John - Flattening Arkansas Stones
(Truck)John, I saw your question about flattening arkansas stones. I've the same issue with new stones - they are often almost flat, but when you get to them with a tool, almost is really seemingly not very close with weird errors like twist or pillow shapes with corners not equally low all the way around.
To flatten those stones, put a cheap 80 grit (red alumina) belt on a belt sander and drag the stone surface where it's high across the idler - it's harsh hard contact, but the idler will work creating a vector on the stone.
You want to pull the stone across the idler on a diagonal and then flip to the opposite diagonal. So when you look at the stone, instead of being straight up and down parallel to the sander's length, you lean it left or right 30 degrees or whatever. And then pull the stone up and down on the idler - but only where the stone is high, of course - linear or in the direction of the belt movement. Same idea as pushing a plane skewed.
This is especially unhealthful and cannot be done without a mask (silicosis!!...you're not likely to get that from one go, but it's the stuff of silicosis). done outside is better, but still with a mask. it will smell weird, like bone.
When you are close enough that the idler just isn't accurate, you can use a diamond hone or loose silicon carbide grit and lap the stone the rest of the way. No stone should take more than about 20 minutes total.
Obviously, the alumina is far harder than the novaculite - so this only works for natural arks, and none of the india or other stones.