A more accurate way of centering the radius attachment to the axis of the spindle is to chuck a piece of bar stock or the piece you want to turn into a ball, and check that the tip of the cutter is equidistant from the front and back side of the bar stock when rotating the radius attachment front to back. Once you have found center, zero your cross slide. Now you can install the work piece adjust the tool bit until it just touches the work piece, while the cross slide remains on the zero setting. Back off the cross slide and start cutting the ball. Stop advancing the cross slide when you reach zero, else you will end up with something other than a ball.
If you are attempting to cut a radius (convex or concave) that is not to be centered to the axis of the piece, then center the attachment per above, then move the cross slide in our out using the dials to center the attachment to the axis of the radius. Always zero the cross slide at the axis center. Then last step is to adjust the cutter for the radius you desire. As before, advance the cross slide until you reach the zero or axis center.
I don’t know how good the instructions are for most radius cutting attachments, but if you google Hardinge radius attachment operating manual (which is the attachment I have), you’ll find the process fairly well documented. The process is materially the same for any radius attachment though the adjustment methods differ between the tools.
