Note to readers: I published a letter from Ray Guilbert regarding skip welding patch panels into place back in the July 2007 "Professor Hammer" column. This has generated more mail than any column in recent history, so this month I'm running a thoughtful letter from a reader about the subject.
Ray Guilbert answered your question regarding skip welding versus continuously welded seams. I would like to elaborate on a few comments made by Ray. I think he was right on in his comments, but let's look at some of the reasons why!
My experience tells me to completely fill in a welded seam with small spot welds; I've always done it this way, and have always had good success, so I'm reluctant to change. As a related point, I am interested in the special epoxies made for bonding panels in the automotive industry, and I plan on contacting the manufacturers of these products in the near future. I'll keep you posted on this, since it may be a whole new alternative to welding, at least for some applications.
Ray talked about the weld parameters being well thought out, and clipping off the end of the wire before each weld to help make consistent spots. I think it is worthwhile to comment on these points, since they are very important! A MIG welder is a constant potential type of welding machine; in other words, when it sees a direct short to ground, it supplies enough current to burn away that short. A .023 wire requires approximately 40 to 60 amps to weld, but when a ball forms on the end of the wire, it might take three to four times that amount of current to burn off the ball. This will probably burn a hole in the panel, or cause so much spatter that sooner or later you will short the contact tip (the copper part the wire travels through) to the shielding gas nozzle.
Ray mentions he tests the parameters on a piece of scrap the same thickness as the panel he is welding. It's better to burn holes in scrap metal than on the finished part! He also mentions proper gun angle; I can almost guarantee if the novice welder drags the weld puddle similar to stick electrode welding that he will have at least an 85 percent failure rate. Nine out of 10 times he will allow the weld to get on top of the weld puddle. You may not see this failure on sheetmetal until after you have finished the car, but surface cracks may form later. It may form cracks right after the weld cools on thicker metals, but your failure rate will be greatly reduced if you push the weld similar to gas or TIG welding. You will be able to see the throat of the weld area, and the puddle will form behind the wire stickout.
I would recommend purchasing welding wire from a reputable source-most .023 wire comes from across the pond, so make sure it has the proper AWS specification stamped on the box, ER 70-S6. Remember that spending $5 more buying welding wire from a reputable source may save you thousands of dollars down the road!
Now, about buying a welding machine-forget the discount stores and building supply houses! Purchase your machine from a reputable welding distributor; you're going to purchase your gas and consumables from them anyway, and they will be able to assist you if you need help or advice.
Yes, MIG welding is great, and it covers up a lot of sins, but weld sensibly, like your life depends upon it! Remember, MIG might stand for metallic inert gas, but in reality it could stand for "maybe it's good!" Good-quality welding takes the right machine, the right wire, the right gas, and setting the right parameters, but in the end there's nothing like practice, practice, practice!
Ron, thanks for everything you do for our sport; I always enjoy your column. And special thanks to the guys like Ray Guilbert, who know what they're talking about!Bob MannVia the Internet