[ardenum2]

So you don't spin a 60" long 16mm screw at 3000 rpm.

1000 RPM + pitch 20 (3:1 belt reduction at best) vs 3000 RPM + pitch 5 (direct drive).

I might be mistaken but...isn't direct drive max m/min in this case lower?

[davida1234]

1000 RPM + pitch 20 (3:1 belt reduction at best) vs 3000 RPM + pitch 5 (direct drive).

I might be mistaken but...isn't direct drive max m/min in this case lower?

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?

[joeavaerage]

Hi,
my suggestion was that OP consider a 2' x 2' machine for which 16mm diameter screws would be a good fit. At that length they could be spun
at high speeds if so desired. The point being that by making a smaller machine not only does the cost go down bigtime, but the choice of components
like ballscrews go up. At 2' x 2' any reasonable pitch could be made to work whereas if the machine is 4' x 4' then 16mm diameter screws would need to be 20mm
pitch to avoid likely whipping. Such screws are available but a much reduced choice.

My experience is that every time you think 'if I just had that extra 100mm of travel.....'. When you analyse that thought then you realise that 'little bit extra' costs
a fortune. You have to match the size of the machine to your budget. The thought process is to decide 'between the size I want verses the size I need (and can afford)'.

Craig

[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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