Machinist School - How Hard Can It Be?

Measuring hardness is a trade unto itself. At a machinist school, students learn to measure hardness for a couple of reasons - to determine what type of cutting tool to use, to determine if an object is hard enough to perform properly.
For a cutting tool to work properly, it must be harder than the object being cut. If you have butter in a freezer, you could remove it from the freezer. Then, using a pocket knife, you could carve the butter into the shape of a knife. You would have a genuine butter knife. You could use the butter knife to cut and spread soft butter that had been sitting on the kitchen table for a while.
In a similar fashion, hard steel drill bits will cut into a piece of soft steel. To cut a piece of hard steel, one needs a cutting tool that is harder than the hard steel. Aluminum Oxide is harder than hard steel. Aluminum Oxide is a ceramic product often used to cut hard steel.
Sometimes a piece of steel is too hard. When a piece of steel is too hard, it is brittle and easily broken (simply by dropping it on the floor). Other times a piece of steel is too soft. When steel is soft, it is not very wear resistant. A knife made of soft steel would become dull quickly. Springs are medium hardness or above. Springs are hard enough to be wear resistant yet flexible enough to bend. (A spring that is too soft bends and does not "spring" back.)
One way to test hardness is to bend an object. If it fractures, it was hard. If it bends and springs back, it is somewhat hard. If it bends and stays bent, it is relatively soft. (If you bang your head against a piece of soft steel, you will find that soft steel is much harder than your head.)
Maybe it reminds you of "Goldilocks and the Three Bears" ..... "this bed is too hard, this bed is too soft, this bed is just right!" Of course, she wasn't talking about the bed of the lathe.
- And the three bears were not machinists!