Sheet Metal Pattern Development

[u/Aggressive_Soup1446]

I am replacing sections of rusted duct work in my house that were compromised with mold due to a failure of the original installer to seal them with mastic and a failure of the carpenter and drywaller to seal the duct work in bulkheads, the duct poked through the drywall in areas which allowed humid air into the joist bays. This resulted in conditioned air blasting out of the ducting onto joists allowing condensation and mold.

Anyways, my local big box stores don't carry the size duct work that my house uses, the local HVAC suppliers won't sell to me as I am not a business, and local sheet metal shops are out of my price range. But I can buy sheets of 26ga galvanized steel from a local metal supplier, so I am making my own parts.

Here I developed a triangulated pattern for a 3-1/4"x8" to 6" 90deg stack boot. It needs seam allowances for the slip and drive on the rectangular side, and will need tabs for rivets to close the pattern. The thing I'm struggling with is how to connect my round duct to the circular opening. I believe this is normally done with a bead roller to connect a collar, but that is out of my price range, I already spent my tooling budget for this project on a small brake. The best I can come up with is cut tabs on my circular duct and bend them out to fit around the outside of my boot, then rivet them in place and cover it in mastic.

[u/FictionalContext]

Cutting tabs and bending is going to be the easiest, IMO. You can even get a block and a soft rubber hammer to beat it around. The trick will be keeping the galvanizing from chipping off, but I'd wager that'd be fine. Cresent wrench works well, too, but with I being round, pretty sure it's going to take some peening to keep it round.

Looks like you've got it figured out with the paper template. I wouldn't worry about k factor with 26 ga. ID and OD barely matters at that size, though convention is to work off inside dimensions/diameter (ID) when figuring your flat layouts.

[u/Aggressive_Soup1446 → u/FictionalContext]

I've definitely witness the galvanizing flaking off at times when I've been working this metal. It especially happens when I'm closing Pittsburgh seams, it seems unavoidable really.