the rails 1200mm and the total travel of the gantry is 940mm.