spindle with encoder

vre wrote:

For rigid tapping i need only index signal from spindle shaft
A-B signals can come from elsewhere the encoder is mounted that is not 1:1 ratio correct ?

forum.linuxcnc.org/49-basic-configuratio...oder?start=20#326827

As I said there, you only need the index on the spindle shaft (or 1:1 with the spindle shaft) for multi-pass threading. 

I have also some backlash to geartrain..
This will be problem for rigid tapping when tap goes backwards ?
If yes then must mount also encoder on spindle shaft..

motor is 7.5kw so it has enough power machine is heavy 3tons knee mill
and gears are strong enough..
If something breaks will be the tap.

How many degrees of backlash at the spindle?
 With this and the thread pitch, you could determine
the Z axis error caused by the backlash to see how serious it is.

I guess you could even make a hal backlash component for the
encoder position if need be. (create a deadzone on reversals)

Just wait a second after the motor starts before reading RPM from encoder.

This would not work with rigid tapping (the OP's question)

1% of 360 is 3.6 degree of error, and that will result in broken taps.