Re: Spheres and cup centers - Pen turning related
Craig Daymon
I usually turn about a 3" sphere. Nice size to hold and to turn.
In an ideal world, one would have perfect cup centers matching the inverse curvature of the spheres they are creating, because that would give you the most contact. I suspect that rarely happens, and turning such a cup is much more challenging than turning a round sphere. (Especially if you are not concerned about an exact size, as is the case with the spheres I turn.)
With my cup centers, I really only worry about a ring of contact on each with the spheres. A conical indentation in the cups would actually provide accurate centering between the cups and that ring of contact for a variety of sized spheres.
Softness of the material is more an issue for me. UHMW turns well, but can be hard and unless the wood/material is nearly as hard or harder, you can get compression marks if you tighten too much. I bought some ureathane to try to turn softer cups, but it doesn't turn or thread all that easy. Plan to make the threaded part (for headstock and for live 3/4"-16tpi center) out of UHMW and then make cups to put on these from the urethane. Haven't done so yet.
