Welding aluminum sheet - Burn through (sag out?) at the ends.

Practicing corner joints and T joints.
Ok... Welding misc thin aluminum sheet (upto about 10 or 12 gauge) I can pretty much go right off the settings on the card on my Miller 212 and
adjust for thickness... except, if I start right at the end and weld all the way to the other end I melt the stock at the start and end. Worse at the end. If I start 1/4 to 1/2 inch from the end and stop about 1/2 inch from the end I get a nice neat weld. Then I can wait for it to cool and go back and do tiny little micro burns to finish up the ends of the joint. Seems to me that would leave a lot of room for a bad weld in that little stretch at each end.
Is that the best way to do it?
If I used some kind of backing at the joint would that work out better or would the backing material just contaminate the weld?
I'm still playing with scraps and drops, but I have started doing some material layout planning for my first boat repair project.
Bob La Londe www.YumaBassMan.com
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That sounds like something worth trying. I never heard of that.
Aluminum is just so damn tricky because it doesn't change color like steel to make it easy to see how hot it's getting. It goes from normal, to slight sag, to big gaping hole in your project in about .1 seconds. :)

Yeah, MIG is really great when you have to do large sections fast. But I never really did get it down for thin aluminum. It really did feel like you had to get up a running start and point the gun at the weld as you ran past because it was so damn fast. Trying to go slightly slower or slightly faster was almost absurd because you were moving the gun so fast to start with it was hard to get any sense of how fast you were really going. I was doing this with a Miller 251 with a large spool gun and 030 wire. Maybe a smaller MIG unit with smaller wire would make it easier? Do they make smaller wire than 030 for aluminum or does it just get impossible to feed when smaller than that?
If you had to do the same weld, every day, with the same material, and the same welder, I could understand how someone could master MIG on thin aluminum. Otherwise, I'd just use the very slow TIG if I had to make a good weld.
A 4 inch mig weld was maybe 1 second of welding. A 4 inch tig weld would be maybe 20 seconds. (haven't timed it and that might be off but that sounds about right).

Yeah, it surely would. If you can master the weld, that's the way to go for speed.

Me too. :) Still trying to figure out how to justify buying it. :) The fact that it could do aluminum is nice as well.

Yeah, building small robots is one of my desired goals and being able to work with aluminum to save weight and battery power is a big win for that. Because of the small size of the projects and the fact it's just hobby work I don't need to make a profit on, the slowness of TIG wouldn't be an issue for me. So for that, I'd like to get TIG (and would end up with Stick as well at the same time). But I'm looking at the $3K range machines for that and can't justify the money yet for the same sorts of reasons you mention. In the short term, I'm looking at MIG mostly for small steel projects because MIG is just so easy and fast and fun for sticking stuff together. There are a lot of little projects I'd like to be able to do that I can't currently do because I have no welding equipment yet.
And, MIG gives me the option to do some aluminum work for an extra $200 or so for a spool gun without having to wait until I can justify the TIG machine. So that too is a win.
I've looked at the harbor freight stuff multiple times just to give me something to use for now like you have done, but I keep telling myself it will be better to wait and buy something bigger and better that I won't outgrow so quickly.

I hope it's of some help. Good luck...

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I would think you could go slower with a larger wire. I have been playing with .030 4043, but I also have a spool of .035 5356 for when I get serial with it.
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For thicker wire, but the same size work material, you would feed the wire slower, but I think you would tend to weld (travel speed) faster. It would depend on actual the settings you used.
The thicker wire takes more amps to melt and as a result, puts more heat (and metal) into your weld quicker. For short circuit mode, if you set the volts and wire speed to get the same "buzzing" rate (say 200 contacts per second), each "buzz" melts back a chunk of wire. With the thicker wire, it takes more energy to melt it back for each "buzz". The result is more heat and wire into your weld as the wire size gets thicker for each "buzz" and for each second of welding (200 contacts).
You can slow down the feed for either size wire, but for a constant buzz rate, you will always get more heat and wire with the thicker wire. The optimal setting for each wire size will likewise give you more heat and material per second with the thicker wire, meaning you have to go faster to keep it from burning through or depositing too much metal.
The smaller wire allows you to weld slower. Think of what it would be like welding with thread sized wire for example. You would have to hold it on the same spot for a long time to get enough metal and heat to build up. And if the thread was too thin, it wouldn't even be able to transfer heat fast enough to your weld unless it was feeding like a bat out of hell.
And think of what it would be like to weld with 1/4" wire on 1" plate. Each "pop" from the burn back after contact would transfer a ton of heat into the plate.
But hey, I just now ordered myself a mig welder so I'll finally be able to do welding projects at home now!
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Makes sense when I think it through.

Which makes me think of another thing. I have been practicing with 4043 .030. I bought it mostly to play with. I have a spool of 5356 .035 which I bought just for my first marine project. If this logic follows through when I switch over to the thicker wire for the thin stuff I plan to weld I'll have more problems with it, and perhaps I should order a spool of .030 or even smaller if available in the 5356.
Dang it dude. Give me 20 or 30 years and I just might get this stuff figured out. LOL.
Bob La Londe www.YumaBassMan.com
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I need to get some more thinner stuff to practice on now. I'm down to just 1/8" and bigger. The rest of my light practice scrap is just that. Scrap. I've been cutting it into narrow strips to optimize how much practice I get out of it. The bigger stuff is pretty easy. I can slow down enough to actually watch the weld. With 1/8 I still get some burn through at the ends so I set up a couple sets to play with clamped to some 3/16 aluminum frame rails left over after I cut down an equipment rack. The pieces had nice sharp corners so it was easy to line up two pieces of 1/8" for a corner joint. The first try using this as backing I did speed up a little at the ends, and I didn't burn through, but I didn't get a good weld either. The second time I tried to burn from end to end just like I would with steel except faster of course. It worked pretty good. I got a bit of buildup actually on the start end, and the finish end was almost perfect. I got the tiniest bit of tack to my backing at each end, but I got a nearly perfect weld.
Like I said, now to pick up some more thin stuff to practice the technique on.
I think it would have been even a little better if I had thicker backing material to clamp to, but this worked out pretty good to test out how it works for me. If I slice up a bunch of pieces of 1/2" thick angle to use for this and just throw them in the scrap pile when they get too scarred up I think I'll be able to get most of the things done I want to for a very long time.
I did try the double weld technique, and I could kinda see how it is "supposed" to work, but I would need a heck of a lot of practice to be able to use that method with any kind of confidence. My results were not pretty.
I guess its time to head back over to Dave's Welding and see if they have some 14 and 16 gauge on the drop shelf to see if I can keep it working on the smaller material.
Bob La Londe www.YumaBassMan.com
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