Lathe CNC Options
It is something of a minefield and lathe CNC is a lot more complex than other machines. Mills simply need to move the cutter around the work while the lathe has a lot more complex movements to handle.
I've a number of lathes sitting here which have both stepper motors and speed control fitted but as yet none of the CNC options seem to work fully so partial options have a lot to offer! It does depend on just what you need to 'automate', and screw cutting can be one of those.
The first 'problem' is monitoring the position of the chuck and this is the key to most screw cutting type operations. I have a Chester 3in1 which should be a good candidate but I've not needed to do anything with it since it does the jobs it needs to. Thread cutting is all done with taps and dies using the 3in1 to keep parts square and concentric.
The chuck has a single sensor on the chuck which feeds the tacho for a speed display, and this also the base for a number of CNC options such as the Mach3 CNC Turn configuration and the ELS. It only works well if the speed of the chuck is constant, something that my 3in1 is certainly not! As long as the chuck does not slow down during the cutting pass it produces good quality work, and to that end the ELS can take a lot more smaller cuts if needs be. John actually suggests that a fixed speed belt drive setup may be better than a variable speed such as the DB8VS which I'm fairly sure is just free running? Linking the pulse from the position sensor to the speed output is a partial help but if the chuck is slowing down it's always playing catchup ... hence the ideal being a stable speed.
One might think that adding a higher resolution encoder would be a solution, and the two machines I have refurbished, a Compact5 and an Orac, have 90 and 100 pulse disks attached to the headstock. This can be linked to a more accurate speed control setup, and potentially allows one to 'rock' the chuck and see the lead screw move. The problem is that when turning at speed, one gets 100 times more pulses, and it may be that you need 10 times as many more to move the stepper driving the leadscrew. So one needs a very fast processor to handle the data and more than a normal PC can handle safely. The other CNC system that pops up here is LinuxCNC, and this provides the very tools that are needed to read the higher resolution from the 100 pulse disks. In addition it can talk to co-processors that can handle even higher resolution encoders.
Probably too much information? But hopefully it highlights the key points, before going on to trying to fit a rotary table to the cross slide to change the angle of the cutter movement ;)
So just how stable do you think your DB8VS is? Having the speed display I presume you already back off cut depth when the speed slows down? If that is the case, then the ELS will probably work nicely. The simplest option is using a single motor on the leadscrew and manually engaging the cutter, hooking the stepper motor up to the right hand end of the leadscrew with a timing belt to give you a 2 or 3 to one reduction and keep the stepper motor out of the way around the back. Adding the second driver and motor to the cross slide is optional.