I'm a newcomer, currently designing my first CNC router for the purpose of ski-building. This requires a fairly long work area (2200mm travel along X direction). The tool would find use in cutting and shaping the wooden cores of skis, as well as long MDF pieces used to form a mold for shaping the ski during vacuum pressing.

I'm limited by my lack of machining skills and tools, so I've chosen to use aluminium extrusion (80x80 for the frame, 40x160 for the gantry and 30x150 for the remainder). My design includes a handful of connecting aluminium plates which I will probably try to have milled for me. For the movements I have selected the following:

X direction (2200mm travel, 40kg driving weight): 20mm profile rails, 4 bearing blocks, racks with module 1 pitch and 30-35mm pinions

Y direction (550mm travel, 14kg driving weight): 15mm profile rails, 4 bearing blocks, 20mm ballscrew

Z direction (150mm travel, 8kg driving weight): 15mm profile rails, 2 bearing blocks, 12mm ballscrew

I selected flanged bearing blocks which are able to be mounted directly to the t-slots in the extrusion.

For the X direction, I chose a rack-and-pinion drive after learning about the problems ballscrews have with buckling and whip as the length grows. I selected 1:5 planetary gearboxes for the pinion drives, with backlash specified as 15 arcmin or less. I managed to design a tensioning mechanism for the motors using M4 threaded rod and rod-end bearings for the pivot. (I'm not sure how it would work in practice but I can actually build it). With this arrangement, a single step of the motor would correspond to around 0.1mm, and the backlash from the gearbox around 0.07mm (on top of backlash from the rack itself).

The design uses a gantry undercarriage, so the frame is unsupported in the X direction except at the ends, which I anticipate could be a problem. Using a beam deflection calculator, I think it could easily sag 1mm or more in the middle. I'm also not able to incorporate any cross beams in the interior of the frame, although the cutting table (probably 16mm MDF) would serve in this capacity.

I've not properly sized the motors yet, but I think I'm looking at around 1.5 Nm holding torque for each X and Y motor, and 0.9 Nm for Z.

I made the model using Sketchup, and uploaded it for viewing here with a web-based 3D viewer:

https://3dwarehouse.sketchup.com/mod...9-73010da7dea6

I would greatly appreciate commentary and criticism on literally any aspect of this design! I educated myself as best I could, but feel that some aspects would really benefit from working knowledge which I don't have.