Preflow for mig?

[1990notch]

Do any mig machines exist that have a gas preflow setting? I typically pull the trigger for a couple seconds the first time I turn it on for the day to purge the line and get the gas to the torch. Even though I do this, I occasionally get some porosity in the beginning of the first bead if the machine sits for a while between beads. I think it's due to lack of a gas shield at the start. I always clean the metal well before welding, so I don't think it's oil or dirt causing it. I got to wondering why a preflow does not exist with mig machines, that I've seen anyway.

[Rocky D]

Yes they do, however a technique will over come this...you strike the arc a little ahead of where you want to start the weld, and back up to the starting point, and by that time, you will have established you arc and gas flow. It's the same technique as used in stick welding....soon you will do it with out thinking about it.

[1990notch]

Yes they do, however a technique will over come this...you strike the arc a little ahead of where you want to start the weld, and back up to the starting point, and by that time, you will have established you arc and gas flow. It's the same technique as used in stick welding....soon you will do it with out thinking about it.

I'll have to try that. How far forward do you strike the weld before moving forward? I would guess about an eighth of an inch would do it.

The little 110v machine I started mig welding with was a Cebora that had a manual valve in the torch body. I used to crack the valve for a second before pulling the trigger further and starting the arc. I used it for many years before getting the TA migs with solenoids.

[Rocky D]

I'll have to try that. How far forward do you strike the weld before moving forward? I would guess about an eighth of an inch would do it.



No. it's about 1/2"...you hafta give it time to establish the welding arc. As soon as the arc is struck, you are already moving it back to the start...then you weld over any boogers that may have formed.

[tigster]

Rocky, thanks for that tip. do you do this technique every time, or just if the machine has been idling for a while without any gas flowing?

[Rocky D]

It's a habit..I do it every time.

[MIKIEweld]

But does anyone here actually know the science behind the theory??
MIKE

[Rocky D]

...I think it's due to lack of a gas shield at the start. ...

It's a combination of things...gas flow and mainly the arc is searching a way to ground...it takes a split second to do all that and this is why when you drag start to get all things, (the arc and gas and stickout) right for welding. Some machines have a hot start which can be used to help get the arc going.

[Roger]

After your making good welds welding schools often have students make a box out of steel plate. They pressurize box with a few pound of LP air to leak test you welds. Great wake up call. Must use Rocky Dś starts to pass this test.