9x20 lathe conversion, pointers wanted

[m_c]

Don't what to get into the old debate re Mach v EMC

Why not? ;D

Quite a few of the external motion controllers can manage that (apparently Mach4 and Darwin/PP can manage it, but Mach4 turn isn't commonly available yet), but for the vast majority of users, they won't be able to tell the difference between a thread cut with a single slot sensor and one cut with a quadrature encoder. I only used my old lathe as an example as it worked perfectly well.
My current lathe has a quadrature encoder connected to a KFlop/Kanalog, so it's capable of the Z-axis tracking the spindle perfectly and even reversing back, but it's got no major benefit during normal threading.