[Hezz]

Thanks Hezz,

Well I could put an encoder in both locations, but it should not be necessary, even with lots of backlash.

The issue with the motor and encoder separated by the backlash came down to servo tuning and the backlash interfering with it. When the servo drive would try to close the position loop it would not know that the motor shaft was moving until it hit the edge of the backlash and started to read movement on the gear box output shaft encoder. By this time there could be significant motor shaft speed achieved. This would create an oscillation that would be difficult to stabilize.

Now that the encoder is mounted directly on the motor shaft no such oscillation is possible. The backlash can now be dealt with in software easily. I am not using the index motion to cut anything, so as long as I can reasonably accurately position the turret, I don't care how much backlash there is. Once the turret is locked, and it will always be locked before cutting starts, the backlash is no longer an issue.

One of the beautiful things about the Hirth coupling is that the servo does not need to have perfect rotational accuracy on index. The thing can take a few deg. of misalignment and correct it in the clamp up movement. Once clamp has occurred, the index servo is disabled. At this point If the servo thinks it is out of position, because of small errors, it wont even try to correct the error until it is re-enabled at the next tool change request.

Hope that cleared up my thinking on this.

Thanks for the reply. I forgot about the Hirth coupling. And I can see your reasoning makes perfect sense.