test ring method for checking sphere
Response To:
Re: Some aids and cheats ()

John K Jordan
>>>The simplest approach I don't understand- Mark StLeger. If too complicated to explain by typing can I call, or you call?

A photo, which I don't have, or a pencil sketch, which I could make if needed, would be best, otherwise we could talk on the phone. When I get time to turn a test ring I'll post a photo but it might not be for a few weeks.

A planar slice through any part of a sphere will be a 2D circle so a perfect 2D circle will rest perfectly anywhere on the 3D surface of a sphere.

The test ring is made with a bevel on the OD of one end such that the working end is narrowed down to a thin line as it near the ID, but left "blunt" enough to not wear quickly. The ring is just a way to hold a 2D circle to check the 3D sphere. The edge made by the bevel is nearly a circular knife edge in cross section, like the bevel on a chisel. The entire edge needs to be in one plane and perpendicular to the axis of the ring. If not perpendicular it will define an ellipse instead of a circle. The edge lets you see the contact between the ring and the sphere.

Mark said he turns the test rings from corian. PVC will work but it will soften if held against a spinning surface for checking. Not a problem if you turn the lathe off every time.

If the sphere is perfect, the narrow edge of the ring will sit nicely anywhere on the surface, in perfect contact with the sphere all the way around the circle. If the sphere is not yet perfect, when placed on the sphere it will contact in two or more points and you can see light under the edge or it will rock a little depending on the shape. By looking at how it sits you can decide where to take off a little more wood. When you slide it around, the sphere is perfect if the narrow edge of the test ring touches all the way around regardless of where you put it.

This method should avoid the dreaded parabola-shaped bead usually made by beginners.

In practice, of course, a sphere on a leg or baluster only has to be close to look right. Since the size is critical, I would turn the cylinder to diameter and length then draw the pencil line in the center of the space before making the first cut. When done, the line should still be visible until sanded. At least that's what I'd do.

Also, I might even round the blank then turn the sphere first before spending time on the other details in case the sphere didn't go well, especially if there was a lot of other detail.


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