1x http://www.aliexpress.com/item/5Pcs-Lot-A210-6-35x12mm-CNC-Motor-Shaft-Coupler-6-35mm-to-12mm-Flexible-Coupling-OD/533891528.html = $22.85
2x http://www.aliexpress.com/item/2-pcs-SBR20-L-600mm-Linear-Rails-4-pcs-SBR20UU-straight-line-motion-block-for-SFU2005/838174537.html = $168.75
3x http://www.aliexpress.com/item/CNC-Single-One-Axis-TB6600-0-2-5A-Two-Phase-Hybrid-Stepper-Motor-Driver-Controller/842624695.html = $81.84
3x http://www.aliexpress.com/item/4-lead-Nema-23-Stepper-Motor-WT57BYGH627-190N-cm-270-oz-in-76mm-3-0A-CE/561715746.html = $96.45
2x http://www.aliexpress.com/item/Free-Shipping-1sets-SFU1605-Ball-screw-L600mm-Ballscrews-ballnut-for-CNC-XZY-NEW/492900834.html = $100.31
Total: $470.21
This parts list does not yet include a frame, the mechanics for a Z axis, a spindle, or a main board.
Ideally, this combination of ball screw + motor will lead to 2.9 kN of force on the X and Y axes. That's wildly excessive but a system that would Ideally produce 1.2 kn is really not that much cheaper. ($40)
I haven't looked very thoroughly for cheaper drivers yet. I like the 5A ones because they future proof my mill in case I ever want even bigger motors.
Assuming the bars are normal ass normal steel, With a 1.2 kN load on the X axis the Y will bend 30 microns and vice versa. I didn't simulate a Z load yet, but the mechanics of CNC milling makes me think those will be pretty small in comparison, and in any case the geometry of those rails makes them strongest in Z, assuming I mount them with the base plates flat. I'm considering mounting them with the bases at 45 degrees or perhaps 60 so they form a V shape, since that would be a be stiffer in X/Y which I suspect is more important.
For reference, the only reading I could find on "normal" axial loads said 700 N (0.7 kN) is the maximum force used to mill steel with a typical tool.
