I am in the middle of both researching 3 axis and 5 axis machines for work and designing a 3 axis machine for home. All the designs seem to raise the gantry off the rails to the height so the z-axis then travels down to meet the cutting table. Raising the gantry to gain this z travel would necesitate a heavier duty block and rail system because of the twisting of the high gantry. Why not keep the gantry low beaf up just the z axis mount, and lower the cutting table the distance of your z travel? On huge cnc machines I have seen the rail be 8' above the floor. Just thinking out load.