Turning Segment Rings

The picture below is of a typical segment ring, in this case it is a pattern ring, but they all have to go through the next part of the process, which is to make the opposite faces parallel and flat and running true in the lathe. You don't need to worry about the thickness, that should be turned when the segment ring has been glued to the segment stack.

In the picture above I have made the faces parallel and flat with the drum sander. Not essential to do both faces at this stage but I like to, it means the work will be close to running true. The problem is how to hold it to do the turning. Unless it is a very small ring it will not fit into a chuck. At a push you could use Cole Jaws, but the grips are rubber and even if you carefully make the ring it will not be a perfect polygon, so you will not get a good grip with every jaw, and they were intended for the light turning of the base of a bowl.

You can also use double sided carpet tape, and it needs to be the really sticky cloth backed high strength stuff. It should be stuck to a face plate mounted sacrificial wooden disk , turned true and flat. Remove the backing and place the ring, which must also have a flat and true face, centrally on the tape. It works very well, and I did this for some years, but getting the tape off after, and cleaning the sticky mess with White Spirit is a real 'faff'. I eventually realized that there had to be a better way.

So, I made some offset steel clamping washers for the Cole Jaws, shown below.

The washer is eccentric and tapered to an edge about 2 mm thick that is intended to bite into the workpiece.

The washers should all be set with the fat side facing out (for external grip) as in the picture above, with the screws just nipped up, and the chuck jaws gently closed. Those washers that do not touch the work when the jaws are gently nipped up should be rotated until they do, and the locking screws should then be tightened. You can then tighten the jaws, the washers will all bite into the work, and give a very secure grip. You can then turn the face flat and true.

The final stage is to hand sand using a flat sanding board whilst rotating the work to achieve an even finish. I lightly pencil mark the area to be sanded. When all the pencil marks have been sanded off your job is done and you have a suitable surface to glue the next segment ring to. Note that I use 80 grit sandpaper; it removes material quickly and gives a very good key for the adhesive. You do not want a super fine finish.

I have finished rings greater than 10 inches in diameter (I have 11.5 inches clearance), in that case you need to grip on the inside. They should be able to stand a significant tightening load, if they fall apart then your joints were not good enough.