Sharpening and Edges
Simply put, sharpening is the process of grinding a groove along the bottom of a skate blade.
It is this groove or "hollow" that creates the edge which allows us to skate.
It is important to accurately grind the hollow so that it is centered along the length of the blade and the edges are the same height. An edge height difference of as little as .002" (2 thousandths of an inch) can negatively effect the way the blade feels on the ice. A flat item (like a coin) laid across the edges will form a right angle with the side of the blade.
It is equally important to only grind as much metal as necessary to achieve sharp edges while at the same time not changing the profile or curvature of the blade. A profile that has been changed or damaged can not usually be repaired. A profile can be damaged by one bad sharpening.
A poor sharpening, one which creates uneven edges, can require the removal of a large amount of metal to get the edge height even again. This can be as much metal as two or more sharpenings.
The sharp edges we skate on are created by the junction of the inner part of the hollow or groove and the flat straight sides of the blade. If this flat side is damaged by rust, a sharp smooth edge can no longer be achieved. This damage can not be fixed.
The hollow or ROH (radius of hollow) is expressed as fracitions of an inch. The most common are 5/8", 1/2", 7/16". These are roughly the diameter as a half dollar, quarter, and nickel coins.