Weld 16 to 20 gauge or 20 to 16?

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As a relative newb, I have what may be a relatively simple question:I'll be replacing the floorpans in a car project, but I need to bake a sunken box to put the floorpans in (I need a little extra foot room).What I have decided to do is create a 4 sided box out of 20 gauge sheet with both upper and lower lips (upper lip to attach to car body and lower to provide a shelf for the actual pan).  Picture sort of a pizza box, with the bossom being made of 16 gauge and the walls being made of 20 (with another lip at the top on all 4 sides to secure it to the existing floorpan (Im leaving about .5" good metal all the way around))The pan will be a square of 16 gauge. I plan to rosette weld the pan to the box.  I also plan to construct the box and pan outside the car, and then drop in it, so hopefully I will be better able to fit and weld when I have a little elbow room.So, my question is whether I should punch my holes in the 16 gauge and have the 20 as the backer or have my holes in the 20 and use the 16 as a backer.I am using .030 flux cored wire (boo hiss -- I know -- just dont have the money to upgrade to full MIG at the moment-- so this project will get done with flux) and have had some trouble burning holes in thinner material before.  As such, which does everyone think would be the most advisable way to proceed?The holes in the 20 gives me the 16 as a heavier material to start my weld on, but may be tough as I could burn up the edges of the 20 as the puddle gets near.  Plus in this situation, the welds will be facing the ground when the boxes are installed in the carThe holes in the 16 gives me heavier edges for the holes, but only gives me 20 to start my weld on, which I may very well burn through.  I honestly have no idea which may be better or if it even matters at all.  ps.  I hope the pictures help.  Sorry in advance for the weakness of my MS Paint skills    The last pic is a profile in case it helps folks to visualize. Attached Images
Reply:Why the two different gauges? Why not make it all out of one?Yup
Reply:It really doesn't matter.  Keep the holes small so you don't build too much heat doing the plug welds.David Real world weldin.  When I grow up I want to be a tig weldor.
Reply:Good question.  I'd like to make it all out of 16 gauge, but I dont have the capability to bend 16 gauge.  Especially at a 16 inchs wide, my rinky brake can only manage about 20 gauge.  Thus the 2 different gauges.
Reply:As for the holes, I was planning on using 3/8", either with a spot weld cutter or with a punch and flange tool.  Actually this punch and flange tool http://www.harborfreight.com/cpi/cta...temnumber=1110 if anyone thinks its decent.  Should I consider smaller than 3/8"?  Thats just about right from what I've read.  Should I consider 1/4"?
Reply:Originally Posted by superkaz661As for the holes, I was planning on using 3/8", either with a spot weld cutter or with a punch and flange tool.  Actually this punch and flange tool http://www.harborfreight.com/cpi/cta...temnumber=1110 if anyone thinks its decent.  Should I consider smaller than 3/8"?  Thats just about right from what I've read.  Should I consider 1/4"?
Reply:Hello superkaz661,Seems like if the 16 flat is punched, rather than the 20 Z, you will retain more strength in the Z's thinner flange; the difference is probably insignificantBut... Do you really need to punch and  plug weld it?How about alternating beads between the heel and toe of the Z's bottom flange?Of course you want no chance of the pan's square edge catching the driver's heel, on it's way to the brake pedal! Whatya buildin' ?Good LuckLast edited by denrep; 07-07-2008 at 06:35 AM.
Reply:Originally Posted by denrepHello superkaz661,Seems like if the 16 flat is punched, rather than the 20 Z, you will retain more strength in the Z's thinner flange; the difference is probably insignificantBut... Do you really need to punch and  plug weld it?How about alternating beads between the heel and toe of the Z's bottom flange?Of course you want no chance of the pan's square edge catching the driver's heel, on it's way to the brake pedal! Whatya buildin' ?Good Luck
Reply:The HF 1110 works well on 16 ga.When flanging, make sure the sheet is seated against the stop.It ain't your grandma's box!!
Reply:Quote:  "Between rust, Lucas wiring and the multitude of other things that the Brits did backwards,"Know why Brits drink their beer warmCuz Lucas makes refrigeratorsI r 2 a perfessional