To fishmouth or not to fishmouth, that is the question

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I am making a structure from square tubing (3/16" thick, 2.5" wide) which will incorporate a tee-joint.  In duty, one member will lay on the ground, and the other member will rise up vertically, resting on the floor member (there are other  members, structurally balancing the thing). As I will be welding the tee joint at the junction of the floor member and the vertical member, I have the choice of cutting a fishmouth out of the vertical member, or the choice of not using a fishmouth, and just filling the gap created by the radius on two sides. Would you cut the fishmouth, or just weld the gap?
Reply:You don't need to do anything but butt it up, and fill in the radius.  It's a full penetration weld on the radius/shoulder, and a good strong fillet on the non radiused butt jointed sides of the tubing.I like to fill the radius in a single pass with a bit of rod manipulation.  But you can fill it in two passes if you feel uncomfortable straying from anything but a straight drag weld.  Filling gaps is easy once you get the hang of it, but is a PITA if you're unfamiliar with your particular rod/wire.  The thin edge is the one to watch.  I like to wash into it, and build slightly over and beyond the edge. Attached Images"Any day above ground is a good day"http://www.farmersamm.com/
Reply:Cool, thanks Samm.  I was leaning towards filling in the shoulder, and I'm leaning a little more now. My main concern is blowing through the gap, but maybe I'll make an initial pass to fill it in, maybe with some 3/32" rod to give me some backing, I dunno.  Thanks for the pic of your weld, and BTW this will be using stick (1/8" 7018AC).Last edited by jonnycat; 07-03-2010 at 08:00 PM.
Reply:Better technique would be to bevel the bottom partially and make use of the leaverage it provides.
Reply:Only reason you'd need to fish mouth a square tube to square tube joint is if the tubes were meeting on the diagonal rather than on the square (corner to end rather than flat to end).You can bevel the side of the end tube that meets the flat side of the teeing tube, if you want a full penetration fillet, but full pen is not usually needed on a fillet.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op