Having been raised with a full machine shop in the basement of the home I grew up in, we were told basic safety items that were necessary in order to prevent injuries.
At about age 6,my father told my brother and I if we ever dropped one of his grinding wheels, to show it to him because it could blow up.
One day, while sweeping up the shop and placing the chips in the barrels in the driveway, we found a spent grinding wheel that had been discarded.
These weren't grinding wheels used on a bench grinder either, they were used on either cylindrical or surface grinding machines.
The entire rest of the afternoon was spent first gently tossing it into the air to see it fall to the ground, then whipping it up against the brick wall of the house for hours on end.
After an entire afternoon was spent trying to get it to blow up, the conclusion we drew was that Dad was an idiot because dropping one, much less than blasting one up against a brick wall didn't produce the desired effect.
Heck, we watched cartoons too, and knew that an explosion would just make the recipient turn black and the others would laugh at him.
Dad never explained that after one got cracked and was mounted on the machine spindle, centrifugal force would make one fly apart violently, hence the explosion he told us about.
After mounting one, it is necessary to let it run at full speed to one full minute while staying out of a direct line of it in case it should fail catastrophically.
A few years later, he showed us how to hold one up by the center bore and ping it with your finger so you could listen to it ring. If it rang, it wasn't cracked and could be determined that it was safe to mount. If it was cracked, it won't ring, it will only make a dull thud.
That's how you know it is time to put it in the trash
