How to center a piece of work on a rotary table?

[Hawkeye]

Once the tooling plate is set up, mark the centre of the arc with a prick punch. I made a couple of centres out of broken endmills with 3/8" shanks, ground to a 60 degree point. With one of these mounted in the mill's collet chuck, you can move the work, loosely clamped to the rotary table and tooling plate, until the point of the centre fits exactly in the punch mark. Then tighten the clamps.

Another advantage of the centre point is that you can press it down into the punch mark and rotate the work piece to get a critical line of the piece aligned with the X or Y axis, as required, then clamp it.