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post #1 of (permalink) Old 09-09-2000, 03:59 PM
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A question on mig welding technique

How do y'all feel about pressing the trigger, staying in the same place for a moment, letting go, moving a few millimeters down then repeating the process. Kind of like tig welding where you lay one small weld nearly on top of the previous one. What are the down sides to doing this.
I know it takes a second for the metal to heat up enough for adequate penetration, but couldn't this be overcome by cranking up the amps, slowing down the wire, and depressing the trigger for a moment longer?
I heard somewhere that this technique was impractical because the slag needs to be chipped of every time you let go of the trigger. I have used this technique on several non structural welds and have been very pleased. I have a lot more control, the bead look a lot nicer, and it seems at least as strong.
Are those welds weaker cause of some hidden impurity? I am using a millermatic challenger 172 with CO2/Argon mix.
I am practicing so I can build a sweet roll cage and am considering using this technique to joint the tubing together. The book performance welding says that some aircraft manufacturers use this technique to joint their thin walled tubing, but they didn't say if they had special equipment or environment (i.e. shielding gas filled chamber) when they did it.
Any practical advice from the pros on this one is greatly appreciated. Thanks

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post #2 of (permalink) Old 09-10-2000, 06:13 AM
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Re: A question on mig welding technique

[img]/wwwthreads_images/icons/tongue.gif[/img] The down side is that you end up with a TON of porosity. I have welded pipeline for years, and one thing we try not to do is start and stop because when you do, you introduce the chance for porosity and leakage and/or corrosion, and of course you are also giving up strength. If you are welding with heats such that you cannot travel and weld without burning through, then one of two things has to change: you need to change heat and/or wire size; or you need to get a better fit-up so the gap is more easily welded up. Normally, I like a little better "Vee" or gap with MIG than with stick; because the MIG doesn't dig in as well and get the weld all the way to the bottom of a too-tight seam; but in any case I am welding as a continuous process. I do use the stop-start technique to weld really thin stuff, but NEVER when strength is of ANY consideration.[img]/wwwthreads_images/icons/crazy.gif[/img]

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post #3 of (permalink) Old 09-10-2000, 04:48 PM
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Re: A question on mig welding technique

Your question is going to generate controversey among two seperate groups of welders. the stitching technique is best suited to machines with a stitch timer that will control the after flow of shielding gas to prevent weld contamination. Several well respected chassis fabricators I know use the sticth weld, their work is beautiful and stands up to crashes. One of the advantages to it is that the stresses created in the weld process seem to cancel themselves out. I follow a different line of thinking that is similar to CJ Dave, I am fast and furious I weld all the way around the tube in one shot, my welds look more like a machine made weld, the disadvantage of this method is joint fit is absolutely critical, a few thousanths gap will be horribly disfiguring.
My best advice is to practice with some 1" .090 wall tubing, try to build an equalateral triange and a square with 18" sides, if you can build these so that you can trace them on the floor with chalk, and flip or rotate them in any direction and still fit the outline then you will have a good lesson under your belt.
good luck, jjc

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post #4 of (permalink) Old 09-10-2000, 05:17 PM
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Re: A question on mig welding technique

My vote is with Dave and JJC.........I WOULD NEVER "STICH" A CAGE!!!!!!!!!
I also only use this method on non-structure and sheetmetal(oh yea, nickle cast too).
My $.02 is that the fit of the tubing on the cage is going to determine how strong it is going to end up.
Sloppy joints=sloppy welds=weak

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post #5 of (permalink) Old 09-10-2000, 05:57 PM
 
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Re: A question on mig welding technique

now this is just from hand Tig aircraft engine parts and EB welding cases together, BUT everytim eyou stop and start you have the potential for a crack, depends on the planet alighnment and the way the arc comes off..

I thought stitching was weaving back and forth while welding?
I didnt realize it meant coming on nd off of hte power?

anything structural If it were me wouldnt be welded in small incrememnts. tacked in small inrements maybe..

but welded in one full penetrating pass

it really I guess, depends on the application and the experience skill of the welder doing the actual work.


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