Sphere Turning Jig
Thursday 26 August 2010
Gabor Lacko shows you how to turn the perfect sphere using this clever wooden version of the Philip Steel jig
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 you can see here.
The wooden sphere jig
The Philip Steel jig is simple to use, easy to set up but it is made in an engineering shop. Since not every woodturner is blessed with a toolmaker friend, I decided to make a wooden version of this jig. The wooden version is similar to the metal design, but I have replaced the very specialised cutter with an ordinary metal cutting tool. This is easier to obtain and being a solid bar - 10mm (3/8in) square - fixing, guiding and feeding all become much simpler. When designing the wooden version of the sphere jig, I had to ensure it was very solid and relatively shudder free in use. This was achieved by using two 6mm (1/4in) threaded rods. These go right through, holding the swivelling base, the toolpost and the clamping head together. The two holes in the toolpost are tapped and the post itself is fixed to the base by two counter bored nuts. It is also glued in place. Slightly undoing the two nuts at the top allows the turning tool to be adjusted. The base of the jig is 50mm (2in) wide; this corresponds to the gap in the bed of the Graduate lathe. The hole for the M12 screw, around which the cutting part pivots, must be exactly in the centre of the gap. This will make sure that the sphere we make will have its centre on the axis of the lathe. This is the only critical dimension in the whole design. The M8 screw with the L-shaped handle locks the jig into position.
Take a 250mm (10in) 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 (9/16-3/4in). Draw a line exactly in the middle, right round, and turn the two corners off at 45 degree. You can now start turning the sphere
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
If the jig hits the shaft, make the shaft thinner or cut a couple of ‘V’s in it
When the turning is finished, you can cut the ball away from the shaft
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. 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
One part of the cup chuck is mounted in a chuck, the other one on a revolving centre in the tailstock
Sand the ball in the cup centres changing its orientation repeatedly. You can then apply the finish of your choice
Here is the completed sphere, sanded and finished