Had a go using motorcalc sheet but does not look right.

[Bob_series1]

With big leadscrews the key thing is screw inertia and acceleration... a big screw operating at high cutting speeds needs high acceleration (short acceleration time) to be able to follow a cutting line.. and that needs torque AT THE REVS IN QUESTION... this is the thing that most people miss... its not bottom end torque you need, its torque at the top end... and as you say, you need to drive them optimally to get that.

Wow, all the hours spend reading the forum and i did not know this. I now understand why you recommended wiring the motors up in a bipolar parallel configuration. Looking at LeeR graph in the sticky, the torque is preserved much better high up the Rev range.

So, since my rails are going to use skate bearings i was going to use the skate bearings to support the lead screw. Turn the end down to 8mm on lathe, place skate bearing on, put thread on the end and lock with a nut+washer. Also, I have a plan at supporting the lead screw at the stepper end.
Wow, playing with the diameter of the screw on the spreadsheet makes a large difference. Will put some more thought into the screw size.

Note also if you are going for dual screws you need to consider how you will drive them... if you use one motor and a timing belt/pulley arrangement you need to add the inertia of two screws together plus that of the pulley arrangement to get the required power... in this case it will be more than twice the single screw arrangement.

So, the plan was to have one motor per screw. I would have the extra motor running on its own driver board and slave it in mach3. From reading a thread on CNCZone it was discussed that if your gantry is rigid enough then the your unlikely to encounter the screws loosing sync.

The build will be fully out of metal so it being rigid is not my worry it is the weight of the gantry.

I can see why people spend money replacing parts on their CNC machine if they do not do the maths.

P.S. That spread sheet is ace.