I want to change from R&P to ball screw on my gantry

I've got a half built machine. The overall plan was to pretty much clone an Avid style one.
I have used a fairly standard chinese gearbox on the Y had some plates cut and have something that should work. Though not very elegant
Attachment 28641
Picture appears to be upside down don't know why.

I've decided that i'd be better off using a ballscrew on the X from the forum a pitch of 10 seems to be recommended.
The beam is1850mm wide though i'm only concerned with being able to have a working area 1200mm wide but bigger the better.
Reading the forum I think it's best to have a 3:1 ratio belt reduction?
What diameter ballscrew should I be going for?
I've got 12Nm closed loop steppers, probably over the top

Attachment 28642

I have very limited workshop facilities, so can I get this going with as many off the shelf parts as possible?
Any suggestions welcome.



Thanks

This leads me on to my Z axis
Attachment 28647

Which weighs 25Kg add this to the spindle and stepper and I will have 40Kgs on a piece of 160x80 profile.
Will this work or will it twist the gantry due to inertia because of it's height (600mm) plus the stepper on top (180mm)


Perhaps I should be thinking about copying the back plate in aluminium to reduce weight?

I'm hopeful that if I can get the machine to a point where I can 'make chips' I can make some of my own parts and make improvements.

I am totally guilty of having gone shopping before getting a design fully worked out, lesson is being learned...