my tack welds not holding and ping free why?

[delcbr]

I have a Clarke Mig welding machine 135TE Turbo and 0.6mm mild steel clarke welding wire. proper BOC Argoshield light.

Whenever I am welding on my cars there can be a space between the new sheet I am placing on top of the original clean shiny metal as its difficult to get a new piece to contour exactly but its close.

So I use one hand to press new metal onto old where I am going to tack weld so there's no air gap and mig weld a tack to hold it not a lot of force is needed to hold in place but there is resistance of course I keep the pressure on it for a further 10 secs to allow it to hold.
But...after 20-60 secs of a single tack weld "ping"!!! and breaks the weld and doesn't hold!!!
Its so frustrating.
Where am I going wrong?

 

[outofthefire]

I keep the pressure on it for a further 10 secs to allow it to hold.
But...after 20-60 secs of a single tack weld "ping"!!! and breaks the weld and doesn't hold!!!

Your not soldering for a start off, if its tacked properly it will hold itself, you dont have to wait for it to cool. Basically your not getting any penetration your just getting a bit of weld on the surface and as it cools it distorts and breaks the tack.

 

[pedrobedro]

If it pings off you've not welded it. You are not getting penetration. Make the plates to fit better so they touch the base metal and weld the join then knock the metal down tight to make it fit better and weld the rest.

 

[mike 109444]

As the other guys have said But I would add once you have sorted your power setting ect then use something like a car jack and maybe a block of wood to hold/press the plate in place to allow tacking. This will then be one less thing you are trying to do whilst under the car ie holding plate in place, aligning the tourch with the edge of new plate, starting weld and seeing what you are doing !!! Yep been there done that....... I use to have a selection of steel rods, wood wedges and long screwdrivers ect then would wedge between ground or wood wedge and new plate, tap with mallet to force it to push up the new metal for a tight fit.

 

[mike 109444]

Like this....

 

[PhillipM]

Generally you want a bit more power on for tack welding than you would use for a seam.

 

[F.J.]

The piece you are welding to old steel must be under quite a bit if tension for it to break after what could be 70 seconds after welding. Try and shape the new section better too before attempting to weld it in place

 

[BillJ]

Generally you want a bit more power on for tack welding than you would use for a seam.

As above - MIG suffers from "cold starts" because it takes a certain amount of time for the cold metal to heat up and all the while you're feeding wire in and trying to weld. Bad enough when you're putting down a seam but when doing a tack, then the whole weld is a "cold start". Hence the need to whack up the power when tacking to get proper penetration.

 

[jonnyBMX]

sounds a bit silly... but is the metal properly clean? if its not this can cause many impuritys in the weld and for it to be weak.

 

[Paul the Wall]

as said before just turn the power up. this will get you the penetration and the tack will hold. I tend to use my thumb (when wearing gloves) or the hammer shaft....hence most of the bottoms of my hammer shafts are...well..charcoaly

 

[hussy]

for most car bodywork i run my mig (3ph powermig3500) setting 2/3 so pretty low, for tacking i ramp it up to normally 4, tacks dont have the benfit of the area being prewarmed, i learned thi sthe hard way whilst building canal boats as a subcontract job, ran a seam then didnt reset the power before tacking on a bent plate (we used a forklift and 10ton pullers to curve the front of the boat) stuck some stitch tacks on and one broke, luckily the forktruck was parked close and it took the impact i got broken ribs and a nice scar to remind me!!

 

[Mid Wales welder]

As all have said, you need a good fit up, clean metal and i allways turn up the power to twice what i would weld at then just give it a very quick blast! This works on most thickness's of metal.

 

[Kafer_meister]

I hate tacking panels to the underneath of cars, it's always a bloody horrible job, especially now that I don't have a nice 2 post ramp. Unless it's mine or a very good friend's, it gets sent to someone else these days.

If you can, start your weld on the sub panel and try and flow it onto the edge of the replacement panel. The panel that doesn't have an edge will take longer to heat up as it'll dissipate more quickly. The edge of your new panel will melt almost instantly as the heat doesn't have as many places to go. You'll find that your welds are always stuck to the edge, but ping off the sub panel I bet?

Are your cars 90's volkswagens by any chance?