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I have a sheet metal (galvanized steel) box on my roof that's currently protecting the return and supply ducts for my old package unit air conditioner. I'm replacing it with a split system, and repurposing the steel box and original holes through the roof for the new refrigerant lines.

I bought a set of Sigrist exit seals to use for the horizontal penetrations of the new refrigerant lines through the wall of the box, but I have no idea how to actually go about CUTTING the 2-1/4" holes they require... the holes are way too big for a drill bit, and (AFAIK) hole saws are only for cutting through wood (and other relatively soft materials).

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  • Aviation snips come in sets of three. Right turn, left turn and straight cut. – user109695 Dec 17 '19 at 7:23
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    A Greenlee knockout punch is a great solution if you can borrow one. Way too expensive to buy for a one-time task. – Mattman944 Dec 17 '19 at 9:43
  • @Jeeped Aviation snips typically are for soft/thin sheet metal like copper or aluminum. Have you had success with galvanized steel? – Carl Witthoft Dec 17 '19 at 15:29
  • @CarlWitthoft - yes to galvanized steel flashings. The OP needs to add the gauge of the steel he is trying to penetrate. – user109695 Dec 17 '19 at 17:53
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I cut holes in sheet metal with hole saw bits all the time. Depending on how thick your material is you might need more than one hole saw to get through it in a reasonable amount of time. 22 gauge is certainly no problem. Not sure how thick your sheet metal box is. You want the ones with the most teeth but generally these are the cheaper ones so you should be fine. I am not sure I'd try on 1/4" plate steel but sheet metal shouldn't be a problem.

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  • Yes, I have cut holes about that size in steel doors with hole saws. You need to buy a tool rated for steel. – Mattman944 Dec 17 '19 at 9:19
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    I find putting the metal sheet onto a wood backing board is very effective. – Solar Mike Dec 17 '19 at 12:14
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    just get a bi-metal hole saw and you'll have no problem... support it with wood like Solar Mike suggested. – JACK Dec 17 '19 at 12:57

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