I started designing a CNC router. The bed and beam are supposed to be cast from UHPC (Ultra High Performance Concrete) [german]. This is a very dry concrete mix with aggregates that increase its mechanical strength, and a plasticiser that makes it viscose enough to be usable for casting again. The end product can achieve mechanical properties comparable to aluminium and has very good vibration dampening properties.
The design uses 20 mm linear rails all 600 mm long. Ball screws on X and Y are 16 mm SFU and 12 mm SFU on Z. All bearing blocks are standard BK/BF style. Motor flanges are NEMA23. Drawn are JMC iHSV57 servos. The design would also allow to use steppers geared down with pulleys.
The main idea behind the bowl shape is to create a partially enclosed volume that allows to use flood coolant without creating a mess. It also raises the y-rails to an optimal height.
All bearing surfaces are metal pads that are glued into the finished casting and anchored with pins. This allows to achieve high position and alignment accuracy without the need for a large mill. If a mill would be available, milling the bearing surfaces after the glue-up would be ideal.
The port holes in the side are mounts for an optional 4th axis to enable mill turning. They also allow to attach dust extraction when milling wood and double as an all purpose bulkhead.
Two threaded rods put the beam under compression along the x-axis to increase its strength.