Carving rings, or, When near enough isn't good enough

To produce rings, the easiest and most accurate method is to begin with a core bit which is just a mm or two smaller than the required inside diameter of the ring.  You core out the centre/hole and then remove the ring from the piece of stone, either with another core bit a couple of mm over the finished size you want, or with your trim blade.

​Figure 1 shows the rings and a couple of pendants with the cored-out centres, having been cut out of the  stock with my trim blade.

After that you grind out the inside to close to the final diameter, using your Vernier Caliper to make sure you don’t go too far, and then grind down the outside so the ring is neither too thick nor too thin.  The only way to define what is the right thickness is experience, aka trial and error – too thick and they are uncomfortable to wear and too thin and they will break easily.
 
​​When grinding out the inside of a ring, near enough isn’t good enough.  The required internal diameter was 20.68 mm and a steel rule just doesn’t cut the mustard.  I used my Vernier Caliper, which is accurate to 0.2 mm to measure the hole in many directions.

​Figure 2 shows the Vernier Caliper measuring the ID  of the ring after the 240 grit stage - it's 20.50 mm, leaving 0.18 mm to lose with the 600, 1000, 6000 and 50,000 grits.

But how to grind out the centre hole to such an exact size?  Having thought of using mini-expanding drums with spiral diamond bands, and discarded the idea because they were too coarse, and diamond files because they would be too inaccurate (the hole needs to be as close to circular as possible), I finally decided on using a ½” sanding mandrel which had a slit in it for the end of the paper.  This was mounted in my point carver chuck.

Figure 3 shows the set-up of the brass mandrel with 600 grit W&D paper in the Jacobs chuck, with the cooling water dripping on the ring.  I rotated the ring to ensure even abraiding of the stone as far as possible.  Sorry the picture is a bit dark!

​I decided on 600 grit Wet & Dry (W&D) paper which would give me a slow grind as I rotated the rings across the paper.  And of course there was a slow drip of water on the work to stop it getting hot.
 
I had to do some additional filing to remove the harder spots in the rock, then went up through the finer grades of W&D until it was polished.  They came up well.

Figure 4 shows the rings after grinding to 600, with the 1/2" mandrel.  Note the rubber tightener for the W&D (an offcut from the sheet of rubber I use to waterproof my bench) which keeps the W&D paper tightly wound around the tool body.

The sanding mandrels can easily be overlooked when carving but they certainly pull their weight at times like this!