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Workshop Wednesdays – Sphere Turning Jig


Monday 9 July 2018

Woodturners always want to turn a perfect sphere, and they usually have their own method for doing this. You can turn a sphere by eyeballing; you can use a template or follow one of the many recipes which are based on geometrical approximations. One method is based on making an octagonal cross section. You can do this by dividing the eight-sided shape into a sixteen-sided body, before smoothing that down to a spherical shape. Any high spots on a sphere can be removed, using a piece of steel pipe sharpened as a scraper. For turning balls, I usually use a Philip Steel jig, a photo of which can be seen in the gallery – image 1.

STEP 1

Take a 250mm square section as thick as you want the diameter of the ball to be. Turn the centre perfectly round to the desired diameter. The length of this section is the same as the diameter. The rest of the piece on either side of the part that will become the ball – the shaft – is turned down to 15-20mm. Draw a line exactly in the middle, right round, and turn the two corners off at 45 degrees. You can now start turning the sphere

STEP 2

Take tiny cuts swinging the jig from left to right and back. After a successful cut, move the tool forward by 1mm or so, and repeat until you reach the pencil line

STEP 3

If the jig hits the shaft, make the shaft thinner or cut a couple of 'V's in it

STEP 4

When the turning is finished, you can cut the ball away from the shaft

STEP 5

If you were holding the shaft at the headstock end in a chuck, then you can cut away the tailstock end of the shaft only and turn off the stub, using the wooden jig – see step 4 opposite. If you are turning between centres you turn both stubs away between two cup centres – see step 5. When you turn the lathe on with the stubs still attached to the sphere, you see them almost as if the stubs were part of the sphere all around. This ghost or shadow image is the contour you follow to turn the stubs off

STEP 6

One part of the cup chuck is mounted in a chuck, the other one on a revolving centre in the tailstock

STEP 7

Sand the ball in the cup centres changing its orientation repeatedly. You can then apply the finish of your choice

STEP 8

The sphere is almost complete; it just needs to be sanded and finished