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Hydro1: After you have pipe dimensions sorted out, cut the sheet like this. All bends will bend more when you press the pipe. My first pipe twisted real bad and ended up in a knot. Leave some extra at the ends for connecting pipes. The material I use is 0.75mm sheet metal.
Hydro2: Spot weld the skins together. At this point this isn't too important.
Hydro3: Dress the skins even. When you have an edge looking like this, it will be difficult to weld.
Hydro4: After dressing, spot weld it again. Now do it good, a spot every 20mm is recommended.
Hydro5: Now the skins have been welded together. I like to use gas torch, then I don't have to anneal the seam. Note the connectors at both end of the pipe, one to let the water in and the other one to let the air out. Never ever try this with air or air inside the pipe as it WILL explode on your face.
Hydro6: The forming begins. The pipe is between stressplates to make it expand more even. If this is done without stressplates (and a press) the belly will expand first and it will cause wrinkles.
Hydro7: The pipe begins to show up. I released the press too early and run into bad wrinkles. Bad welding also causes worries like this.
Hydro8: Looks hopeless... But you can repair this! Begin hammering the wrinkles from the outer edges and they will disappear. Note there has to be pressure inside the pipe when doing this. If it doesn't help, try it again after you have high pressure inside the pipe. I use pressure up to 60 bar and would use more if my crappy pump could handle it.
Hydro9: This will happen if you have to weld the pipe many times at the same place and don't wait until it has cooled down completely. Holes like this aren't dangerous as long as you don't have air inside the pipe. On the left you see a wrinkle that has been hammered down. This one was real bad and usually they look better.
Hydro10: At this point I was happy with the pipe.
Hyrdo11: Next step is to fit the pipe into the frame. Usually it needs a few cuts, but I managed to get away with only one. The weld is better on the inside than on the outside ;)
Hydro12: There it is. Just needs a paint and a silencer.