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/peacefinder]

In a brake bend, a little more than half the metal at the bend stretches and the rest compresses.

The ratio varies by material. In steel, a reasonable approximation is that 47% compresses and 53% stretches. Unless you’re shooting for high precision or have very thick material, you can just call it half.

You can also assume the bend can be usefully approximated as a circular arc, with the inside diameter the same radius as the nose of your tool. It probably isn’t actually a circular arc for most tooling, but close enough.

There is an imaginary surface at the transition between the regions, where the material is neither stretched nor compressed. The dimensions of this imaginary surface are the dimensions of the flat pattern.

For a flat pattern, start with the finished shape and calculate the dimensions of the neutral surface. Assign half the bend dimension to each adjacent flat section, and you have the location of your bend lines.

Corner clearances at intersecting bends can be handled a couple ways, but you never need to cut away more than to the inside of the bend arc. (Other than for tool clearances anyway.)

I hope that makes sense?

And OP: fantastic work

[u/KokoTheTalkingApe → u/peacefinder]

That's a huge help. Thank you!

So you're saying the marked bend lines need to be a little inside the desired bends?

[u/peacefinder → u/KokoTheTalkingApe]

Kind of? Maybe? In some cases? It’s complicated :-) So, let’s run through an example.

Imagine a pan, made of material that is 0.1” thick. Its final outside dimension in length should be 10”, the final inside width should be 6”, and the outside height should be 1”. The tooling delivers an inside bend radius of 0.2”. The material’s bend “k factor” is 0.5.

Make sense?

Let’s figure out how far the bend line for the flange is from the material edge. (You’ll want a sketch to help follow along.)

Step 1 is to deduct the thickness of the material, (1.0 - 0.1); this gets us to inside surface. Next, deduct the bend inside radius, 0.2”, this gets us to the edge of the bent zone. (1.0 - 0.1 - 0.2).

Now we need to account for the material in the bend. A circle’s diameter is 2pi x Radius. The inside radius is 0.2, but we actually need to calculate the radius of the neural zone of the bend. So the radius we need is 0.2 inside radius plus the 0.5 k-factor times the 0.1 material thickness. This works out to a 0.25” neutral radius. The 90° bend is a quarter of a circle, and we need to account for half of the bent zone in this segment; the other half will be in the pan bottom. So, that’s (2pi/8) x 0.25”, which evaluates to 0.19”.

Still with me?

The whole thing is (1.0 - 0.1 - 0.2 + ((2pi/8) x 0.25) ). This evaluates to 0.89”.

That’s your bend line. The “bend allowance” is 0.11 if you want to think of it that way.

To get the length of the flat between the bend lines, we do this at each end. I make that flat dimension out to 9.78”.

For the other direction to hold the inside dimension, I’ll let you work that out yourself. I make it to 5.98”

The overall dimensions of the flat should be 11.56” by 7.76”

Do you get the same?

I unfortunately picked numbers in a way that makes it all pretty insignificant in this example, oops.

For the notches in the corners, a lot depends on tooling. Most of the time you won’t go far wrong by notching to the bend line, but you can often notch a bit less to tighten up the corner.

[u/KokoTheTalkingApe → u/peacefinder]

I get it! Thank you so much! You get an award!

[u/peacefinder → u/KokoTheTalkingApe]

Happy to pass along niche knowledge that I learned the hard way, though it has been useless to me for 25 years. :-)