Connecting additional heat duct

[Helper]

I want to run additional length of 6" flexible round duct from an existing duct that is soldered to a sheet metal rectangular box. To do this I thought I would cut a 6" diameter circle in the sheet metal and use a prefab 6" collar to connect the new duct to the box.
1. How do I cut a nice, even 6" hole in the sheet metal?
2. The collar and an existing connection have tabs as if one is to intertwine the established box to the new duct. How do I cut these?
In other words, is there a specialized tool that I can purchase? How else can I make the cuts?

[Ed Imeduc]

Just draw the 6"circle on that box. Now with a screw driver tap on the blade of it as you hold it 45o to the sheet metal. slice down across the circle. Then take a wise aviation snips and cut out on the line.


ED ;)