Cutting a metal duc t-- what tool to use?

I own most of the tools and would hate to buy a new one for this project but take your best shot at the best way and tool to do this and I will go from there.
I have not actually measured it but I think the main trunk line I need to cut is 12x18, might be 12x24. Standard gauge metal HVAC duct from 40+ years ago. I have reasonable access on all four sides. Well that one side is going to be a b**** but I can do it once if I must. Most likely, I want to return the duct to its original position after the project is completed.. I might choose to step it up to make it easier on the next fat guy. But that part I can figure out.
I can't unzip it at the seams without a lot of digging in 60+ year dried dirt as the next seam is resting on the ground. Hey, I did not do the original install. If I had it would be far different.
So what I want is a clean way to cut the duct without destroying the metal on either side of the cut. As an example: much like cutting the four sides of a wood box with a table saw.
Now to inject a little humor to the thread; the reason I want to do this is because my belly has grown to big and my back to bad to scoot under the tunnel I made years ago in this crawl space. Did I mention I hate tight crawl spaces.
Suggest a diet and die. There isn't time or the willpower to make it happen. I got to get over there to figure out where the water leak is coming from. Go ahead and make your fat guy jokes if you must. Paybacks can be awesome. -:)
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Colbyt
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On 10/10/2010 8:52 PM, Colbyt wrote:

I cut all kinds of metal with an angle grinder, something like:
http://www.lowes.com/pd_36866-79992-PC750AG_4294857497_4294937087_?productId08961
Mine is an old B&D made in the US. Only B&D product I still like. I see cheapies everywhere. The sparks fly, just like on Letterman.
Get a thin disc, remember they don't turn, or you won't have a disk.
I suppose tin snips would work *after* you got a start. Perhaps you could start on a corner with a jig saw, you could cut it all with that too.
Just my non-professional opinion. I don't do duct work. I may be rambling...
Jeff
I have reasonable access on all four sides. Well that one side is

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What about a sawz-all with a fine-toohed/metal cutting blade???
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That's going to be the best answer. I use a metal shear from Harbor Freight, but it sounds like the OP doesn't have much room to work with.
Duct work doesn't necessarily have to be steel. He could use plywood to replace the metal that he takes out. Some duct tape to cut down on leaks.
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Christopher A. Young
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Use a framing square or a plywood chunk as a way to mark the cuts on the duct with a Sharpie.
Use a 4" grinder with a thin metal cutting abrasive blade. Cut the sides & the bottom first. Do a partial cut on the top, both cuts. Use some aluminum tape on the side cuts (make sure to leave a "peel tab"),
Finish the top cuts, peel the side tape, remove the duct section & deburr. The section can replaced with aluminum tape.
cheers Bob
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I'd recommend drive cleats followed by duct tape, but that's just me. Since you are going to have butt joints on all four sides, you might want to support them with little tabs of metal screwed in with zip screws as well.
nate
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I'm also suggesting a dremel (with diamond blade cutting attachment), since that is the best tool for confined space. If it were sitting on your work bench, I'd probably use an angle grinder.
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-snip-

I'd go with bob's method- but use a Dremel and [many] disks instead of the angle grinder. I always feel like I have more control with the Dremel.
Jim
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On Sun, 10 Oct 2010 20:52:53 -0400, "Colbyt"

How about and air nibbler?
Video:
_Harbor Freight Air Nibbler Reveiw and Demo _

http://www.youtube.com/watch?v=HZbBwJm7Ew8

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wrote:

That would be about 3 times the continuous CFM rate of my better compressor but I sure like what I saw. Breaks are not a bad thing while working.
Colbyt
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Looks like the dremel or angle grinder is the clear winner here. The diamond versus the abrasive disc not yet so clear. I am concerned about the sparks in a confined crawl space but more than likely I can drape a tarp to catch them.
I own both tools and will test these on some scrap metal sometime in the next few days.
For whomever said it, if I rejoined these as it was I planned to bridge the seam with some screwed or pop-riveted metal and then tape the whole joint with the better metal tape. Bending new tabs for a seam strip would shorten the duct by two inches so that is only an option if I can offset the last 5 feet.
What I really want to do is raise that last 5 foot of trunk by patching in an offset since there is a good 8-10" between the top of the trunk and the floor joists. There is no valid reason for it resting on the dirt the way it is now. Just a lazy HVAC person. Also they way it is now precludes the possibility of insulating it.
I will update this thread for the benefit of those who may follow it later.
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Colbyt
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Depedning on the gage of the metal, tin shears might be the way to go. Drill a couple holes with a "step bit" and start from the hole. Plywood can be used to make duct, though some of the HVAC guys here would not be happy with me writing that. I'm thinking you could make something that comes out later, next time you need to go make repairs.
Some supply houses have offset pieces, so you can run the trunk up or down a couple inches. I doubt any of the premade pieces would offset 8 inches, though.
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You want to use the diamond cutting Dremel blade, not the regular cutting blades. My reasoning is that the diamond ones don't break where as the regular ones break like crazy. In a confined space, you want the one that isn't going to break.
Specifically, something that looks a lot like this:
http://images.lowes.com/product/converted/080596/080596012274lg.jpg
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HVAC pros and sheet metal journeymen use double cut snips. See Wiss HM41V at Amazon or air powered version 98680 at Harbor Freight, around $35 for either. Cutting duct work with real tools will always work best. There are electric models also from several manufacturers, but for the DIY person a double cut nibbler like the Grizzly G4956 might be easiest to use in a confined space. None of these will make a mess like an angle grinder or Dremel.
Joe
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I want to thank again all those who made a serious reply in this thread. The job is finished and I am back to report the results for the benefit of others who may visit the thread on one of the republication places. I plan to post an article with photos on my site when I have time to write it.
Due to my inexperience with sheet metal, angles, and math I actually managed to cut in place and on a work bench so I tried all the tools except the nibbler at least once.
I found the Dremel using the right angle attachment and abrasive cutting discs to be the tool with the best control. One should use that if 98% accuracy is required. Do expect to use a lot of discs. A standard weight one will cut about 2-3" of duct before it is gone. Using the heavy-weight discs did not improve the length of the cut because of the increased stock removal but they will be required for cutting the 4 layers of metal at the joints.
The Dremel with a diamond bit was worthless. Diamonds are for tile and stone not metal.
The angle grinder with a thin kerf cutting disc was the best for overall speed and disc durability. It is somewhat harder to control and makes a lot of sparks but it is the tool I would use next time if I had the room.
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Colbyt
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What about a jigsaw with a metal blade? I don't do much hvac, only diy. I use my jigsaw for cutting openings in metal ductwork. First mark with the sharpie. The air powered double cutters are the best but I don't do enough to justify one of those. A dremel would just take forever and use a ton of those little disks.
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