Initially I planned on just a a DRO for the cross-slide as that's where I believe I need it most. As I got to thinking more about it, I figured out a way to also include a carriage position DRO using just the one sensor. The sensor in this case is a draw wire optical encoder; think of it as a precision tape measure.
If I do this, the sensor will indicate the total wire draw from BOTH the carriage (Z) and the cross-slide (X). This seemed like it might be a bad idea at first but, I found myself being more interested in relative movement rather than absolute movement, and in no case have I felt the need to accurately track more than one axis at a time. So lets say I just finished a turning operation and want to advance the tool in by 0.002", I don't really care what the absolute reading of the DRO is, I just want it to tell me when I'm 0.002" more (or less depending on direction) from where I am now. Similarly, if I'm using a boring bar and want to plunge it 3/4", it doesn't matter what the current reading is on the DRO, just tell me when it's 3/4" more than the current reading, from there I can set a carriage stop to finish the hole.
Since the encoder is connected to a computer with a monitor, a value would be prominently displayed which indicates the current movement from the last waypoint, no need to play the differential algebra game.
There are at least a couple reasons I'd do it this way. 1) It saves money, each encoder with an interface is about $175. 2) There's not a lot of left over real estate on a 7x10 lathe, keeping the parts count down means less things to get in the way.
So team, shows promise, certain to flop, consult a medical doctor?
