Why leave a gap between pieces to be welded?

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I'm a total newbie, still teaching myself to weld and I am trying to work up to building my own bumper for my jeep at the moment.My question is, why do you leave a gap between pieces to be welded together instead of just putting them up to one another? I use a mig welder, won't it sputter all over ever time it passes over that gap with no metal to make contact with?For example, like this. (I am assuming you only do that for an outside edge and inside edges get filet welded. On such a project, I am assuming the entire length of the material gets welded to ultimately make one enclosed piece, and then grinded smooth?My second question, is how do you gauge how wide the bead needs to be for the material used? Say 3/16" plate for example.Last edited by BrooklynBravest; 12-12-2013 at 01:54 PM.
Reply:To ensure 100% penetration. You have to learn what gap and land you can handle for each process. Watch this video on open root welding. Don’t pay any attention to meI’m just a hobbyist!CarlDynasty 300V350-Pro w/pulseSG Spool gun1937 IdealArc-300PowerArc 200ST3 SA-200sVantage 400
Reply:Originally Posted by CEPTo ensure 100% penetration. You have to learn what gap and land you can handle for each process. Watch this video on open root welding.
Reply:You don't HAVE to gap or bevel material, it just makes getting a full penetration weld easier with less power. You have to watch that you don't crank up the power so high that when you penetrate all the way thru, it all drops out. Problem is if you turn down the power, you may not get full penetration. It's some what of a balancing act and much of it comes thru experience and skill.As far as how much bead to lay, general rule of thumb is your weld doesn't gain any more strength once it passes the thickness of the thinnest piece being welded. So in your example of 3/16" material, the size you are shooting for is 3/16" on each leg of the weld. With small material, that often gets tough to do, so in welds under 1/4", especially with mig where you can't adjust your filler addition separately like you can with say tig, 1 to 1.5x the material thickness is a good gauge. So for 3/16, you'd be shooting for roughly between 3/16" and say a tad over 1/4" on your legs..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Here is a 2 1/2-inch open root pipe I did with Mig.For some reason I can't upload anymore pictures of the open root pipe.  Attached ImagesDon’t pay any attention to meI’m just a hobbyist!CarlDynasty 300V350-Pro w/pulseSG Spool gun1937 IdealArc-300PowerArc 200ST3 SA-200sVantage 400
Reply:Here is the 12-o'clock position of the pipe weld with Mig. Don't know why it wouldn't let me upload it before. Attached ImagesDon’t pay any attention to meI’m just a hobbyist!CarlDynasty 300V350-Pro w/pulseSG Spool gun1937 IdealArc-300PowerArc 200ST3 SA-200sVantage 400
Reply:On something like that bumper where you want to grind it smooth, if you don't put a gap you're not gonna have much weld left after you get done grinding.
Reply:If you assemble the pieces to provide outside corners, then you can achieve full penetration without needing gaps.  Of course fit-up will not always be perfect, so learning to fill gaps is part of the process.  You will learn to weave the electrode from metal to puddle to metal to avoid slipping through the gap.  It comes with experience, and you learn on every project.