Rotating Ball nut

[Jonathan]

I tend to put two grubscrews in each pulley, with a small brass cylinder piece under each grubscrew so that they don't mark the shaft. Do them up tight and use threadlock and the difficult bit will be getting them off, not keeping them on. Also your problem with the 12Nm motors is getting enough speed, so adding a 2.12:1 reduction is only going to reduce that speed (although it will improve acceleration). With the 4Nm motors then I'd go for 1:1 if you're mainly cutting wood since you don't need the extra resolution. If you keep the motor mount and ballnut mount as separate parts then it's easy to change the size of the motors, but more difficult to cover the belt.

I've seen this:



Its from a Techno LC router (google search), which is a similar kind of design I was looking at. I know you can't see any detail of the ballnut and bearings, but there is a quite a big ass pulley... This particular machine is servo driven, I wonder if that's why this design works for them?

It looks like what's happened there is they've designed it with a large pulley, realised that that requires more power to drive it and just stuck a reasonably large servo motor on to compensate. Not the greatest design. They've also used one ballscew on the X-axis, which is pretty poor as it will cause the gantry to deflect/rack when the tool is near the ends. Lets look at how they justify it on their website:
'The placement of the ball screw in the center of the axis of travel eliminates the possibility of racking.'
So they've tried to justify it by saying it avoids a problem which doesn't exist with two motors done properly, and in fact they've made racking become a potential problem by using one. They've also used THK SR bearings and made out that they are excessive, when in fact they have lower load ratings in the direction they're using them and in the first pages of the datasheet is says you're better off using different bearings with equal load ratings if they're mounted upside down, as is the case here. You can do better than this company...