I intend to design and build a vertical (or near vertical) 4′ x 8′ CNC router that is sufficiently beefy to mill aluminium or take deep and wide cuts in wood.
I currently have a 2′ x 4′ flat bed router but my workshop is too small to go 4′ x 8′ flat bed, hence the vertical design.
I am greatly inspired by this build by Dean (JazzCNC over on mycncuk forums) but I hope to make it a little more solid and also faster if I can.
So to get started I have chosen my spindle and drive motors.
Spindle: I intend to use a 3KW ATC water cooled spindle. Going ATC is hugely important for me. I have a 2.2Kw spindle now and I never use all it’s power so I feel confident the 3KW will be enough even with a more ridged machine.
Drive motors: I intend to use 3 -phase closed loop Nema 34 steppers with either 9nM or 12nM torque. Dual on the moving gantry axis.
I did consider AC servos but advice to me was that their tuning can be very sensitive and easily get upset by backlash or resonance in indirect coupling like belts. I plan to use (admittedly short) belts to couple motors to ball screws and rotating ball nuts. The closed loop steppers are more plug and play and will easily provide the speed and acceleration I need while leaving more money for things like spindle pneumatics and vacuum table. Plus 3-phase motors are pretty smooth and quick.
I’ve started by calculating inertia and acceleration for the dual 9Nm drive for the moving gantry via rotating ball nuts. Each motor has a rotor inertia of 3Kg so 6Kg total (I am ignoring rotating ball nut inertia). I intend to use 2525 ball screws (25mm pitch) which means a gantry mass of 40Kg would provide an equal split of inertia between the drive system and the moving mass. I see no harm in pushing the moving mass inertia slightly above the drive system inertia, so I think this gives me a target total moving gantry weight inc. Z axis of about 60Kg-70Kg. The spindle is 14Kg so I want to aim at a gantry beam mass of about 30Kg-40Kg.
I have made some static stress simulations for 1.5m gantry beams to see which shapes and materials give the least deflection.
Out of the ones below it seems that ali box section tube with additional wall thickness added by ali plate epoxy bonded to the main beam gives the best results. Damping as well as stiffness is also desirable so I am keen on the constrained layer of epoxy adding damping. Ali tube with epoxy granite fill was my favourite before trying that.
So now I have my gantry beam design I will start designing the X and Z axis.
I quite like the idea of using HGH35 rails with 2525 ball screw for the X axis at least because they are large enough to naturally give clearance for the ball nut. Is there any downside to using larger rails like this rather than say HGH25, other than cost? More friction?
I also want to ask, will it be a bottleneck to use slimmer rails on the Z axis than the X axis? They will be shorter rails but that won’t change how strong the block grabs the rail. So I think if I use HGH35 on the X axis I should use the same on the Z axis. Correct?