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Ball cutting jig
Turning a sphere freehand will test your skill, particularly if you need two the same size. If you want more than one or two and don't want to do them freehand, you can use a simple homemade or commercial jig. I make lots of balls in a range of sizes, and for me an effective jig that lets me produce them quickly and accurately is essential. 

A jig is not hard to make. The most basic is simply a clamp to hold a cutting tool, with a pivot to allow the tool to swing through a quarter circle or more, all fixed to the lathe bed.  The cutting tool can be adjusted in the clamp, or there may be a sliding adjustment so the clamp itself moves. But the manufactured jigs I have seen are all a bit flimsy or lack easy adjustments. The cutter usually has to be advanced by pushing it forward by hand and there may not be an adjustable stop to control the finished ball size. 

I made my own heavy duty jig. It does an extremely good job cutting balls and can also make hemispherical hollows. It was easy to make, but would not have been so easy if it was not based on some scrap parts. The key component is a machine slide such as top slide from a metal turning lathe. These can sometimes be obtained from dealers in old tools, or from Ebay. Mine has a rack and pinion lever feed, which is ideal because it is fast to operate, but the more usual screw feed would be fine. The rest of the jig is made of bits of scrap steel. The construction details depend on what machine slide you have so I can't give more than guidelines. Assembly would probably only involve drilling, tapping and bolting together. 

If you would like to make a jig like this but have never done any work with mild steel before, don't worry. Think of the steel as like very hard wood. You can saw it with a hacksaw, or if it is too thick, drill a line of holes close together and saw through them. You can drill holes with a twist bit in a drill press (it is difficult to drill freehand, even with a power drill). You can cut screw threads in it with a tap by screwing it into the correct size hole.

The jig (see outline plan below) consists of the following components, starting at the bottom:
  • A clamping plate that will hold the jig to the lathe bed, allowing it to slide and lock
  • A locating block that fits the lathe bed so the jig can slide along parallel to the bed without sideways movement. This makes locating the pivot point with reference to the ball centre much easier as you can just slide the jig along to the right position. You have to make sure the centre of rotation of the jig passes directly and accurately under the turning axis of the lathe. The short bed on my Graduate lathe has a slot that is offset a little, so the jig also had to be offset to match. The locating block is not load bearing, it just positions the jig before locking down, so hard wood would do.
  • A pivot block sitting on the lathe bed. The pivot block should be thin but rigid. The thinner it is, the more clearance space you have and the larger the ball you can make. A steel disc about 10 mm thick and 100 mm across would be ideal. Mine is much thicker, because that is the scrap that I had at the time. 
  • A slide support platform fixed above the pivot block. It needs to be rigid. The mating surface between the pivot block and the underside of the slide support platform is the pivoting plane, so a smooth clean surface to each is needed. The ideal for the support platform would be a strip of steel of width to suit the slide, and 12 mm thick. If the strip is too short it limits the ball size unnecessarily. I rounded the end of the support platform but that may not be necessary.
  • A pivot pin. The clamping plate, guide block, pivot block and slide platform are all locked together with the pivot pin. The ideal pin would be a 12 mm or larger steel rod, threaded where necessary. It must be firmly anchored in the pivot block, for example by being threaded into a tapped hole in the pivot block and securing with thread lock compound. This provides two separate clamping actions - one from below to hold the jig onto the lathe bed and another from above to adjust the swivel tension. The pin passes down through clearance holes in the guide block and clamping block. This allows a nut and washer to pull the pivot block down onto the lathe bed, positioned by the guide block and held by the clamping block. Above the pivot block, the pin passes through a snug-fitting clearance hole in the slide platform. Above that are a washer and two nuts that can be locked against each other so the pivot tension can be set. A Nyloc nut would be better. The pin should preferably be unthreaded where it passes through the slide platform as this is a bearing surface for the pivoting movement, but even a screw thread should give adequate guidance, assuming the hole is a snug fit without play. The top of the pin needs to have its centre marked as this is the point that must be exactly beneath the centre of the ball when in use. I turned a point on mine. If the jig is not lined up properly, the ball it makes will not be perfectly spherical.  
  • A machine slide, bolted to the slide support. 
  • A tool holder carried by the moving slide. The tool holder on mine came from an old machine, but a similar one could be made just by bolting three bits of steel together. It stands on a raising block to bring the tool to the lathe centre height. Having it a little low allows different cutters to be used with shims. If the cutting edge is not on centre height, the ball will not be spherical.
  • An adjustable stop for the slide movement. How this is achieved will depend entirely on what slide and other parts you have. I used a bit of threaded rod with two adjustable lock nuts, passing through a clearance hole in a bit of metal attached to the slide. The cut stops when the metal contacts the adjusting nuts. By counting movement of the nut flats, I can adjust the cutting depth and size of the ball very accurately.

The locknuts act as a stop to the slide travel

The maximum size of ball you can make depends on the clearance above the top of the swivel point and the travel of the slide. Don't forget that if making a ball from a cylindrical blank, and even more so if from a square blank, the clearance needed is more than the radius of the ball. And you have to allow for projection of the cutter.

The raised tool holder allows a larger ball to be made

To use the jig, the swivel axis is positioned directly beneath what will be the centre of the ball. I normally make hemispheres with the blank held on a screw chuck, so locate the swivel pin beneath the face of the chuck. I glue the hemispheres together to make perfect spheres. The cutting bit, which could be an ordinary scraper, is advanced using the slide, pulled round to make a cut, then advanced again for the next cut. When set up, I can quickly make lots of identical half balls. 

A small half ball quickly made in mdf

Jigs of this kind cannot make a whole ball in one pass as the blank has to be held in the lathe and the cutter cannot reach all the surface. It could make most of the ball for finishing off either by hand or by moving the ball in a chuck so the uncut parts can be reached with the cutter. If the blank is held in a chuck the cutter could do 75% or more of the circumference in one pass, leaving just a single chucking spigot. If the blank is held between centres, there will be two smaller spigots left. 

A traditional way of hand turning spheres is to put the roughed out ball between two female cone centres with the spigots projecting sideways for turning off. Using this method, the ball with one or two spigots remaining could be finished using the jig, leaving a perfect ball if set up correctly.

Plan of jig

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