Thanks for your comments guys. I am in contact with Harald.
If I CNC the 4L, it will be a reversable change (as it is on the 7x12). I am not sure if that is the path I take with the 4L though. I just talked to my dad and he has a spare set of change gears he was given a few years ago. No idea what machine they are off, but if the size is near enough, and bores are small enough to be bored out, then I might end up adapting that set (if he brings them over from NZ in summer).
The 4L is a very simple machine. This makes it ideal for CNC, as all the wonderful mechanically tricky features which make something like a 5LZ or 1EE so cool, becomme redundant once a computor spins the wheels.
Remove the 4L lead screw, and install a ball screw mounted on bearing blocks and a nut carrier which pick up the existing mount holes.
Same for the the cross slide.
Top slide is replaced by a tool riser block.
Two inductive sensor, one the spindle cog for rpm, and another on a 1/rev feature for indexing.
The T-Slot for sliding stops on the bed will be excellent for mounting limit and home switches.
The motor is a standard, industrial, flanged motor. It is only .75/.9k. I am not sure if the place I bought has 3ph in the cellar. If not I will probably need a 240 3PH motor, and will run it off a 1PH -> 3PH VFD like I have my current lathe.
Pekka, 1/rev indexing worked fine under T-CNC. Since I swapped to LinuxCNC (EMC2) I only have the index signal set up to provide RPM indication and havent done any threading.
On a CNC lathe, you can cut the thread with dozens of light cuts, and at much higher spindle speeds than you would comfortably do manually, so the spindle doesn't really load up or slow down enough to make a significant effect on thread accuracy (obvious it depends on the desired accuracy). I have cut two M40x1.5 (er32 collet nut) threads on 7x12. It was just on the wrong side of the machines capacity, and turned out ugly on one, and worked on the other.
I think you only need a Px oiling system if you use the mchine for hours each day. I just squirt oil on the ways and screws regularly (like every time I use it). Given the cost of 7x replacement parts, it is probably easier just to replacing things if they start getting worn.