How do you get curved edge drilled holes?

I have seen the holes drilled into frames and the bomber seats that have a nice curved edge. how do you get these? They are two different types of holes, one in steel and the bomber seat is softer metal so I'm guessing a die punch or a drill but not sure. Let me know Thanks

 

you mean somthing like this
http://mittlerbros.com/aircraft-punch.php

 

Punch & Dimple Dies

 

There are more expensive ways to do it but heres a simple one.http://www.jalopyjournal.com/forum/showthread.php?t=463343&highlight=dimple+dies

 

Someone did a very good tech thing on that within the last year. It was done with simple hand tools. Search around..I know it's out there.

 

I will have my eye out now, when I'm looking through stuff like that. Good idea, thanks!

 

Metal_man_sweden has a good video on Youtube on that subject. The poor man's version.