Turning a sphere

At the demo by Fred Holder, I attended he turned a sphere and used a set of sphere calipers. They are available online but are pricey, so I did some more searching and found a great article by Al Hockenbery. It involves a bit of math but is a fairly quick and easy way to turn a sphere on the lathe. This article describes and illustrates the process.

Edit: Since writing this article I have purchased a set of the Soren Berger Sphere Calipers.  You can read my post on using them at this link ~ Instructions for Using the Soren Berger Sphere Calipers.

One of the nice things about the spheres is that they show off the end, face, and side grain in a small area. They also look so cool and feel so good to hold!

I find a nice size piece of wood to start with is 3″ x 3″ x 4 1/2″.

You can click on any of the images to see a larger version.

First the math. You first need to turn a cylinder that is round in cross-section. Then you will turn that into an octagon and finally turn that into a 16 sided polygon before blending all the angles into a perfect circle. The pictures below illustrate this and you can see that the sides of the octagon are 0.414 x the diameter of the initial square.

Mount a piece of wood between centers. A 3″ x 3″ x 4 1/2″ piece is a nice size, yielding a sphere about 2 3/4″ diameter which fits in your hand nicely and feels good to hold. Grain orientation is not important, although it will affect your choice of turning tool. I like to use a bowl gouge to rough turn the cylinder.

Mark the center point of the cylinder and find the diameter at that point. Layout that dimension on the cylinder, centering it about your first mark.

Now turn tenons on either side of the cylinder. Turn the tenons down to a diameter of 0.414D. (D = diameter of the cylinder)

Now layout two more lines on the cylinder that are 0.414D apart and are centered around the centerline of the cylinder. Make a straight cut from these lines to the edge of the tenons. You should now have a shape that is an octagon in cross-section.

Mark the center point of each of the newly created flat areas. These two marks, along with the first mark on the centerline of the cylinder, will be on the surface of the sphere.

Turn the tenons down a bit more. Then mark the center points of each area between the lines and the edge of the flat area. These are the red lines in the image below.

Make straight cuts between adjacent red lines to turn the octagon into a 16 sided polygon. Try to make the cuts as straight as you can, you can see in the picture below that one of mine ended up a bit concave.

Using a bowl gouge or a skew held flat like a scraper, blend all the flat parts in, creating the sphere.

In order to finish the sphere and turn off the tenons it needs to be mounted between a set of cup centers. To make the cup center for the headstock side chuck a piece of scrap in a jaw chuck and hollow it out slightly. Make sure that the sphere fits against the edge of the concave are and does not bottom out in the hollow.

Next mount another piece of scrap in the jaw chuck and hollow out a v-shaped hollow. Place a 60-degree live center in the tailstock and check that it fits snugly in the v-shaped hollow. Turn a tenon on the end of the piece.

Turn the scrap piece around and remount it in the jaw chuck using the newly created tenon. Hollow a cup on this end.

Mount the sphere between the two cup centers and carefully turn the tenons off. As you can see I cut the majority of the tenon off with a handsaw.

When sanding the sphere I put a piece of high friction router mat between the cups and the sphere to protect the wood. I also re-orientate the sphere between the cup centers a number of times in order to get the sphere as round as possible and to sand the whole surface.