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square to round layout

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  • square to round layout

    Hey all I'm building some duct work with about 2" pipe that has a fan on it. The fan end has a square opening
    3 1/2" squared opening. And the pipe circumference is 7 1/2" od I'm just wondering if there is some sort of formula /procedure to lay this out so it looks right rather than me just notching hammering and filling with weld. I assume its similar to laying out an orange peel cap end. Just can't fund anything on this. Thanks

  • #2
    Do a search on the web for "square to round layout". Many sites available to explain it. Here is one.

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    • #3
      I have built many of these from stainless. The biggest tip I can give you for a first timer is patience. You will get the first one wrong, but they are always a PITA.


      • #4
        Is this a common axial fan, like the one on the front of a truck engine? Can you run the round pipe right up to the round fan blade opening?

        Circumference is pi (~3.14) x diameter, so 2" OD pipe has an outside circumference of ~6.3". If it's actually 7.25", then the OD is actually ~2.3".

        Either way: a 4-sided 3.5" square has a perimeter of 4 x 3.5" = 14" (about double the pipe's). So you'll need to 1) square the pipe end by hammering or driving in a square mandrel, and 2) fabricate 4 identical trapezoidal panels...
        ' ___

        ...whose short sides (TOP) are slightly less than 1/4 the pipe's circumference (to allow for corner bends), and whose long sides (BOTTOM) fit the way you want around the fan (smaller to go against or inside; larger to go around the fan). The angled sides (or the distance between the long & short sides) depends on how much room you have to work with, and how much material you want to use. Longer panels will squeeze the airflow more slowly, allowing the fan to move more air.

        With more effort, the panels don't have to be identical or trapezoidal, and can offset the center of the fan from the center of the pipe, and can incorporate a bend. But that can turn into a LOT of work & frustration if you don't get it right the first time.
        Last edited by Steve83; 01-20-2013, 02:02 PM.
        Walk softly & carry a BIG SIX ! ! !
        MM211 + SM100


        • #5
          This is how I build a square to round Cut and tack the pieces as in the drawing, Now cut a thin piece of card board to the approximate angle to fit the pipe, Hold the cardboard tight around the pipe and the rest of the offset. Have someone mark the inside of the cardboard. cut each side of the template about a 1/16" wider on each side. Trace out on the metal you are using and cut. Then open your vice about a 1 1/2" or what ever you need and use a ball-peen hammer to form the piece to fit taking into account the angle for the pipe. With a little patience and sanding you will have a nice looking square to round. Try not to get to many union labels in the piece when forming. If you are using tig to weld the offset leave the 1/16" on the peaces and use it for filler, otherwise grind or sand the peace to fit. Lay the piece lengthwise over the vise jaws and start forming at the pipe end.

          1 Make the square the size you need it

          2 Make four trangles the with of the square to a point the length you want it

          3 Tack the pieces together

          4 done
          Attached Files


          • #6
            square to round layout

            Thanks guys I figured it out and if I could figure out how to upload pics I would show ya but basically I found the perimeter of the square piece and took the circumference of the pipe divided into 4 and just centred those on the quartered perimeter. Bent the stretch out into the square and then bent the top in and weld.