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I've cut a few supply and cold air returns into my main branches and it's been a PITA. Need to cut another 6" hole for a return, it's just standard rectangle ductwork, probably 14" wide x 10" high.

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    What did you use last time, and what made it a PITA? – brhans Mar 7 at 13:56
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    Protip: Cut away, fold away, or cover all shards left behind from your cut. I'm currently healing from a full-depth finger slice on someone else's work, which was hidden behind a duct connector. – isherwood Mar 7 at 14:29
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    Also when cutting only turn your attack angle while squeezing the snips, never go back to make a turn or adjustment. If you do you will end up with very sharp curled up pieces of metal colloquially known as meat hooks. – Joe Fala Mar 7 at 14:50
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    Anyone want to address "why not a Dremel?" which would be my first go-to for something like this... – R.. Mar 8 at 2:01
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    @R.. Wouldn't that be a bit small for a 6" hole? It would definitely help with getting rid of the sharp edges afterwards though. – Mast Mar 8 at 12:38
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You might be able to rent a tool from a rental yard called a "nibbler"; you would start with a drilled hole and then the nibbler takes little semi-circular or rectangular bites out of the sheet metal. There are also inexpensive ones now that attach to a drill motor, one is shown in this video. He has troubles with it because he is trying to use it on corrugated sheet metal, you would not have that issue. https://www.youtube.com/watch?v=Wt4O62qnHsk

  • That things actually pretty cool and looks inexpensive. I have several other styles but I think I may buy this one. +1 – Joe Fala Mar 7 at 18:25
  • 15 Canadian dollars! Sold one click purchase. – Joe Fala Mar 7 at 18:28
  • Air shear are also very similar to a nibbler and inexpensive but usually air driven. ~30 CAD – Franck Mar 7 at 18:40
  • As a tin banger snips are still the best, fastest and most accurate way to cut. But as I've gotten older I've been reaching for the power shears due to golfers/tennis elbow in both my arms. I used to be able to literally crush a walnut in my hand. Now I can barely hold a cup at times. I'm only 37. – Joe Fala Mar 7 at 18:45
  • Ah, an Electric nibbler. The hand powered ones used to cut holes for switches in metal plate, would work, but would take roughly 1 year per hole. Don't get the manual tool for holes that size. – Wayfaring Stranger Mar 8 at 0:18
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STEP ONE

Using a big flat screwdriver orienteded edgewise and a hammer. Strike the screwdriver swiftly and accurately to puncture the duct. enter image description here

STEP TWO

Insert a Red pair of snips into the hole and nibble around turning to the left (Greens are pictured only because that's what was in front of me) in an expanding circle until you get to the line. enter image description here

STEP THREE

Cut along your line all the way to the corner, with force turn and continue while gently pulling up on the scrap. Every 2-3 inches turn in off the line and make a relief cut and go back to the line. Once you get to the next corner you don't need relief cuts anymore. enter image description here

COMPLETE THE CUT Now it's easy, just cut along the line gently pulling up the scrap. If you are left handed, or only have greens feel free to use Greens but reverse the images. Yellows are designed to cut straight but are capable of cutting to the left like Reds

FOR CIRCULAR HOLES

The same process applies just without the relief cuts.

  • The best tool is a plasma cutter. Fast and clean but not many homeowners are going to shell out +1K for this type of cutting tool. – Ed Beal Mar 7 at 17:15
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    harbor fright sells cutters for about half that. probably not the biggest workhorse around, but should be enough for duct work... – dandavis Mar 7 at 18:17
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    Life's too short not to own a plasma cutter. – Sidney Mar 7 at 20:41
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    What's the difference between red, yellow and green snips? How would this translate internationally? – Mast Mar 8 at 12:40
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    @Mast A yellow cutter cuts in a straight line, a red one is designed for cutting toward the left, and a green one cuts toward the right. In this case, in the cutting direction shown in the pictures, a "red" - i.e. left curved - snip will leave a clean right edge and a messy left edge, but the left edge doesn't matter because you are removing that piece. The reason not to use a "yellow" - i.e. straight - snip here is because you don't have room to work with it because the metal doesn't move out of the way of the body of the tool.... – Moshe Katz Mar 13 at 17:28

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