I've had this idea in my head for a couple weeks now. Tonight I had to give it a try. I started by turning a sphere. The blank was cut slightly longer than the width (3 5/8" long, 3 1/4" wide) and mounted between centers.
The blank was turned to a cylinder and layout lines were made. The important line is the one in the middle, the others were no measured, just drawn on as a guide.
The corners are knocked off back to the center line. This is where the bulk of the material will be removed. Almost no material will be removed at the center line as that is the largest diameter of the sphere.
Each side is then turned to a half sphere shape. I prefer to do the left hand side first because it's my weak side and I can better match the curve on the right hand side.
A couple more cuts and the basic sphere shape has been formed.
The blank is then rotated 90 degrees and put between wooden drive centers. If you look at this photo closely you can see the "shadow" of the nubs left by the drive center on the original axis and the rest of the wood to be removed to make this a perfect sphere.
The sphere is repeatedly repositioned on 3 axis (X, Y, and Z) so the entire surface can be trued. Here you can see that the sphere is on the Z axis. The X axis is noted by the single line on the top. The Y axis is the double line on the near side. The blank is turned on eash axis about 3 times, each time smaller and smaller cuts are taken. If a cut is too deep the entire surface will have to be turned down to that new depth.
Once turning is complete the entire surface is sanded with each grit. Once per axis per grit.
Next the sphere is moved off azis and a groove is turned. The was repeated after rotating the sphere 90 degrees.
There are some flaws. Chipout on some of the edges where the wood was unsupported during the cut and a couple of areas where the soft wood was crushed by the wooden drive centers.
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