Davek0974,
Link below just for thoughts....it's for the Z on an Atlas mill.
http://www.machsupport.com/forum/index.php/topic,7396.msg127816.html#msg127816One pic shows the Z driven by using a small stepper ( 80 in oz) using the Z fine feed.
That was no good / don't do as you end up with backlash. Similar to what Hood posted is a 500 inoz 23 stepper mounted to a plate which is attached to the mill head. There is a preload bearing in the mounting block and have no backlash.
Acceleration can become important...ie; For 3d milling you can a have a lot of small
moves in combination moves.
Size wise, the experience from reliable users for stepppers / motors which provide satisfactory service for "what you want to do" for a "very" similar sized machine is good advice.
If one were to "design" what they want, they would define desired preformance,calc's would be done, consider experience of others, etc. That will provide confidence in what will be done. It gets rid of the subjectivity in doing the conversion.
Today there are manny programs to assist in doing design as compared to years past
where all was done manualy.
Just some thoughts....
RICH