When I was a graduate student making something in the machine shop, another user of this (excellent) departmental facility had just spent most of the day settin

When I was a graduate student making something in the machine shop, another user of this (excellent) departmental facility had just spent most of the day setting up his CNC milling job on the oldest, shittiest (and therefore least-time-pressured) 3-axis mill in the building. Whilst I was manually turning something on the lathe about 15 m away, he shut the door and pressed the 'go' button. There was an absolutely almighty BANG, of the 'an explosion has just occurred' variety that made me very glad I wasn't actually cutting metal at that precise point in time. His g-code had an off-by-one error (I think) and it had dutifully picked up a ~20 cm HUGE flycutter, spun it at the speed of of a ~5 mm bull nose cutter, and driven it directly down into a piece of aluminium. The flycutter had broken and effectively welded itself to the aluminium, which had sheared down the middle, spun out, and hit the protective guards that completely encompass the machine. The person driving it (either a postdoc or graduate student) was maybe 40 cm away and saw this ~2 kg piece of metal bounce off a thick reinforced poylcarbonate guard directly at the level of his head. Safety equipment saves lives.