[phomann]

What Ger21 is saying is that the direct drive needs to spin the screw at 3000 rpm vs just 1000 rpm for reduction.

How is the direct drive screw rpm lower than the reduction rpm?

They are referring the speed of the axis, not the ballscrew.

It’s because of the ballscrew pitch. Turning a 20mm pitch ballscrew at 1000rpm will move the axis faster than turning a ballscrew at 3000rpm

The maths;
For a 20mm pitch screw, at 1000rpm, it will move the axis at 20m/min.

For a 5mm pitch screw, at 3000rpm, it will move the axis at 15m/min.

So, turning the 20mm pitch screw at 1000rpm moves the axis faster.

The advantage of the coarser ballscrew is that it rotates slower for the same feed rate, compared to a fine pitch ballscrew. This reduces the amount of whipping the screw will experience.

The down side is that the resolution of the axis will be less on a coarse screw, compared to a finer pitch screw.
That said, with the 3:1 reduction on a 20 pitch screw, the resolution will be slightly worse than the direct drive 5mm pitch screw as the 20mm screw is effectively a 1:4 gain over the 5mm pitch screw.
These days with the high count encoders,its not an issue.

I’m not a that is clear, but it is what it is.

Cheers
Peter


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