Fergus is correct in what he said ,this was originally designed for the myford size lathe .i have had mine for about 40 years .the bolt that is used as the centre pivot should have a hole in it.a 3/16 rod with a point on is inserted in, then the cross slide y axcess is advanced until the point is on the centre of the bar that is being used to make the ball .the apron x axcess is then advanced so the pointer is half the ball dia , lock off x and y ,take out the pointer ,the feed is by the micro adjuster on the top ,for the finishing cut ,make sure that the nut that secures the tool bit in the holder is nipped up ,
Hope this helps ,I might have forgot a few things ,as it is a long time since I have used mine .
Regards Peter