Weld Penetration on 20 ga sheet metal

[reedjinnette]

First time poster here and new to the metal working game. I'm in the process of restoring a 65 mustang convertible. My goal is to do everything myself. So far, the driveline has been completely rebuilt and I have a few more small projects left on the mechanicals before I start tackling the fun stuff (body work).

I've already begun practicing/experimenting with the sheetmetal welding on some old scraps that I've picked up. I've got a 110 V MIG machine running .023 solid wire and have had pretty good results with butt welding 20 ga sheet metal.

I've been able to dial in the speed and the heat to achieve nice flat welds. When I adjust the MIG to achieve these welds, the tacks look identical from the front and the back of the weld. When I slow the wire speed down to reduce penetration of the weld, the weld becomes a lot more proud of the of base sheet metal and in some cases, almost resembles a ball. My gut tells me that the first setting is the correct one, but that then leads me to a question.

Will the protrusion of the weld on the backside of the material cause problems when trying the stretch the metal back out after welding? I'm using a method outlined in one of the forums in which I apply overlapping tack welds for about an inch. Then, I dress the welds and stretch the metal before repeating the cycle.

My thought is that the dolly would need to contact the base sheet metal on the backside, and not just the protruding weld, in order to properly stretch the metal.

I don't want to have to clean up both sides of the weld, because when I actually get to the car, it may be difficult or impossible to get a die grinder to the back side of the weld.

Any insight would be much appreciated.

By the way, this forum is unbelievably valuable resource to use newbies. Great job.

[Noplacetosail]

Cool car ! Think about it like this when setting a mig

voltage = arclength (but you can think of it as weld width)
IPM = penetration + filler material

What gas are you using?
CO2 = most penetration least cost worst weld flow and appearance
75/25 = average penetration middle cost good weld flow and appearance
92/8 = least penetration highest cost best weld flow and appearance

Most body shops that use mig use .023 wire and 92/8 gas

good luck
www.weldguru.com (http://www.weldguru.com)

[reedjinnette]

I've got the Hobart Handler 140, which has the 4 tap voltage control, and it's set on the lowest setting. In addition, I believe my feed rate is about 35 ipm. I'm also using the 75/25 shielding gas.

I'll guess I'll play with the wire speed a little more to see if I can't reduce the penetration a little bit.

[JD in SoCal]

I've been able to dial in the speed and the heat to achieve nice flat welds. When I adjust the MIG to achieve these welds, the tacks look identical from the front and the back of the weld. When I slow the wire speed down to reduce penetration of the weld, the weld becomes a lot more proud of the base sheet metal and in some cases, almost resembles a ball. My gut tells me that the first setting is the correct one,...
Your gut is correct.

Will the protrusion of the weld on the backside of the material cause problems when trying the stretch the metal back out after welding?
Yes, you will have to grind off the protrusion (if it is substantial) in order to work the metal with a hammer and dolly. If the bead is reasonably smooth on both sides, you can stretch the metal (counter act the shrinking that the weld caused) without grinding first.

JD

[Noplacetosail]

In spots where you can reach the back you can use a heat sink pushed against the back side to help minimize burnthrough and flatten protrusions. I have also noticed this tends to make spots broader and flatter. I actually have one I made out of an old chunk of copper that I made the face flat with slightly rounded edges and I brazed a handle on it. I just stick it on the backside and spot on the front.

just my 2 cents
Don
www.weldguru.com (http://www.weldguru.com)