Here is a fixture I have built and used successfully to drill & Tap a .500 inch diameter Brass Ball used in Fly ball Governor construction. I ball end milled a cavity in a piece of 1" CRS and drilled & Tapped 4 holding screws at 90 degrees. You need to ball mill the cavity about .030 to .050 inch deeper than the mid point on the ball to successfully hold it in the fixture. The brass screws have a portion of the tip turned down to minor diameter and smoothed to grip the ball without scaring or damage. Locking nuts are provided to maintain the pressure on the ball during turning. Don't need a lots of tightening to hold the ball in place for drill and tapping, of course you can over do here and scar the brass with a brass screw if excess pressure is applied. Tap was a 5-40 tapped about .400 deep. Balls came out without a scratch!