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I wish to cut the duct work without damaging my hands or my wife's kitchen scissors. So what tool should I be using?

(Trying to get the ends squared using a freehand hacksaw does not look easy.)

enter image description here

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up vote 5 down vote accepted
  1. Leather gloves and safety glasses.
  2. Wide rubber band wrapped around the duct at the position you want to cut (a wide band stays in position better than a narrow one). Leave a little extra to allow for smoothing out any irregularities at the end.
  3. Permanent marker to mark the line you'll be cutting along.
  4. Hacksaw to start the cut. With the line marked, it's easier to keep the cut square, but you could also use a pair of tin snips to complete the cut.
  5. File to smooth out the cut end and make it square if you've got any irregularities.
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2  
When cutting metal, always wear leather gloves to protect your hands from cuts. It's also a good idea to wear safety glasses. – Tester101 Sep 1 '11 at 17:36
    
@tester: excellent points, incorporated in my answer. Thanks. – Niall C. Sep 1 '11 at 18:13
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+1 for the tin snips, no need to over engineer this one. – BMitch Sep 1 '11 at 20:02
    
I brought a set of tin snips to do this, but in the end found that a pair of heavy duty kitchen scissors were easier as the aluminium is so thin, the rubber band worked well. – Walker Dec 18 '12 at 18:51
    
Instead of a file, there's an item called a 'deburring tool' which is a handle with a little bit of a hook-like tip on it that can freely spin. You just hold it at a 45deg. angle to the cut edge, and use it to scrape off the sharp bit. Then tip it towards the other sharp edge, and run it down that side, too. Of course, you might still need the file because if you mis-cut, and leave a little spike of metal, the or don't have an otherwise smooth edge, the deburring tool won't quite work. – Joe Feb 22 at 15:21

A note on tin snips, which I suggest for this task: if you have a hard time cutting a straight line, make sure you have the proper style. An offset handle helps keep the material from digging into your knuckles. Look for one that is angled up and possibly to the side. Long cut versions (with longer cutting blades) are good for smooth straight cuts in things like siding.

But most importantly is the color of the handle. Typically, yellow will cut straight, green will cut a curve to the right, and red will cut a curve to the left. For short cuts on thin material, the effect is minimal, but if you want a straight cut around the tube and the metal is somewhat stiff, you'll have a difficult time if you don't have the straight cut version.

For more details, there's the wikipedia article on snips, particularly the compound action section.

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Good answer - I find myself in a situation where I need to shorten existing ductwork (replacing would be a serious chore, pretty sure this was installed before the subfloor above during initial construction) for a new boot, and found my green snips could not cut a straight line around the tube. – Snowman Feb 22 at 4:45
    
@Snowman : just be careful of the red handled snips -- Wiss also makes heavy duty ones that have a red handle. If the cutting portion is rather shorter than normal, it's probably heavy-duty, not the ones intended to cut left. – Joe Feb 22 at 15:22

Aviation sheers can be tricky to cut straight if you're not used to dealing with them. What you might want to look for instead are tools called 'nibblers'. They take out a section of metal, maybe 1/8" to 1/4" (cutting it twice).

It can be a little bit slower as they don't cut a lot in each squeeze, but because you don't have to force the opening large enough to fit the tool through, you can get a much straighter cut.

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Found a hand powered nibbler at HD for ~$27.00. Corded and pneumatic nibblers go for ~$200-400 (a little expensive if you're only using them once). – Tester101 Sep 1 '11 at 20:34
    
@Tester101 FYI - link in your comment is still live, but points to a discontinued product. For the time being, it at least shows what a typical product looks like. – Snowman Feb 22 at 4:42
    
@Snowman : It looks like what I've used before are actually 'super nibblers'. Here's a video showing the difference between the two : youtube.com/watch?v=zREw9gfry7k – Joe Feb 22 at 15:28

It probably depends on the location and how much room you have to work with. I'd try using a jigsaw with metal blade or tin snips to start with.

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Cheap but won't give a very straight edge: tin snips

Expensive but will give a very straight edge: band saw

I've cut aluminum rail with a miter saw (equipped with a special blade, of course,) but never rigid aluminum ducting. I'd imagine that it would work great, but finding the correct blade might be difficult.

I've never tried a rotary tool. I'm sure it would work, but I doubt it would work well.

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2  
Snips can give you a good cut, if you're careful and do it properly. – Tester101 Sep 1 '11 at 17:37
    
It's definitely a lot more difficult than cutting paper, and yet people still use guillotine cutters with paper to ensure straight cuts. Ultimately, these things are subjective and vary from case to case. Of course, if he has neither tool, he should try tin snips first. Even if it doesn't work, they come in handy all the time and are worth keeping. If he finds the cuts to be unsatisfactory, there's only a few tools that will do a better job, but they definitely do exist. Band saw with a clamped-down guide for sure and maybe a miter saw w/ a special blade. – Michael Sep 1 '11 at 18:06
    
I am using semi flexible aluminum ducting not rigid aluminum – Walker Sep 6 '11 at 8:05
    
@Walker It was unclear to me what exactly you were cutting, but the picture you added after I gave my answer clears that up. I've cut enough of that annoying crap in the past month to know that you're never going to get a really straight cut on that stuff with tin snips. It'll be "ok" but never "good." You're better off using a utility knife and then cleaning it up with tin snips... or if you want it to be really clean, a miter saw or band saw. – Michael Sep 6 '11 at 14:42

A reciprocating saw with a stubby metal blade works great.

Make sure you clamp the workpiece down or get a friend to hold it.

A long blade will bottom out on the inside and bend, so watch the blade length.

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