CNC milling a soapstone ring

After some experiments with the Snapmaker 2.0 CNC module using wood, I decided it would be fun to try with soapstone instead and I’m quite happy with the result.

Here are some details about how I achieved the result.

The 3D design for the ring has been done in Blender using two circles with a Screw modifier and a Curve modifier applied to them.

After sawing off a slice of soapstone from a bigger piece, I prepared a spoil board and fixed the flatter side of the soapstone onto it with double-sided tape.
With the spoil board securely fastened to the Snapmaker bed, the X/Y starting position of all calculated toolpaths/g-codes would stay the same for the rest of the project (in the lower left corner of the rectangle), and I only had to adjust the Z starting position after each step.

I used this image, transformed to the proper dimensions of the rectangle, as reference to create an outline of the stone, using 8cm x 7cm as my stock material size (depth didn’t matter just yet), and generated a tool path to mill down the thickness by a small amount.
By varying the Z-starting position this allowed me to experiment with what depth of material I would be able to mill away in one go without damaging the CNC bits. I ended up using a maximum depth of 0.4mm for the remaining toolpaths. I probably could have gone deeper but wanted to stay on the safe side and I wasn’t in a hurry to finish the project.
After several steps of flattening (using a 3mm flat end mill) I had a perfectly flat surface.

With both sides flat enough (if the other side would have been too uneven, I could have repeat above process for this side as well), I measured the thickness of the remaining stone (15.1mm for me), flipped over and reattached the stone with double-sided tape, and took another photo as reference to prepare and calculate all the necessary tool paths on both sides, now using the final stock size of 80mm x 70mm x 15.1mm.

On the top side I used one roughing pass (using a 3 mm flat end mill), one finishing pass (using a 1.5 mm flat end mill, since it was the smallest bit I had, and I haven’t received my small tapered ball nose bits which would have given a much better result…), and created two small holes (using the same 1.5 mm end mill).

After milling a mirrored version of those small holes into the spoil board, I was able to use small pieces of wooden skewers to fix the stone in exactly the right position for milling the other side.

The other side also needed a roughing and a finishing pass, and then a final pass with the 3 mm end mill to remove the remaining material in the center, and cutting out the ring, leaving small taps so the ring wouldn’t fall down during the final milling.

Afterwards I cut out the ring with a fretsaw, cleaned it up using a Dremel and sandpaper, and applied some mineral oil.

Since this worked out so nicely I’m already planning my next bigger soapstone CNC project, but I will have to wait for some CNC bits (especially the tapered ball nose bits I mentioned above) to be delivered first.