A Czech machinist friend of mine was forced to work for the nazis, he was in a ball bearing factory he used to take a fine file and make a tiny nick in the surf

A Czech machinist friend of mine was forced to work for the nazis, he was in a ball bearing factory he used to take a fine file and make a tiny nick in the surface of a hardened ball in each box of 1 kilo. He dared not make more. He figured each engine had about a 1/3rd chance of getting a bad ball - which would fail and ruin that moving part, perhaps even in the air? These were tiny balls. The larger ones were watched closely. Life was tough, worker barracks,low grade food, work 10-12 hours/day, 7 days/week.