Blowing through on sheetmetal, reduce voltage or wire speed???

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Hey everyone, new to this board and have a newbie question. I am welding on an automobile project and having a little trouble.  I am welding in some 16 guage sheetmetal to change the contour of the inner fenders, and the sheetmetal in the car is probably 18-20 guage. The problem I'm having is welding the 2 pieces together, I am blowing through the thinner stuff in the car even stitch welding.  I have a Millermatic 135 and am using .024 wire with argon/co2 mix.  I currently have the voltage set on "3" and the wire speed on "50".   If I reduce both, it starts welding badly (popping and sputtering).  What should I do to reduce the heat on the thinner metal to keep from blowing holes in it? I can't access behind the panel  to back it and don't know whether to reduce just the voltage or wire speed.  Which affects the main portion of the heat?  I am new to welding a practiced on some scrap pieces of the 16 guage and weld those fine, but then melt the thin stuff in the car while attaching them.  Thanks for any help.Mike
Reply:You want to use the lowest voltage you can and still get a good arc.Can you position the part to weld vertical down? That would help alot.Maybe you can increase your stickout and use as much push angle as you can.I guess you are doing butt joints? Another option but really should not be needed with your machine/wire size is to get a different argon mix-say c-10 or argon/oxygen mix.  Can you take some pictures?"after a long day of doing nothing...its gooood to kick back" Pumbaa
Reply:Thanks for the reply. I have tried to reduce voltage and wire speed down together, but that basically doesn't let it weld at all. I haven't tried to reduce just voltage, I stopped before it started getting excessive and decided to check for some information since I am so new to this.  I'll have to give that a shot and leave the wire speed up.  The part is the front of the main inner sheetmetal of the fender so I can't turn it, but I do see where that helps as I am basically welding in a piece the shape of a rectangle, and on the sides I am welding vertical down and no problems with it blowing out.  Yes, I am welding butt joints with it.  If the voltage reduction doesn't seem to get it, I'll try and snap a couple of pics to let you see what is going on. ThanksMike
Reply:If you are doing that type of joint on thin sheet metal you'll need to practice at the bench first with some scrap pieces. Choose a low voltage and play with the wire speed until it almost sizzles. Hold the gun perpendicular to the joint. You will get nowhere fast if you have even a slight push or are trying to do a weave. The joint will have to be near perfect and only run stringer beads. Another trick is to back the joint up with a thick (approx 1/8")sheet of brass if you have room. It reduces burn through and acts as a heat sink.
Reply:I've read this a couple times, and since I've never run a 135, am kind of shooting in the dark here.Let me start by repeating what I've said too many times; THE SETTINGS on the dials are only a starting point, a suggestion, some engineers idea.  While those settings might mean something in the lab, they are just a starting point in the real world.  On a 120 volt machine those settings don't mean much because minor variation of the input voltage have major effect on the output side of the machine.You said you can make the weld on the bench, but have problems when you try on the car.  You really aren't making the same weld, under the same conditions.  Are you adding an extension cord when you move from bench to car?Now that I have that out of my system, I'll get to the most important thing in MIG, CLEAN steel.  Any thick to thin weld is a bitch, and if the steel isn't absolutely clean, you multiply the problem by a factor of 10.That said, you state you have practiced on the bench, and can make your weld on a piece of the 16 ga.  That establishes where you want to be with the power level, the rest is in your wire speed and travel speed.What you need to learn is the technique of welding thick to thin.This is pretty much a matter of concentrating your heat onto the thick piece and sort of whipping the weld onto the thin piece and back to the thick piece quickly enough so you don't burn thru the thin one.  The technique isn't one you should be learning on the car, but rather in your practice area.Another thing that could be your problem is joynt design.  Making such a weld on a T joynt, or where the 2 pieces overlap is fairly simple, but if you're trying to make this weld on a joynt where the 2 pieces just butt up, it will be extremely difficult for a new guy.It also sounds like you're chasing the hole.  One of the most important things in welding is knowing when to STOP.  The tendency with ner guys is to keep welding when you blow thru.  While I might be able to get away with leaving a porcupine on the back side of a weld, a new guy can't.  I've done a lot of buildups so I could have something to weld to, and pretty much know what I can get away with, you don't.   When you blow thru, STOP, and move over.There was also a reference to stitching.   In my opinion, stitch timers are the most useless device ever put on a machine.  Every weldor I ever met was born with a stitch timer, it's called a trigger finger.  Pull the trigger, make a short weld, and let go of the trigger.  Automatic stitch timers are for robotic welding.You will probably need to make this weld with a series of interrupted welds, going back between them and filling in, both to minimize warping, and to eliminate blow thru.I'd suggest you go back to the practice bench and play with making the exact weld there.Appreciation Gains You Recognition-
Reply:You also might try turning the wirespeed/voltage down and making a few practice welds on the bench. The machine should be able to weld smoothly with lower settings. If it can't, you're possibly fighting a too-tight wire spool brake or another problem. I've run into this with my machine. It didn't want to weld at low settings. Loosened up the spool brake a bit and it's right again.Just another .02Can we fix it? Yes we can!!!
Reply:Addressing the Wire spool tension. I have a 135 and had the tension problem. The fix for the 135/175 is to cut 1/4" off the spring. That will give you the ability to set the proper tension.
Reply:Thanks everyone for the help. In reference to the above posts, I have discovered a couple of things.  I ended up picking up a piece of 22 guage and welded that to the 16 guage on the bench. This helped tremendously on setting the welder.  I then started back on the car, weaving a small stitch (No stitch timer here, just my trigger finger      In using this process, it started building back in quite nicely and I ended up just taking the time to let the metal cool enough between spots to get it all filled in. Earlier,  I was actually stopping when I would blow through, but instead of letting the metal cool enough, I was basically moving down 4 inches or so and starting again. I think that the residual heat was just too much which was causing me to blow through in other areas where the metal was already heated due to the previous weld.  This resulted in about 3 places around 1/2 inch to 3/4 inch long that had to be filled.  Things worked out nicely this afternoon though and I got them all filled without any more problems using advice from several of you. Thanks again for the help as you all gave some great advise and set me straight. Mike
Reply:I do alot of this type of welding, and the thing that has worked out well for me is to when you strike the arc, move across the joint and then stop right away.  Continue this across the area that you want to weld, but every weld you make, go from one end to the other end to keep the heat from concentrating in one area. The thinner material on your car is hss and this is hard to not burn through.There's a method to the madness, disregard the method and the madness begins!!