Re: Manual Lathe to CNC Conversion
That's going to be quite a project. I assume that the machine has babbitt spindle bearings, so is not a high speed spindle.
You would need a ball screw at least as long as the Z travel that you need, plus extra length for mounting, plus the length of the ballnut assembly. They are available but not cheap in the size you need. I would most likely use a 4040 multistart.
Normally the existing carriage would support the ballnut. I think you could turn the ballscrew over that length with no problem, would make things a bit easier to set up. I have a 4040 that is 14 ft long that was on my router and turned it at about 500 RPM for rapids, fixed ballnut.
If you're clever, another way to drive the carriage is using the existing rack & pinion. You could add a pneumatic backlash eliminator and it should work well. Might require linear scales depending on how accurate you want to be.
On a machine that big I question the use of steppers, servos might be a better choice. You have a lot of moving mass on a machine that size.
Your BOM seems OK for a start.
Jim Dawson
Sandy, Oregon, USA