First Build 5 x 12 Steel Frame CNC router

[Scott Damman]

The fact you even considered 5mm pitch screws and Steppers on such large machine shows you need to do much more research.

Sometimes I forget about the speed of the screw also being an important factor. I put together a little table for resolution based on ballscrew, based on my calculations I need to go with a higher pitch or up the gear ratio to keep the speed ~80% max. Upping the ratio of course hurts resolution so Im looking for a 20 pitch screw. Higher pitch also helps keep the ballscrew speed down, but of course hurts resolution per mm.



Thoughts on this?

[Scott Damman]

From my table above I am looking at a 2525 screw for the long axis. If I do a 2:3 ratio I get 37.5 mm travel per one motor revolution. 37.5/10,000 pulses = 0.00375 mm per pulse. This would be the best resolution. Or 0.0001476 inch resolution. This seems plenty good to me but thought I would ask

This reduces the speed of the screw considerably to reach the same machine speed and actually gives me some headroom on the motor max speed as well.

At the moment I am planning on rotating the screw and putting in multiple ballscrew supports. Something like...


If I choose to do rotating ballnut I will need to recalculate the inertia and just make sure everything still matches up. I am leaning the other way as my parts are coming back around $300 usd per side....

Does this sound like a reasonable solution?

[Scott Damman]

From my table above I am looking at a 2525 screw for the long axis. If I do a 2:3 ratio I get 37.5 mm travel per one motor revolution. 37.5/10,000 pulses = 0.00375 mm per pulse. This would be the best resolution. Or 0.0001476 inch resolution. This seems plenty good to me but thought I would ask

This reduces the speed of the screw considerably to reach the same machine speed and actually gives me some headroom on the motor max speed as well.

At the moment I am planning on rotating the screw and putting in multiple ballscrew supports. Something like...


If I choose to do rotating ballnut I will need to recalculate the inertia and just make sure everything still matches up. I am leaning the other way as my parts are coming back around $300 usd per side....

Does this sound like a reasonable solution?

Got my gear ratio backward. I need to redo my calculations...

[Scott Damman]

Alright new table now that I got the gearing correct the ballscrew and motor speeds were correct but I took the ballnut travel per motor rev the wrong way.



So based on these numbers still looking at 25mm lead screw. With a ratio of 2:3

Ballnut travel per motor rev 25*0.66 = 16.5mm
Precision mm = 16.5/10,000 = 0.00165 (is this real? i.e. with a quadrature resolver will I truly get 10,000 counts per motor rev?)
1600 rpm ballscrew speed to get to 40,000mm/min (~1500ipm)
Motor speed of 2424rpm (67% of max motor speed)


So am I looking good here? The precision seems great and the speeds would be very acceptable with some headroom on the motor speed.

Thanks