Excel Chart thumb

There is an Excel table that you can enter the diameter of your sphere, in this case 65 mm, and it will show the distances for the points of a polyhedron. Here you set your compass points 34.1705 mm apart, and plot your points. Starting at one of the endpoints, scribe a circle with the 34.1 mm radius; then anywhere on this circle set a point and not changing your compass, mark out 4 more points 34.1 mm apart, the last should meet up exactly with the first point. Then from every two adjacent points scribe intersecting arcs; these are the next 5 points. From each of these points, a 34.1 mm arc should meet up for your 12th point, exactly opposite the first. This last point should meet up exactly also, or else re-do your drawings. We found that marking the centers points with a little push of the compass point or even a push pin, left a small precise hole that was not sanded away. Then using your compass draw a circle around each dot marking what will be your 12 holes.

With your turned sphere, and 12 points marked out it is time to turn your Chinese Ball.

The real secret is having the right tools and setup. We were able to simply and quickly turn the jigs needed to turn a sphere, but the special tools needed for the Chinese Ball were provided by the school and Jean-Claude and we simply learned to use them.

First you need a sphere jig. Jean-Claude designed a sphere jig actually based on the old Plumier book, made out of plywood which is ideal as it is very stable and won’t move. Each size Chinese ball you want to make must have its own specifically made-for-it sphere jig. So these were designed for 65 mm spheres, and had a perfect 65 mm hemispheric cavity turned in it to hold the sphere without any play, yet easily turned. A flange is then employed with an inside curve matching the sphere which is tightened to hold the sphere in place. Many sphere jigs employ a flange with 4 bolts which are tightened to hold the sphere. Jean-Claude’s sphere jig has a threaded ring that tightened the flange with a simple twist. He threaded the plywood to screw on the lathe spindle, and for the ring to tighten the flange. He said it was no problem to turn threads on the plywood with a threading jig. This method works great to apply pressure evenly to your sphere.

11 Flange thumb 12 Secured Sphere thumb

 

Other special tools: the tool rest is replaced with a little steel table. The tool holder slides on this table perfectly level.

13 Steel Table thumb

 

Special cutters must be made to undercut each inner sphere you want to cut, each with the precise radius and depth of the inner sphere. Jean-Claude supplied cutters for a 5 sphere Chinese Ball, and a 7 sphere Chinese Ball.

14 Cutters thumb

 

The tool holder must allow for precise placement of each cutter each time. Jean-Claude’s tool holder is machined to hold the cutters at the exact place each time. Simply slide the cutter in the slot to the very bottom, tighten the grub screw, and go. Its leading edge is also machined with a 32.5 mm radius so that at the correct depth it rubs the outside of the sphere, allowing precise and repeatable placement of the cutters each time.

15 Tool Holder thumb

 

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Monday the 19th. Thanks for visiting Woodturners Unlimited.