Quote Originally Posted by tintin View Post
i was told by an old machinist that granite inspection plates were made completely flat by rubbing two together. i guess they cancel out each others imperfections. not sure if it was true or not.
Rubbing two stones together will make them complementary: the greater the difference in hardness, the greater the curvature. For really flat, you need three stones and that will create a totally flat face. For what we do, two stones will eradicate 'saddle' and 'twist' and thus get 'pretty flat". Also, by happy accident, the complementary curves will typically be a convex soft stone and a concave hard stone. But, in sharpening the soft stones wear in the middle much faster than the hard ones. So the route TO complimentary stones passes THROUGH "flat". Thus, when they 'stick', you're done.