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Heavy duty ball cutting jig

 

Heavy duty ball cutting jig

I often turn spheres, and for me a ball cutting jig is essential. There are some manufactured jigs available, but the ones I have seen are all a bit flimsy or lack easy adjustments. The cutter has to be advanced manually and there may not be an adjustable stop to control the finished ball size. I made my own. It does an extremely good job cutting balls and hemispherical hollows. It would not be easy to make from scratch, The main component is an old machine slide such as a metal turning lathe top slide. These can sometimes be obtained from dealers in old tools, or from Ebay. Mine has a rack and pinion lever feed, which is ideal, but a screw feed would be fine. The rest of the jig can  be made of bits of scrap steel. Obviously the construction details would depend on what machine slide and bits of steel you have so I can't give more than guidelines. Assembly would probably only involve drilling, tapping and bolting together.

 
The ball cutter set up on the lathe. The turning blank goes on the screw chuck. 













 





 Another view of the ball jig. The depth stop is two lock nuts on a threaded rod fixed into the vertical block at the front.




 

The component parts
You need a clamping plate that will hold the jig to the lathe bed, allowing it to slide and lock.

The jig will need 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 and along 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.

The guiding block is fixed to 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, which cuts down the ball diameter. The mating surface between the pivot block and the slide platform is the pivoting plane, so a smooth clean surface to each is needed. A disc is neater, and I rounded the end of the support platform too, but square would work.

A slide platform is fixed above the pivot block. It needs to be rigid as it supports the cutters. The ideal would be a strip of steel 50-100 mm wide, or to suit the slide mechanism, and 12 mm thick. If the strip is too short it limits the ball size unnecessarily.

The clamping plate, guide block, pivot block and slide platform are all locked together with a pivot pin. The ideal pin would be a 12 mm or larger steel rod. It must be firmly anchored in the pivot block, for example by being threaded from above into a tapped hole in the pivot block.  The pin passes down through clearance holes in the guide block and clamping block, then through a retaining washer. This allows a nut below the 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. The pin should preferably be unthreaded where it passes through the slide platform as this is a bearing surface for the pivoting movement, but mine uses simple threaded rod and works OK. 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, the ball will not be perfectly spherical.
 




The top of the pivot pin has two lock nuts to adjust tension and the central point corresponds to the centre of the ball.
The small hole is for oil, and normally plugged. The small screw is just there to close another hole.






The slide itself is bolted to the slide support.

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 just below lathe centre height. Having it a little low allows different cutters to be used with shims. If the cutter is not on centre height, the ball will not be spherical.

An adjustable stop is needed for the slide movement. How this is achieved will depend entirely on what slide and other parts you have. You're on your own for this bit! 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 accurately.

In use, the jig is swivelled through a quarter circle or more depending how it is set up, cutting as it goes, then advanced for the next cut, repeating till the limit stop is reached. The jig is capable of taking a cut heavy enough to rip the blank off the screw chuck if I am not careful.

To use the jig, the swivel axis is positioned directly beneath what will be the centre of the ball. I normally make hemispheres and locate the swivel pin beneath the face of the screw chuck holding the blank. The cutting bit, which could be an ordinary scraper, is advanced using the slide then pulled round to make a cut, then advanced again for the next cut. I use a tungsten carbide cutter bolted to a short steel bar. 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.

I normally make halves and glue them together to make perfect spheres. When set up, I can quickly make lots of identical half balls. Jigs of this kind cannot make a whole ball in one pass as the blank has to be held on its axis and the cutter cannot reach all the surface. If wanted, 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.

 

 
 
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