Cross slide screw

I currently have my carriage off my Viceroy to fit a stepper on the crosslide. Should be up and running today to nearly complete the
installation of my ELS <http://www.autoartisans.com/ELS/
In the process I recognised that the existing screw is a bit worn, not as straight as it should be, and would be better longer for the new setup.
What do others suggest as material for the screw? It will be 450mm long 3/8" diameter for 134mm, 0.7" for 34mm, screw portion 1/2" x 200mm by 2.5mm pitch square thread then 8mm for about 60mm. All approximate dimensions. In my opinion do not need a ballscrew!
One thing that does surprise me a little is that the 2.5mm screw thread is assymetric in that the thread width is 1.3mm and the "gap" is 1.2mm. Considering that the nut has worn greater than the screw this appears to be the wrong way around if anything.
Any helpful tips on machining the square thread (1.75mm deep) to give a good finish on the flanks, or would I be better going Acme as I will need to make a new nut anyway.
Richard
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I have no direct experience but everything I have read indicates that an Acme thread is much easier to cut than a square thread. I imagine that is why Acme threads seem to be used so much more.
Don Young (USA)
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