How Many Welds?

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Hi Folks-A little background to my questions:I am a Technical Director working for the scene shop of a theater company. We typically do a fair amount of MiG welding on thin wall (.065" or 16 gauge) tubing (1"x1", 1"x2" etc). My education in welding such materials has been that welding "opposite" sides (i.e., if you are butt welding one piece of 1"x2" tube into another, for example, you need not weld all four sides, but only two opposite sides) is more than sufficient to create a strong welded joint for a number of reasons. Heat build up is one factor, resulting in warping of the thin tube. Another is that I have been led to believe that it's actually possible to *weaken* a joint by welding too much on it.I am curious to hear the opinions and experience of others here. I am trying to teach my young carpenters to be efficient in their work for a great many reasons, and having them weld only what is needed and no more is one step in that direction (less wire, less gas, less time spent actually welding when extra welding is not needed and may be, in fact, detrimental). But I want to make sure I'm not steering them in the wrong direction.Any thoughts would be greatly appreciated.Last edited by Technical Director; 08-25-2009 at 11:45 AM.
Reply:It's entirely dependent on the material type and it's intended use.  Without a little engineering, it would be hard to give you a definitive answer on just what the bare minimum is.
Reply:So long as HAZ is not excessive, heat input is not excessive, then with all things being equal, a fully welded joint is far stronger than any partial weld. Negative aspects of a partial weld are: potential lack of overall bond strength. Cutting it short may actually yield a weld that is "Too small, not enough grab" and the result is a weld that is not as strong as the tube itself. Another big piece of the puzzle is the fact that in a stressed member, the starts and stops on welds are where failures tend to get their start. A junction of tubes that are only welded on 2 sides will have good resistance to stresses in one set of directions, but lateral or radial deflections will have little realized resistance. This can lead to a rapid failure of the junction. I understand that for building a set, or a small stand a partial weld is much faster, and strength is not of primary importance- often it only needs to be strong enough to hold together and form a skeleton for a prop, etc. For these applications, I would not disagree with you. An overgrown tack weld will do. But....there is a reason that all structural welds are generally 100%. There are NO partial welds on rollcages. There are no partial welds on structural beams. If you have people actively supported by the structures you build, or if the structures you build are expected to carry loads or support things above people, it is cheap insurance to use the best welds you possibly can for the application. Were something to happen, and God knows ....S*&T happens, it is better to point to the weld and say " It was a full penetration weld, and a 100% welded joint." If you ask me, the partial welds we see on stuff like lesser quality weight bench sets, playground sets are sales driven lawsuits waiting to happen. Every decent built playground set you will see has solid welds, every well built weight bench set has solid welds. 2 easy examples we see often enough, but pay little attention to.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:Excellent reply, thank you. I can generally say that most of our work is "light duty", i.e. walls covered in fabric or wood and which are usually "decorative"; the most stress they receive comes from moving them around; and platforms which are weight-bearing but usually used in situations where they are stationary and are adequately supported by legs or knee walls.
Reply:For the most part I would just have them weld all around the joint for the 16g 1"x1" 1"x2" etc. You'll still have to educate them on the behavior of the metal when welded- distortionyer not going to weaken these pieces realistically and like you said the Scenery gets pretty banged up just from setting up and breaking down or in transit.Clipped a Tree on the way inEd Conleyhttp://www.screamingbroccoli.com/MM252MM211 (Sold)Passport Plus & Spool gunLincoln SP135 Plus- (Gone to a good home)Klutch 120v Plasma cutterSO 2020 benderBeer in the fridge
Reply:That's a nice Murray lowbed that them stairs are riding on.Vantage 500's LN-25's, VI-400's, cobramatics, Miller migs, synch 350 LX, Powcon inverters, XMT's, 250 Ton Acurrpress 12' brake, 1/4" 10' Atlantic shear,Koikie plasma table W/ esab plasmas. marvel & hyd-mech saws, pirrana & metal muncher punches.
Reply:If you know the direction(s) of force applied, example torsional,compression,shear ect, on the piece to be welded, never start or finish a weld in that area with the maximum applied force. Always wrap (weld) your corners.Welds are strong if done correctly.1/2 and 1/2 weld. Wrap the end welding inward on one side 1" of a flat bar or tube. Then do the same on the opposite side, wrap the end welding inward.Main thing is good penatration on both pieces to be welded.. More heat is better than too cold of a weld joint. Really depends on the situation as for as safety is concerned.Good luck!THE OLDER I GET! THE  SMARTER MY DAD WAS!Thermal Arc 185 inverter Tig welderABITIG 26  tig torchLotus foot pedalLincoln 225 stick welderClark 185SG Mig spoolgunwelderVictor torch/regulatorsHF36" shear,bender,roller.
Reply:This is a good point about light gauge welding.  A good starting point would be to show students the amount of welding on regular chairs and tables used for everyday use.  The approach taken by furniture manufacturers reflects a need for economy with serviceability.  100 % welds would bankrupt the manufacturer and make little difference to the consumer.