Quote Originally Posted by irving2008 View Post
Hi and welcome to the forum.

The sizing of the screw depends on three things.. Firstly the speed you plan to rotate it and how its physically mounted - too thin a screw will 'whip' at speed. on an 800mm length of 20mm fixed in bearings both ends the critical speed is 2700rpm, thats 13.5m/min traverse on a 5mm pitch - I dont think that an issue!

Secondly, what is known as Euler buckling... the tendency of the screw to bend under the load (think of it as a column with a weight on the top). Again for a 800mm long, 20mm screw in a fixed bearing at each end held in tension the load is around 7000N (7 tons), again not an issue for a mill of this size (max cutting force probably a tenth of that)

The thrid factor is simply the load rating of the ballscrew and nut... can it transmit enough force through the thread of the screw.. looking at a typical manufacturers catalog (Nook), a 20mm x 5mm nut can handle 10800N dynamic load for 1million rotations or 21000N static thrust. The screw will buckle long before the nut fails!

I'd go with the 20mm...
Thank you for your welcome and your clear and informative answer. Using a 20 mm screw will make my life a little easier.
Thank you once again. Once I get going on this project I will let you know how do