Wobblycogs CNC Mk1

[Jonathan]

I think I'm just starting to see how the multitude of factors play off against each other.

Which is precisely why I linked you to the spreadsheet instead of just telling you the answer , but it seems that's what you're after so...

So, to all those who know a lot more about this than I do, what would you do?

I'd use RM1610 and rotate the ballnut instead of the screw. From using the spreadsheet it should be now be obvious why it is such a big advantage to not rotate the screw.
The X-axis on my machine is 1700mm travel, with RM2510-2094mm ballscrews and rotating nuts. The highest it seems reliable at with 3Nm motors is 12m/min (so about the same as Jazz's but with substantially smaller motors), although it will intermittently run at 15m/min still with no signs of vibration/whipping problems. Tenson here has RM1610-1500mm and rotating nuts, so you may wish to ask him how it is performing as that should be a good indicator of the feedrate that configuration would get with 1800mm.

If sticking to a conventional setup it's not so clear cut. RM2510 and Nema34 is probably the safer option, but your table suggests there's a significant chance that the RM1610 and 3Nm motors would outperform it quite significantly you could go for the latter as it is the cheapest option and if whipping is a problem change to a rotating nut at a later date and keep the same motors/drivers.

[JAZZCNC]

The highest it seems reliable at with 3Nm motors is 12m/min (so about the same as Jazz's but with substantially smaller motors),

Not much difference really because I have single motor turning 2 screws and you have 2 motors, plus 34's are stronger and will hold more torque higher up the curve.!!

Don't see the point of going to all the expense and trouble of implementing a rotating nut design and all it's complexity's when the standard setup will still allow everything he needs anyway.? The extra expense of components and having someone (Namely You) machine the rotating nut assembly cancels out extra cost of 34's etc for what is affectively wasted speed has he'll never cut at those speeds and Rapids are only usefull in quite limited circumstances.!

If speed is required then yes I'd take the rotating nut assembly route has it clearly will allow higher feeds but if cutting below 7mtr/min which most materials are then why go to all the trouble.??

[Jonathan]

If speed is required then yes I'd take the rotating nut assembly route has it clearly will allow higher feeds but if cutting below 7mtr/min which most materials are then why go to all the trouble.??

I agree - clearly the only reason to do it is if it works out cheaper or if you especially need the higher speed and (perhaps more importantly) acceleration. Personally I would do it even if the cost is slightly greater since the factor of safety will be much greater, so you will have a more stable system. Perhaps ask Chai for the price separately for two RM1610-1800mm and RM2510-1800mm so we can compare the relative cost more precisely?