I was asked this so I will pass it on. This is used with a BPT VMC (1000, 800,760), and on the TC (Torque Cut) machines.
Try watching a rigid tapping short program in Maintenance page. The F5 key. On the lower right side of the screen you ahve position data for 4 axis, X, Y, Z, and S. This includes position error. I am assuming that you are turning rigid tapping on in your program. You can call up a program and run it from the maintenance page. Run it and watch the position error of the S axis. I can usually tweek it to about a degree of error. This is done in the axis gain parameters. Typically, the set of these has a value of about 250. Change the S gain to be closer to 325. Change it a little bit at a time untill you get the error down to as low as you can. Password is 111257 (if I remember correctly). The parameters are at the top of the page. Commands are at the bottom. I believe 9 is labeled edit parameters. Arrow down untill you find these parameters. Write down the original value. Press enter to see a red block come up for a new value field. Enter new value, esc to get to a screen that asks if you want to save values. Say yes, try the change, repeat untill you get a very small S position value. If rigid tapping is enabled, the spindle will orient before starting to tap. I usually start the tapping a few turns above the part so everything settles in. Keep me posted.