Thread: How to make a short rectangular duct out of steel?

Buy some flat steel the same width and thickness as the square - so 1/8" x ?1". Bend it into the square shape with a hammer and a vice and weld the join. Then weld to the round plate. It's gonna distort though. And the bends may not be sharp enough. You could use separate pieces for each side and weld each corner. More welding means more distortion. Kind of a job for a welder-dude really.

Edit: Not square, rectangle.

Is space allows I'd use 4 pieces of angle iron. Wont look any better than Bryans indea but it should make clamping and sizing easier. May help a little with distortion also.

Well ok if space allowed I make a real sq to round transition. But in 3mm without the right tool that wouldnt be much fun at all.

Stuart

4 pieces of flat, as has been said previously, but TIG weld only on the outside of each bend and on the "outside" angle of the joints between the flat and the plate. Technically, a "corner" weld.

By using TIG and by doing an outside corner weld (as opposed to internal fillet welds) you can control the heat, dramatically reduce distortion and you will only need low amperages (< 50A for 1/8" MS).

In the area where you join the duct to the flange it also gives you metal to file/grind back to smooth out the transition a little bit.

Lots of tacks on the long runs possibly with stepped welding and you should be OK.

Vancouver is probably a bit far for me to come and do it for you though

I was originally thinking around the lines of a separate up-stand but have several people have pointed out, the potential for weld distortion is high.
A couple of off the wall ideas –
1. Cut this out with the waterjet and then weld 4 corners in. Provided you are careful (tack before welding) there should not be much distortion. The down side is that the bends will be tricky.
2. Cut out two of each of the shapes, bend the upstand and then weld. In reality the distance from the corner to the edge is only 1.25", so the heat into the flange will be far less than welding flat into the centre
Michaeltransition.jpg

Justin even though you've explained the purpose, I'm afraid it doesn't mean that much to me. However I wonder whether you need to have a continuous square section? Square to round adapters I've seen, say in HVAC ductwork, they just mitre the corners and bend them up to accept the duct. The "tabs", if you like, that stick up are there only to support the duct and don't form part of the duct itself, therefore it makes no difference if there's a gap at the corners. The square duct then butts up against the round flange, with the right angle tabs simply there to provide a fixture.

Not sure if that makes sense, I'm not readily equipped to draw diagrams and post them.

Pete

You say coming from a blower to a silencer?
Is the rate of flow important (ie do you want to maintain a maximum flow rate)?

If flow rate is important then that flange design being so abrupt will introduce some turbulence and hence restriction to the the flow.
To minimize this effect transitions should be tapered. A longer taper is better but often that is not possible.

Have a look on line here for the Duct Evolution.xls spreadsheet which shows how to lay out a tapered transition.
Then you can bend it up out of lightweight sheet metal and either weld or even just pop rivet it together and add some sealant inside the joint.