[GDK_1968]

I feel that no question is a stupid question, only un-asked ones are, so here it goes...

I have had two differing methods when it comes to MIG welding new pans, whether floor or trunk. The first being weld the entire seam and then seal top and bottom with sealer. The other is to stitch-weld by welding the new pan 1 or 2 inches then skip 3 or 4 repeating this the entire perimeter of the new pan then seal top and bottom with sealer. The second method I have been told allows the new pans to flex and give.

Which is the proper method?

[GDK_1968]

I feel that no question is a stupid question, only un-asked ones are, so here it goes...

I have had two differing methods when it comes to MIG welding new pans, whether floor or trunk. The first being weld the entire seam and then seal top and bottom with sealer. The other is to stitch-weld by welding the new pan 1 or 2 inches then skip 3 or 4 repeating this the entire perimeter of the new pan then seal top and bottom with sealer. The second method I have been told allows the new pans to flex and give.

Which is the proper method?

[gtoguy1967]

im not a pro but i like the skipping method as it gives it time to cool and you get less warpage.
Don

[ June 18, 2002, 05:12 PM: Message edited by: don ]

[Greg J]

I welded my pans in by stitch welding an inch or so, then skipping several, and continuing around the perimeter until I had a continuous weld. Be careful not to overheat or weld too much continuously in a single area as you can warp the pans and possibly twist the body. Having the body still on the frame helps.

[GDK_1968]

Thanks guys. Looks like the stitch method seems the way to go. So the entire seam should be welded then, correct, just not in one continuous attempt? Must allow for the metal to cool to prevent warpage and melt-down. Sounds like a plan.

[gtoguy1967]

safty first have a fire ext close by and mabey a bucket with water and a rag. undercoating and even paint will burn.
Don

[Marty Phipps]

Here's another option for you.
Allow a 1" overlap between the panels.
Drill a series of 5/16" diameter holes, approximately 1/2 " down from the top edge of the new panel.
Space the holes 2-3 inches apart.
Thoroughly clean the overlapping old floor section, apply weld through primer.
Set the new floor pan back in.
Start "plug-wleding" the two panels together.
after you're done, clean and dress the weld's.
apply a thin coat of primer.
THEN seam seal the inner and outer edges.
Now before any one questions this method, I'll explain.
Any time you overlap and weld two panels together.
Regardless of continuous welding or stitch welding.
You get a certain amount of shrinkage.
If the factory spot welds aren't 100%, you can actually cause premature failure to these, by "stressing" them.
Also, a fully welded piece goes through a series of "work-hardening"
In mild steel the metal actually decreases in strenght at the welds.
By "plug-welding" you don't add undue stress at the factory spot welds, plus there's enough room for the metal to expand and contract.
Add in that since you're not fully welding the panel, you're not changing the molecular structure of it.
So no work hardening.
Plus a proper plug weld is very, very strong.

[Skip Fix]

Man , that method sounds like something even I cn do.

[larry davis]

Marty's technique is the most structurally sound. But I would still skip around in doing these "spot" or actually plug welds.

Of interest is that regardsless of the joint design or metal thickness, any proprietary welding process produces extreme metalurgical stress's in all base metals.
This is usually most pronounced at the & along the toes of the weld, in thw heat affected zone (HAZ).
In this zone is where most defects or failures will develope. Assuming the actual weld bead is sound, the HAZ is so traumatically affected by the rapid super-heating, fusion, & cooling process's that there is little if any actual stress relief. The desired properties that were present in the parent metal has now been changed. To whay extent is dependant on carbon % of the weld metal & parent metal, temperature(s), time at that temp., speed of cooling, etc...

In welding sheetmetal for our car hobby, it is not practical to stress relieve body panels. An argument could be made for peening welds and weld zones w/ a needle gun, but that is splitting hairs & would probably add to the body work required to finish acceptably.
As stress relieving is not practical, I use a blow gun to cool my stitch &/or spot welds immediately afterwards. I will weld 1"-2" then cool. This does not allow the heat input to spread into the surrounding panels too far, thereby inducing warpage.

Need to know anything else about welding or related process let me know. [img]smile.gif[/img]