Machine tools, like lathes and milling machines, are big and heavy for a reason. In order to ensure precision, they need to be incredibly rigid. Even the slightest amount of flex or twist anywhere in the machine will result in loose tolerances. That's why manufacturers construct the frames of their machine tools from steel or even good ol' fashioned iron. Well-built CNC mills can, for example, cut within tolerances of a thousandth of an inch. But does that mean it is impossible to build a practical machine with weaker materials? To find out, Ivan Miranda decided to build a CNC knee mill using aluminum extrusion and 3D-printed parts.
Miranda has built several CNC routers using similar materials in the past and some of them were even able to cut aluminum. But those were all large routers and they didn't have the rigidity necessary for serious machining operations. In his newest video, he built a CNC knee mill. While it is much smaller than his previous machines, it is far more robust. Miranda obviously put a lot of thought into making this machine as rigid as it could possibly be with the given materials. He also ditched the usual handheld router in favor a real spindle motor.
The frame of this machine was constructed with a lot of T-slot aluminum extrusion and some ABS 3D-printed parts. He used heavy duty linear rails and bearings for the ways, which only allow movement in a single axis each. This is a three-axis mill, with a NEMA 23 stepper motor actuating each axis via proper ball screws and anti-backlash nuts. The Z axis only has a few inches of travel, but the table is vertically-adjustable to handle a variety of workpiece thicknesses. It's equipped with a 1.5KW ER11 spindle motor driven by a VFD (Variable Frequency Drive). The CNC motor movement is coordinated by a Duet3 controller.
There is almost certainly some flex in the machine's frame, but it still seems to work well for low tolerance milling. In the video, Miranda demonstrates cutting both aluminum and steel. That is quite impressive for a machine constructed from aluminum and 3D-printed plastic.