Quote Originally Posted by Jonathan View Post
There's no point getting 20mm screws if you're not exceeding the critical speed of a 16mm screw - it will only serve to reduce your maximum feedrate and acceleration.

With angular contact bearings on one end and standard bearing on the other end with 16mm the critical speeds will be 7000mm/min and 3000mm/min. With 20mm 8500mm/min and 4000mm/min (approximately).

Remember that with 10mm pitch screws (RM1610 for instance) you will of course get twice the above feedrates. Since it's not that useful to have one axis (except Z) a lot faster than the others I think 10mm pitch, 16mm diameter on Y and 5mm pitch 16mm diameter on X would be a good setup (7000mm/min and 6000mm/min ... assuming your motors have enough torque etc...)
Hello Jonathan, I still have to learn about feedrates, I would be mainly working with wood and some aluminium, what would the advantage be of having 10mm pitch on the Y axis, I'd rather go for "resolution" than speed, I will be getting a System 4C with 3.1Nm motors setup from Roy at DIYCNC

It might be worth doing a rotating ballnut set-up like I did on my 1700mm (2092mm screw) X-axis on your longest axis. Depends how fast you want to go.
I definitley will be using rotating ballnuts on all axis with angular contact bearings on both ends, I've seen many on this forum recomend "Linear Motion Bearings" on eBay, I also thought I'd get a 2.2Kw Spindle from them

A 350mm Z-axis makes things tricky ... see my router (400mm Z)! You will want to either make it shorter or not have gantry sides.
Did you take a look at the P2 model from the link I gave, he uses 15mm thick AlMg4,5Mn Aluminium and the gantry sides are 440mm High!!
I'd like to have some height as I'm contemplating adding a 4th rotary table later, do you have a link to your router?