FCAW lap joints on 18-22ga sheet

[Jehannum]

I'm fairly competent with my welder (a basic FCAW Lincoln 35-88a unit), but I'm going to help a friend tomorrow make a replacement for the lower air guide in her 3rd gen Firebird. It doesn't have to be pretty, but it would be nice if I didn't blow any holes in it. I figured my best option would be to weld the duct into the guide using lap joints.

So, any sage advice for FCAW on 18-22ga sheet doing lap joints?

 

[mrolds88]

Personally I would do **** joints. Keep gap tight and tack every several inches then tack in between and continue until fully welded. Let it cool a bit in between sets of tacks. Practice on some spare pieces to get amperage correct. I personally don't care for lap joints because it leaves places for moisture to collect.

 

[PugetDude]

Personally I would do **** joints. Keep gap tight and tack every several inches then tack in between and continue until fully welded. Let it cool a bit in between sets of tacks. Practice on some spare pieces to get amperage correct. I personally don't care for lap joints because it leaves places for moisture to collect.

+1 on this. Practice on scrap so you can tack without burning through.

 

[MoonRise]

I'm fairly competent with my welder (a basic FCAW Lincoln 35-88a unit), but I'm going to help a friend tomorrow make a replacement for the lower air guide in her 3rd gen Firebird. It doesn't have to be pretty, but it would be nice if I didn't blow any holes in it. I figured my best option would be to weld the duct into the guide using lap joints.

So, any sage advice for FCAW on 18-22ga sheet doing lap joints?

Advice?

Yeah.

Do GMAW with C25 and a **** joint. Use 0.025 solid wire.

Lap joint WILL trap crud and moisture behind it, leading to rust from behind.

FCAW on automotive work will usually lead to failed paint from FCAW residue.

FCAW burns 'hotter' than GMAW (due to the higher current density through the electrode and the chemistry/physics of the arc and the metal transfer). Which leads to a much easier result of blowing holes through the sheet metal. 0.035 FCAW wire can blow a hole through 1/8" thick steel with a 'small' welder not even on MAX.

Use the 'right' tool(s) for the job and results are usually better than using the 'wrong' tools.

FCAW and 22 gauge sheet metal is the wrong tool for the job. IMNSHO.

Even with GMAW and 0.025 wire with C25, you should practice a bunch before hand. You can still blow a hole right through 22 gauge steel with GMAW.

And the procedure for welding thin sheet metal is NOT to run a bead along the joint seam.

You line up the pieces in a **** joint, clamping them as appropriate. Make a weld dot that actually achieves penetration/fusion into the base materials (not just a superficial 'tack weld'). Go to the other end of the joint, and make another weld dot. Then make a weld dot in the middle of the joint length. Let the metal cool off. Maybe you gently grind down the weld and also planish the seam. Then you make a few more weld dots. Grind and planish. Repeat until done.

Look up MP&C's project page here on GJ for some pictures and descriptions of welding automotive sheet metal.

 

[Jehannum]

Advice?

Yeah.

Do GMAW with C25 and a **** joint. Use 0.025 solid wire.

Lap joint WILL trap crud and moisture behind it, leading to rust from behind.

FCAW on automotive work will usually lead to failed paint from FCAW residue.

FCAW burns 'hotter' than GMAW (due to the higher current density through the electrode and the chemistry/physics of the arc and the metal transfer). Which leads to a much easier result of blowing holes through the sheet metal. 0.035 FCAW wire can blow a hole through 1/8" thick steel with a 'small' welder not even on MAX.

Use the 'right' tool(s) for the job and results are usually better than using the 'wrong' tools.

FCAW and 22 gauge sheet metal is the wrong tool for the job. IMNSHO.

Even with GMAW and 0.025 wire with C25, you should practice a bunch before hand. You can still blow a hole right through 22 gauge steel with GMAW.

And the procedure for welding thin sheet metal is NOT to run a bead along the joint seam.

You line up the pieces in a **** joint, clamping them as appropriate. Make a weld dot that actually achieves penetration/fusion into the base materials (not just a superficial 'tack weld'). Go to the other end of the joint, and make another weld dot. Then make a weld dot in the middle of the joint length. Let the metal cool off. Maybe you gently grind down the weld and also planish the seam. Then you make a few more weld dots. Grind and planish. Repeat until done.

Look up MP&C's project page here on GJ for some pictures and descriptions of welding automotive sheet metal.

There may come a day when I buy a GMAW, but it's not tomorrow.

 

[bigguns69]

Copper backer plate. Can make out of a chunk of copper pipe 1 1/2" or bigger, pounded flat or Copper/Brass door hinge with the pin mounts cut off. clamp in place or can hold in place till gets hot.

It acts as a heat sink and supports the **** weld joint particularly on thin material. Good luck. A lot of new cars use lap joints for panel connection so wouldn't be opposed to that either, makes it easier. My 2 cents.

 

[Jehannum]

Jumped the gun, and thought I had much more to do. Turns out I just had to repair a headlight bucket, and the shroud for the bottom is just two bends and two cuts. Easy peasy.

 

[Jehannum]

Copper backer plate. Can make out of a chunk of copper pipe 1 1/2" or bigger, pounded flat or Copper/Brass door hinge with the pin mounts cut off. clamp in place or can hold in place till gets hot.

It acts as a heat sink and supports the **** weld joint particularly on thin material. Good luck. A lot of new cars use lap joints for panel connection so wouldn't be opposed to that either, makes it easier. My 2 cents.

Cool idea, thanks!

 

[MP&C]

The issue with lap joints is you have one layer of steel on one side of the joint, two layers on the other. The differing thicknesses equates to differing expansion and contraction times during exposure to the sun, I would think especially in the NM area. This leads to a ghost line in the paint finish that shows exactly where the seam is located. See this video for an example, this is a tailgate on a station wagon that had lower edge rust repaired using lapped seam. (not my work)

.

 

[Jehannum]

The issue with lap joints is you have one layer of steel on one side of the joint, two layers on the other. The differing thicknesses equates to differing expansion and contraction times during exposure to the sun, I would think especially in the NM area. This leads to a ghost line in the paint finish that shows exactly where the seam is located. See this video for an example, this is a tailgate on a station wagon that had lower edge rust repaired using lapped seam. (not my work)

.

No doubt that would be a consideration if I were working on a body panel. I was just making an air guide that bolts to the bottom of the car between the fascia and the radiator core support.

I had thought there was a duct that ran up to the radiator, but that turned out not to be the case, it's just a simple sheet with two bends at the end.