A lot of builds here have the bed with rails along the bottom and the gantry sitting on top of it. I have been looking at 3D printers and see a few designs where the 'gantry' runs on rails above the build table. Is there a reason CNC routers dont adopt this design more? At first glance these seems like a good option because the motors and other components would be above the work surface and out of the dust. The downside might be that the gantry above the bed might make adding material on the bed would be harder or blocked by the gantry support. Can anyone comment if they have tried this?
The key issue here is rigidity. 3D printers require very little, generally only enough to keep the frame from flexing from the inertia of directional changes of the elevated moving parts. CNC milling is quite different however. The forces required to push the bit through the material are rather substantial. This in turn causes flex in the system which leads to two problems. The first is an errant path to the cut, i.e. crooked lines, jagged edges, etc. The second and more important though is that flexibility allows the bit to glance off the cutting surface rather than biting into it. This leads to burning of the material and heating/gumming/wearing of the bit. So rigidity is extremely important to CNC machining. And the best path to rigidity is to keep the system as low and lean as possible. In the better designs you'll find the cutting surface is used to brace the side rails and the gantry beam is kept very low to the side rails. This keeps the flow of counter forces on as short a path as possible. This is not to say that taller systems or systems that hang down from above are not possible, but to achieve the level of rigidity as a low and lean system drives the cost up substantially (astronomically). For foam cutters, overhead systems are fine. But if you want to mill anything that fights back you need to go with something that is as low and stiff as possible.
