Neanderpundit

But there is a sort of a “dot” in the image below. This is the face of a car part immediately after machining.

The “dot” is a clever bit of differential tempering. Outside the dot, the only tool that will touch the surface is Cemented Boron Nitride, or CBN. Basically, the closest thing in hardness to diamond. Inside the dot, you can cut it with a pocketknife. The reason is, after heat treat, holes have to be very accurately drilled and reamed in these spots. Can’t be done prior to heat treat, because the holes would distort. This is the capability of modern manufacturing, and it borders on witchcraft.