Dee,
I'm busted! I was avoiding it too. However, you busted me so politely that I guess I owe you an effort.
Mig is not my strongest area but its one that has been thrust upon me recently, I also like whistles and bells (it' a weakness) which are common to mig power sources, so the last semester has been a big learning/teaching experience for me.
I'll try to be clear. The resistance built in the wire between the contact tip and the arc (stickout) is critical to clean operation in short circuit and spray transfer modes. I’ll talk more about the hands on stuff (effects) for short circuit and leave to arc physics to another poster. (Hopefully the mighty Submariner)
In short circuiting transfer mode, 100% CO2 is most active and less forgiving so it makes the best example of the effects we are discussing, however the effect is similar with mixes. As the length of the wire "stickout" increases the current (amperage) decreases whilst the voltage remains constant, this is what one ought to expect from a "constant voltage" power source. So if a procedure is set with the assumption that the nozzel to contact tip distance is, let’s say even, and you were to recess the tip ½ inch into the cup you would loose amperage. For instance if your data sheet suggested 150 inches per minute or 95 Amps with that even tip/nozzel configuration and you put the thing back ½ inch, your current output to the work could drop as much as 20 amps (this is pretty significant.) In MIG in either transfer mode or Flux-cored, consistency is real important. The above scenario might not make much of a difference in bead profile but it will effect side wall fusion and penetration, cause cold laps and blow a guided bend exam. Spatter problems are more often related to improper arc voltage settings.
In short circuiting transfer mode in wire diameters up to 0.045 the close tip/nozzel setup should not present an overheat, spatter or melted tip condition if the arc voltage is set properly. (Process control is everything when looking for MIG consistency)
For mild steel short circuit I prefer the single piece copper cups and for spray transfer and flux cored I like the two piece heavy duty Nozzel.
BTW, there are different sized contact tips, however you can also just keep dressing old ones down until they are significantly shorter and then put them to use for spray.
We can talk about spray and fluxcore later if you want but this is about enough for one post.
Lawrence,
You were quite clear and in your typical good form to start off the new year. I appreciate your fine effort.
I guess "spatter-free" actually is a bit subjective... I'll try to quantify:
I do get some spatter beads (round, like tiny ball bearings) when I weld. Unlike hot or oblong spatter I've seen from grossly misadjusted power supplies, these can usually be brushed away with a single wipe of my gloved hand. At the end of the day (probably translates to several good hours in a real high production shop) a tap or two of the nozzle will release a little bit of a spatter residue from the inside of the nozzle, and I take the opportunity to dust off and generally clean and inspect everything under the nozzle as well just for good form. I find a cheap 1 inch paintbrush (or my gloved fingers when I'm lazy) with half-length bristles usually suffices.
Compared to problems I've seen and heard other guys dealing with I have the impression I've achieved a functional level of mediocrity. When I describe my experience with spatter it's not dealing with anti-spatter sprays, creams, gells, chisels, grinders, or special vocabularies we use when ladies aren't around. I bought my first can of anti spatter spray before I ever came up on the board and it's still quite full. Sometimes I shoot a bit inside the nozzle (I think so I can get rid of it someday) but unless I really overheat the torch it seems unnecessary to reapply. It seems the antispatter itself causes the spatter I described to cling to the moist nozzle parts if I use too much of it.
Every job has a tit or two that adheres a bit better than I'd like (usually near the start of the bead) but scrapeing the side of a hammer along the surface generally knocks the toughest of them off very easily.
Fact is, my weld technique is a little sloppy (I guess I've been busted too!) and I was under the imprssion the human element was the culprit. I'd like t pose a serious question; please dont take it rtrtorically or as a wisecrack... how clean can you reasonably expect a manual (GMAW) short arc weld to be in the real world?
I'd really like to see some kind of research article about nozzle design and gas flow characteristics published in the Journal sometime.
Thanks & regards
D
D,
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