Need some help getting mig weld started

[stonebreaker]

Searched the forum and didn't quite find what I needed to know. Anyway, I just bought a MM175. I'm in the process of building my welding table out of some pieces of plate and angle scrap I picked up for nothing at a local iron works. My problem is, that when I stop welding and then try to restart on the place I stopped, the weld doesn't look clean. I scrub and scrub with a wire brush till it's clean and shiny before I start, but where the new bead starts, I'm left with what looks like brown soot coming out from under the edge where the two welds meet. Once I get to the unwelded joint, the bead looks OK to my somewhat untrained eye for as long as I want to run the bead. It's just starting that is giving me problems. I have had to grind out a couple of spots where I got an inclusion (dang bug flew into the arc - can you believe that?) so I know I'm getting adequate penetration.

I have tried starting a little ahead of the last weld and going back to it, although I'm not sure I'm doing it right. As I said before, I use a wire brush to clean the metal as much as possible before I start. Also, I'm grinding the metal down to shiny everywhere I'm going to weld.

This problem really makes no difference on the weld table, but one of my projects this summer involves putting a V8 in my son's sonoma, and will include quite a bit of welding on the rear end and suspension setup; I absolutely cannot fubar the welds on THAT! I will post pics when I get home from work, but posted this now in hopes that someone could help me before then.

Thanks in advance.

[smithboy]

Moths are the worst...they just come out of nowhere and annoy the heck out of you. You might be just letting the weld get cold before starting up again and need more juice to form a puddle on the now thicker piece of metal (with bead). lots of soot is generally associated with poor gas coverage. You might be running your gas on the low side or not getting the tip of the gun close enought to the metal. Turn up you heat and gas a bit, get close, and see if it helps on the starts...

[MAC702]

When posting a problem, let us know some more specifics about the process.

Wire type, size, AND BRAND.
Machine polarity.
I'm assuming this was GMAW and not FCAW?
And the biggie for this one maybe: What shielding gas and flow rate were you using?

Looking forward to the picture. Does the start on top of a previous bead look somehow different from a start next to other CLEAN, SHINY, NO-MILL-SCALE metal?

[gnm109]

I presume you're using fluxcore......? That could cause brown soot. Regular MIG (GMAW) is cleaner.

[Sandy]

Moths are the worst...they just come out of nowhere and annoy the heck out of you.

Around here I get a lot of wasps and mud dobbers. They come in at mach II. Sometimes there is a pretty good flash so they must be carrying a lot of fuel or itty bitty bombs maybe!!!

[stonebreaker]

Originally Posted by MAC702

When posting a problem, let us know some more specifics about the process.

Wire type, size, AND BRAND.
Machine polarity.
I'm assuming this was GMAW and not FCAW?
And the biggie for this one maybe: What shielding gas and flow rate were you using?

Looking forward to the picture. Does the start on top of a previous bead look somehow different from a start next to other CLEAN, SHINY, NO-MILL-SCALE metal?

Um, off the top of my head, the wire is Weldcraft .030 copper coated mild steel wire (not sure of the exact alloy number, will check when I get home; the sales guy threw it in when I told him it was for general welding).
Machine polarity = DCEP
GMAW, with flow rate set at 20 cfm with 75/25 Ar/CO2 mix. The flow rate, btw, isn't actually a real flow rate - it's just a reading on the oulet pressure gauge of the regulator that came with the welder.

I'm welding quarter inch plate, and am using Miller's recommended settings of V=10 and wire speed =7.5 (the knobs on the machine just have a scale of 1-10, not actual values).

[Roy Hodges]

Back in the 70's , i worked for Kaiser Steel (rip) . In the summer , sometimes these bugs , with long wings would come into our hoods & into the welds , by the thousands , so bad i wanted to go home .What a mess . maybe it was because we were only a few feet from the Napa river. Maybe they was some kind of knat ?

[stonebreaker]

Originally Posted by smithboy

Moths are the worst...they just come out of nowhere and annoy the heck out of you. You might be just letting the weld get cold before starting up again and need more juice to form a puddle on the now thicker piece of metal (with bead). lots of soot is generally associated with poor gas coverage. You might be running your gas on the low side or not getting the tip of the gun close enought to the metal. Turn up you heat and gas a bit, get close, and see if it helps on the starts...

I'll try turning up the gas.

[prop-doctor]

the worst here is the brown june bugs at night in the shop they love the light

[MAC702]

Originally Posted by stonebreaker

Um, off the top of my head, the wire is Weldcraft .030 copper coated mild steel wire (not sure of the exact alloy number, will check when I get home; the sales guy threw it in when I told him it was for general welding).
Machine polarity = DCEP
GMAW, with flow rate set at 20 cfm with 75/25 Ar/CO2 mix. The flow rate, btw, isn't actually a real flow rate - it's just a reading on the oulet pressure gauge of the regulator that came with the welder.

I'm welding quarter inch plate, and am using Miller's recommended settings of V=10 and wire speed =7.5 (the knobs on the machine just have a scale of 1-10, not actual values).

Okay, all of that looks good, so we'll rack our brains elsewhere.

Though that really IS a flow rate. That low-pressure gauge is calibrated for a specific orifice size, so it converts the pressure to a flow for you. With typical atmospheric pressures as its reference, it is more than accurate enough for us.

[smithboy]

Just remember, once you have a bead on the metal, it's now thicker at the bead than 1/4 inch.

[stonebreaker]

Originally Posted by smithboy

Just remember, once you have a bead on the metal, it's now thicker at the bead than 1/4 inch.

OK, so how do I deal with that?

Here are the promised pics:



This second one shows the amount of soot that runs ahead of the bead before I clean it up.


And here's the tag from the welding wire:

[mopar74]

try cutting the tip of the wire before restarting, (get rid of the little ball on the end) Kevin

[MAC702]

Try Kevin's idea. The MM175 is known to have not quite as good an arc start as the Lincoln SP-175+, even though it is still one of my favorite machines. At least we'll remove this variable from the equation.

But, I'm leaning toward the cold start theory. With the extra thickness at the start of a new bead on top of an old bead, the start may be just a little cold before it starts to penetrate after a few seconds.

[stonebreaker]

OK, how do you deal with a cold start? I saw some of the students who were doing MIG in the welding lab while I was learning TIG, and their starts didn't look nearly as rough as mine. Of course, they were using bigger machines...

[Rojodiablo]

Originally Posted by stonebreaker

OK, how do you deal with a cold start? I saw some of the students who were doing MIG in the welding lab while I was learning TIG, and their starts didn't look nearly as rough as mine. Of course, they were using bigger machines...

You can lead (LEED)the junction by 1/4 inch or so. Start on the steel, and then back up quickly to the junction to make the connection with the existing bead. Then, motor forward a little fast to get past your start point and after that, cruise... It will allow you to get heat flowing, and a good arc started. And with the Miller 175, I have little problem running over my bead. It works well at that. But the hot start arc is maybe not as good as it could be. Just don't stop once you start!!! Let the steel burn, and it will be good with a little practice.

[smithboy]

To me, the first junction doesn't look that bad. But, the second looks like you started away from the first weld and ended up on top of it. Is that what you did? Also, about what is the angle you are holding the gun nozzle as you move? That dark soot is generally a sign of a gas coverage problem, but the soot is always on one side (the left) of your junction, so the angle of attack may be at issue as well as the gas flow rate. Increase the flow rate to about 22-25cfh and try holding the gun closer and at 90 degrees (perpendicular to the workpiece) on the start and then proceed into your normal drag angle as you start to move. If you starting the weld away from the initial bead and are moving toward it, try moving from your normal drag angle into a 90 deg angle as you approach the tie-in point (maybe even a bit past 90, to push the gas ahead to the tie-in as you stop).

Just some thoughts...hope we helped.

[stonebreaker]

Thanks guys. Sounds like what I really need is more practice.

[TEK]

All the above are good answers but think about this. When you pull the trigger, do you get gas coverage instantly? It takes just a split second to get a complete envelopement in which time you have your problem. My $.02....

[Pat the Welder]

I've used guns (Binzel on a Murex set) where by half pressing the trigger you get a pre-flow of gas before the wire comes. Maybe try that?
Not sure if the Miller does it though.

[Sandy]

I going to throw in the observation that I don't see one single really really clean spot in the pic you did post. Not to be chastising but the gas and solid routine would like the base metal a little cleaner.

Do an experiment for yourself and try the same operation on a piece that's polished up good. You won't see as much slag or smoke dust. Not that you absolutely have to polish everything up to a mirror finish but it helps demonstrate the difference. I may be dreaming but I believe that with clean metal more of the heat gets used for wetting out and blending the add mixture and not so much used in burning out the crud.

Good pic BTW. I wished I could get close ups like that.

[stonebreaker]

It's hard to see in the second pic, but if you look closely in the first pic, you can see where I ground all the mill scale off down to the shiny metal. It's just covered by soot. I took the pictures with my cell phone, btw.

And Tek, the gun that came with the welder won't start the gas before the wire. Wish it did, tho. Wouldn't mind a pre-flow timer, either, but hey, I'm on the cheap here!

Incidently, I got my welding table finished. As I mentioned before, a buddy let me root through his scrap pile, so the top is made from 5 separate pieces of steel - two pieces of channel and three smaller pieces of 1/4" plate. The legs are 1/4" x 3" angle. Looks like it would be right at home in Fred Sanford's yard, but thanks to your suggestions the welds are sound - I should be able to set an engine block on that thing no problem!