You are likely to get various answers depending on who you ask but (assuming you don't have access to a Lindab SR Cutting Machine, which of course you are unlikely to have) we find the best method is to use a quality jigsaw with a blade suited to cutting thin sheet metal (a good blade choice might be, a Bosch T118G or a Dewalt DT2162-QZ).
Place the duct on a clear flat surface. On the floor is fine. Mark the duct at various points around the circumference e.g. if you want to cut a 1m length you measure from the end 1m and place a mark. You then make further marks around the duct 50-150mm apart depending on the duct diameter. Marking all the way round is important. If you don't it's likely that you'll find yourself cutting off line. When you get back round to where you started, you may find you have 'missed' and have to do a sharp turn to meet up with the initial cutting point. Drill a suitable size hole (8-9mm) to allow you to insert the jigsaw blade and slowly and carefully cut around the duct following the marks you made earlier.
We strongly recommend that you use protective eye wear when using a jigsaw. The swarf typically flies out in a forward direction but it's not uncommon for it to come upwards towards the face. You might want to use protective ear wear too, as this method is very noisy.
Another simple and clean, albeit rather slow, method is to use a hand nibbler (see pic below). These are inexpensive and made by Draper, Monodex, and Sealey. As with the jigsaw, you will need to drill a hole to start and you may need to use a hacksaw on the seam, but they are ideal if you are only making a few cuts.
Contact: Yuan Jun
Add: Add : No.82, Jing ju Community ,Dalingshan Town, DongGuang City, GuangDong Province, China