Wait a moment the encoder is channel A , channel B and channel I ( index connected to the encoder index input in the TB4 mesa 7i76e card)
To further elaborate..
For a _lathe_ you absolutely need the index to happen exactly once per spindle revolution to allow single-point threading to work.
But, having checked with the mailing list, you can rigid-tap on a mill with an index that is not synchronised to the spindle, just as long as the spindle encoder scaling takes in to consideration the gear ratio _and_ as long as you never try to do more than one rigid-tapping operation in the same hole.
