4' x 4' x 8" Router Build

[Neale]

Decent drives, and I would be happy using 55V AC with those.

I have a home-built CNC router, roughly 1500x750mm cutting area, and the gantry is about 1m long and fairly heavy - especially with a decent spindle motor on it! I drive it with 2 20mm ballscrews. 5mm pitch as 10mm wasn't so common back when I built it, and given that most of my work is smaller but needs a lot of XY movement (i.e. not just long straight cuts) actually that's not too bad a compromise, and I can still get 5m/min out of it which I find acceptable. Personal choice there, and reflects the kind of work I do. Classic "your mileage may vary" situation! However, when you do the sums, you might be surprised to see just how big a component the rotational inertia of the ballscrews contributes to the torque needed to accelerate and decelerate it. Very roughly, on my machine, the ballscrews need about as much torque to get them turning at speed as the gantry itself needs. Surprising result, but this is exactly why some aspects of machine building are not intuitive. 25mm ballscrews would significantly reduce performance purely due to inertia, and why NEMA34 motors often reduce and not improve performance over NEMA23/24 for this size machine.

What you might want to consider when you get to that point is stepper motor current. There are endless arguments about whether the data sheets talk about average, RMS, peak, etc, current. All are different, and usually you don't know which the motor manufacturer is quoting. I would go out on a bit of a limb here and upset the purists by saying that those data sheet values are only a starting point. Basically, the more current you can put through the motor the better it will perform, but the hotter it will get. So my technique is to start with data sheet values, then check the motor casing temperature after the machine has been used for a bit. If you can comfortably hold the motor, then crank the current up a notch. If it's just a bit too hot to hold, turn the current down a bit. Stepper motors typically run hotter than you might expect. Once you have the current about right, then you can finish tuning the machine max acceleration and speed parameters, knowing that you are getting about the best performance available. Other opinions are available...