Cup Centers for Sphere Turning
Dennis in Southern Oregon
Craig Damon asked if I used cup centers to finish off spheres turned using my “Simple Aid” technique. I answered that I have had trouble getting the rough sphere to center reliably in cup centers. It occurred to me later that maybe the sixteen-facet approximation produced by the second stage of the technique inherently is a bad fit to a circular cup. As an experiment, I turned a baseball sized sphere through Stages 1 and 2 and then marked up and turned the central half of the sphere for Stage 3. Voila! The sphere then fit rock solid and squarely in a pair of cup centers. Unfortunately, I still have problems seeing and tracking the fuzzy double image so I resort to using what John K. Jordan refers to as a “tube gouge”, only mine are made by grinding off the teeth of a hole saw and replacing them with an outside bevel as shown below.
By the way, I also recently discovered that marking up for Stages 3 and higher of the “Simple Aid” technique is very easy—you don’t even need my graph for modest size spheres. Beginning with Stage 3, the distance A from an existing corner to the cut line is almost exactly one-fourth the width of the current facet. Therefore, you can estimate by eye the midpoint of the facet and then the midpoint of each of the halves, and you have the location of the cut lines in about five seconds. I also discovered that when making these short straight cuts across the corners, a flat scraper works very well.
