MIG pre-flow?

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First of let me say I'm new to mig welding. On my mig it takes a fraction of a second before the drive rollers is starting to feed the wire and you can hear the gas hiss and the contactor activate before the drive rollers are moving. If you do a fast on-off on the trigger you will only get gas for a fraction of a second and the wire will be completely still.Is this some kind of pre-flow function for the shielding gas or is there another reason for this?The trigger on the gun only has one switch. The entire gun and hose is detachable.The only settings I have on the machine is volt, wire speed and inductance. It's an inverter mig so it's "computerized".Last edited by Pete.S.; 04-13-2013 at 04:18 PM.
Reply:It may be intentional pre-flow since it's an inverter. Generally, only larger professional-grade MIGs have pre-flow (or post-flow), at least on this side of the pond.JohnA few weldersA lot of hammersA whole lot of C-clamps
Reply:It's a Lincoln and as you said I now see that Lincolns bigger inverter migs actually have pre-flow and other settings. They probably just set it to some preset value on my little machine.
Reply:the old style MIG machines don't have pre and post flow.....the newer machines like the Miller 350P does which makes it a good machine for stainless welding.......  Miller Dynasty 350Twenty Six HammersThree Crow BarsBig Rock
Reply:double post...
Reply:Originally Posted by B_Cthe old style MIG machines don't have pre and post flow.....the newer machines like the Miller 350P does which makes it a good machine for stainless welding.......
Reply:Originally Posted by Pete.S.So pre/postflow is most useful for stainless? Why is that if I may ask?Also what would typical setting for pre and post flow be? Are we talking about seconds like for TIG or is it much faster?
Reply:Those pretty colors that you see with stainless (straw, salmon, blue) are actually from oxygen hitting the stainless while its still hot.  There are actually charts out there that talk about what temp creates what color (blue being the highest, straw the lowest).  That's oxygen contaminating.In addition, stainless is sensitive to carbon - or at least the chromium would rather bond with carbon than stay next to iron atoms.  And by carbon I mean any kind, CO2, high carbon steel drill bits, and even steel wire brushes.  That why stainless is welded with extremely low CO2 - like 4% max.By using postflow and/or preflow you can minimize the chance for these contaminations.On TIG, post flow protects the glowing hot tungsten as well - that's where the minimum time typically comes from.As for post and pre-flow on MIG.  I have my pre-flow set at .4 (that's 'point 4') seconds.  The problem is that if I set it higher, and don't pick up the torch for some days, I'll pull the trigger, then pick up the gun and wonder why wire isn't coming out.  By that time it starts coming out and I now have to cut it back.  If I wasn't wearing a helmet I would be smacking myself in the forehead.  Looks like 3-stooges but there is only one of me .  Bottom line, it throws off my internal timing.  The only time I have every run into a gas related contamination issue was when I was trying to figure out how fast I could move the gun - seems like I was out-running the gas coverage.Flip over to the Multimatic 200...   I don't leave a bottle hooked up so I have to purge the lines every time and I don't always do a good job (rushing).  There is no pre or post flow that I can tell.  The welds always look clean with no visible signs of porosity (or anything else that I can see that would indicate contamination - I'm not an expert though).Using flow gauges, I use 25 cfh for steel and 35 for aluminum ('normal' size nozzles - Bernard centerfire type).Con Fuse!Miller Dynasty 350Millermatic 350P-Spoolmatic 30AMiller Multimatic 200Hypertherm PowerMax 1000G3Miller Maxstar 200DX
Reply:Thanks for the explanation., very interesting!