MIG Welding Sheet Metal

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Hey guys, I've heard from various places that when welding thinner sheet metal (like 18ga or so) to do small tack or spot welds at different places along the whole length of the piece so that you don't over heat and get warpage or burn through.  Can anyone shed some more light on this?  Is this a decent way to do it?  Whats the technique for making sure there are no holes in the weld?It would go like this (*=spot weld; - = joint):     1) *---------------------*2) *---------*-----------*3) **--------**--------**4) ***------***------***etc until eventually its just  ***********Thoughts?
Reply:Hello Chuck86, the techniques that you are referring to are sometimes called back-step welding or possibly skip-welding. Those techniques will definitely help with distortion and warpage. Along with those methods you might consider using a backing bar or some other form of a heat sink to help with distortion/warpage control. Your backers could consist of a copper bar, brass bar, or possibly even an aluminum bar. Other shapes are used depending on the area that needs to be addressed. Angular shapes, round bar, and other bits and pieces often will help out. If you have large gaps the aluminum might not be the best choice, copper or brass might be better choices. Tacks every so often and spaced out will help with alignment, the progression might simply include welding from one tack to the next by starting with a particular gap and then making the next weld so that it ends where the first one started and then repeating this. Other methods might include a progression that somewhat emulates the torqueing of an engine head or similar sort of thing. These are just a few suggestions, trial and error often times will lead you to a technique that will work best for you and your particular style or understanding. I am sure that others will give you plenty of suggestions to choose from. Good luck on your attempts and best regards, Allanaevald
Reply:I do that alot with autobody repair. A bead will warp most automotive sheetmetal from my experience. My burb came from FD and I was too cheap to buy new fenders so I welded up the old ones where the holes were from the lights.Like Aevald said, just zig zag around like tightening head bolts so it doesn't warp. Not sure of the gauge but but pretty thin stuff. I'm still getting the hang of mig myself. When I worked in a bodyshop years ago we brazed everything. Same technique though to prevent warpage. Attached ImagesLast edited by acesneights1; 02-11-2011 at 08:22 PM.Lincoln Ranger 8Lincoln 140c MigLincoln Tombstone(doesn't everybody have one ?)Miller 200Diesel/Heavy Equipment MechanicLead Plant Mechanic Municipal Water
Reply:Aces has given you a good example, don't leave any pinholes(keep the same gun angle every weld) or your paint will bubble.If you can get behind with a dolly as you work on-dolly the hot weld relives stress, alot like hot forging.It is going to warp, if you see it gets out of hand pull it back with a unispotter, or weld a spotter nail in low places and hammer weld, keep weld profile minimal.Good Luck
Reply:Thanks, that makes me feel good. I wasnt so confident when I did it.Lincoln Ranger 8Lincoln 140c MigLincoln Tombstone(doesn't everybody have one ?)Miller 200Diesel/Heavy Equipment MechanicLead Plant Mechanic Municipal Water