How to turn Perfect Spheres

If you want to turn geometrically perfect spheres, then make or buy a sphere turning jig. However, the commercially available jigs, good as they are, are not cheap. I fabricated a DIY one based on a David Springett design and although it worked well enough, it was slow, and the finish especially on open grain timbers was a little course.

Sphere turning jigs often cut by scraping, and even when fitted with a cup cutter the finish may not be as good as a bevel rubbing cut. We all know what makes a perfect cut.

However, there are other methods which will, with care, result in pretty good spheres. Probably the most common technique is the template system. A hole is cut into thin plywood ideally turned on the lathe to the required diameter, is then cut to a crescent, not quite a semicircle, which is then used as a guide over the revolving blank between centres. The sphere is then shaped until the template is a perfect fit over the top with two small remaining spigots where it is held between the centres. The big drawback with this method is the need to keep having to release the gauge, pick up the template, and offer it to the work piece, over and over again.

Taking a clue from the production turners finger jig, I came up with a similar equivalent for sphere turning. The system comprises a bracket attached to my lathe bench. The bracket has an adjustable arm which allows space for banjo movement. The adjustable arm is fitted to another hinged arm, onto which the template is fixed. It is important that the arm is adjusted so that the template falls exactly in line with the revolving centre. You will notice from the photos that on my lathe the bracket is fitted onto a rail screwed to the lathe bench. You must be a bit creative and engineer the best solution for your particular lathe. The jig template holder is angled to present a clear direct view of the work piece.

Equipment Required:

Jig with template, roughing gouge, parting tool, spindle gouge, Vernier callipers, revolving centre, small drive centre, (mine is a self-made steb type) cup chuck, push rod, abrasives and a suitable blank.

In use the blank is held between centres. The jig template arm must of course fall directly over the blank. The template arm at first is positioned clear of the revolving blank.

The blank is first rough turned and then carefully turned to the correct diameter to suite the template size. The template arm is then lowered to swing over the revolving work and then, gradually, the shaping proceeds.

The spigots at each end of the sphere can be turned really small. Three millimetres or less if you are careful and confident.

At this stage you would normally turn off the spigots with the work piece held between cup chuck and cup centre. However, if the spigots are turned down to as small as you dare, then trimming with a sharp blade and a little hand sanding will suffice.

Full sanding down to 320 or 400 is the next stage. The sphere needs to be held on a good fitting cup chuck. However, sanding and relocating the sphere to cover the whole area can be a bit slow and tedious.

A quick way is to have a suitable push rod through the headstock with which to push and relocate the sphere in the cup chuck without stopping the lathe. Of course, the cup chuck will need a hole for the push rod. (photograph of the Cup-chuck, cut in half for illustration)

WARNING. The push rod left unattended with the lathe running would be dangerous because it is likely to swing out and fly out of control. I keep mine captive, but free to spin without the danger of a spinning missile in the workshop.

To avoid damage marks on the sphere I have glued a piece of cork to the end of the push rod.

I like the simplicity of this method, as it involves being ‘hands on’ with proper tooling techniques. If you are a beginner, try turning half a dozen spheres, by this method and I can guarantee your gouge skills will be improved. This method of sphere turning is probably not the most accurate, but nobody would know unless carefully checked with a micrometer.

Even a geometrically perfect jig-cut sphere will almost certainly distort a little in time. Well, it is wooden isn’t it?

One quick way to check the accuracy of your sphere is to roll it on a flat surface. If it rolls straight it is good. If it rolls with a wobble it is not so good.

You probably have many small off cuts of different species of wood, too small for most projects hiding in your workshop. Try turning a collection of say 30mm spheres.

So, there you have it, a good way to help you endure detention.  Happy turning and stay safe

Mike Haselden

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