Mushroomed hammer head eye injuries

Tom cites "three or four cases" of chips breaking off mushroomed hammer heads and blinding the user. Used to motivate the discarded tool as a teaching artifact.

If you want to see another example of deformation where things are folding over, here is one. [Tom produces a steel bar with a mushroomed head, cut down from a larger piece.] This was a tool that was probably in the MIT machine shop for fifty years. When they were working on the milling machine they just had a big steel bar — it was bigger than this before. I was walking through the lab one day and I saw this thing with the mushroom head. So it's mushroomed, and you can see the deformation, you can see cracks forming. I've had three or four cases where someone was using a hammer that had a mushroomed head and the piece broke off and flew in their eye, and they're blind in an eye now. In my horror, I thought that thing should have been thrown out about forty years before, when it started to mushroom. But it wasn't, so I confiscated it, and then I decided I'd just cut it off and pass it around class and show people deformation. There it is — deformation, and rolling over as something deforms inhomogeneously.

So, heat treatment of steel. This is the contents out of the ASM volume on heat treatment, okay. We have bulk heat treatment, we have surface hardening where we just heat treat the surface. Files are not heated all the way through — they harden the surface, because you want the core to be nice and ductile. If this whole thing was as brittle as that top surface, you could hit it with a hammer and it would shatter, go right in your eye, and you have a lawsuit, okay. In fact, people do that with hammers all the time — they put little shards of hammer in their eye. And it always goes to the eye for something, anyway.