Of course it doesn’t need to be perfectly cubical, either. The QuadRap design makes it easy to customize to your needs. The dimensions you choose won’t affect any other parts, except the length of belts and the Z drive-rod. The tubes have to be drilled at their ends to accept Ø6mm bolts. Once you’ve align the edges of the tubes and tensioned these bolts you’ll have a perfectly aligned frame, presuming the cut lengths are all the same. The frame acts as both the supporting and the directing structure for the movable components. Not only that, but it plays the role of a heat sink for the stepper motors, so long as they are connected with aluminum plates to the frame! In six months of regular use I have no worn out parts yet, but if the aluminum tube in the X-Y gantry should wear out from use, it can be replaced quite easily. In fact, since the frame tubes have the same construction as the gantry, you can just swap the gantry tube with any of the frame tubes, and if the frame has a cube shape, you can do this up to 5 times!
The Drives The stepper motors are connected to the frame with an aluminum plate that sinks the heat to the frame, and instead of using toothed gears on the motor shaft, we use a piece of rubber hose from an air compresser. This small rubber drum drives a belt made with reinforced packing strap. The friction between these hoses and straps seems to be completely sufficient to perform movements with high accelerations up to 900mm/s^2! Even when you try to hold the moving part with your hand, the stepper will lose steps before the strapping will slip. The absence of friction means there’s no wear to the hoses or straps, and because packing straps are designed to remain tight for a long time, the strap doesn’t lose its pre-tensioning. Since there are no teeth, its movements are more precise, and the small diameter of the rubber drum reduces the torque required on the motors.
For frames over a half meter in length, it is helpful to add diagonals made of twine or packing tape. The packing tape can be nicely connected with an extra washer and links of bicycle chain.