Question on Technique for Long welds in 3/16" plate

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I'm a relatively new welder and currently fabbing a gas tank skid for my Jeep from 3/16" steel plate.  The skid is basically a shallow, open-top box, with dimensions of ~ 35"L x 18" wide X 5" high.  My welder is a MM 251, running 75/25 and .045 wire.  Voltage and feed settings will be close to what's on the welder for this thickness using .045.  I don't have them in front of me, but I think it was ~ 17V and 200.  I realize .045 wire is a little thick for this material, but it's what I have to work with.My question is on how to do the welds for this skid.  My plan would be to weld the seams on the interior of the box first, but I'm wondering if I can just run continous long beads on each side, or do I need to weld shorter length beads and skip around to avoid too much warping?  After the interior is done, I would weld a bead on the exterior seams.Here's a crappy cell phone picture of the skid in progress.  Any inputs from the experts appreciated.  I have to finish fabbing up the front piece (it's a little tricky since it has to be bent at a slight angle) and the attachments that bolt it to the Jeep today, and hopefully will have it all tacked and ready to finish weld by this evening.Thanks,John
Reply:I would backstep it with a MIG...... 6" right to left...... move right 12" and do another 6" right to left....That's what I would do....... there are probably many ways to skin that cat though...... and I don't really like MIG much anymore anyway......Precision TIG 185 and MP 210Bobcat 225NTCutmaster 42O/A tanks.... 2 Argon tanks...... 2 C25 TanksFacebook...... F2 Metal WorksETSY....... https://www.etsy.com/shop/F2MetalWorksF2MetalWorks.com....... http://www.f2metalworks.com/
Reply:Your settings sound good, maybe a tad more voltage. .045 wire should be just fine for that thickness.What you have in the pic looks good, but you might want to make your tacks a bit closer together, ideally not more than 3" apart. Clamp the main piece to your table while welding and leave it clamped while you weld and until it cools. Skip weld to distribute the heat evenly.HTH,JohnA few weldersA lot of hammersA whole lot of C-clamps
Reply:Thanks for the inputs!  I'll skip weld as mentioned.  The setting were a little low as suspected, what I was remembering were the setting for 1/8", which I had welded yesterday for a friend.  The 3/16" was 18V & 230.
Reply:If that is a steel table then turn that assembly over and tack weld it to the table.  Skip around doing all the corner welding on the outside.  Nothing longer than say 5 inches.  Let it cool then grind the tacks and remove it from the table.   Welding on the inside would be optional if you fitted everything with an open corner on the outside.
Reply:Do people even use the SEARCH feature?ZTFab has spent a great deal of time describing the EXACT process the OP would use for welding up his project.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:Originally Posted by SundownIIIDo people even use the SEARCH feature?ZTFab has spent a great deal of time describing the EXACT process the OP would use for welding up his project.
Reply:back step,and weld opposite corners, 3 to 6 inch welds....Miller bobcat 225g  - spoolmatic 1Mm250 - 30a spoolMiller spectrum 375Thermal arc 95 Everlast Ex 250As the boiler turns, these are the days of our lives
Reply:Think if you check out the "mig like tig" thread, you'll find everything you need regarding setup and weld sequence.As mentioned, backstepping will help to reduce warping.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:Whats your joint fitup on the outside of the box?Corner to corner? Butt? (which plate overlaps which)I would weld the outside first.  Their are way more shrinking stresses on an inside fillet than a outside corner weld.If you weld the inside first, it will suck the sides of your box in.  The weld around the outside will not be enough to try and pull it back into square.How i would weld this, would be all the outside corners first on your pieces that are going vertical, off the bottom plate.  Then i would weld all the other outside corners.  Then weld the inside fillet.I wouldnt weld the inside all the way around either, their is no need.  You wont gain any strength from welding short sections then all the way around, but it will definitely warp more.Just my 2 cents.
Reply:Thanks for the pointers!  Fit up is the vertical pieces rest on top of the horizontal bottom and outside edge of the vertical pieces are flush with the outside edge of the bottom plate.I did it this way to keep the bottom completely smooth, so the exterior welds will not be the lowest point.The point about sucking the sides in is a good one and I like the idea of not welding continous beads on the inside, as you mention it's overkill for the this thickness plate.Thanks,John Originally Posted by drivethruboy54Whats your joint fitup on the outside of the box?Corner to corner? Butt? (which plate overlaps which)I would weld the outside first.  Their are way more shrinking stresses on an inside fillet than a outside corner weld.If you weld the inside first, it will suck the sides of your box in.  The weld around the outside will not be enough to try and pull it back into square.How i would weld this, would be all the outside corners first on your pieces that are going vertical, off the bottom plate.  Then i would weld all the other outside corners.  Then weld the inside fillet.I wouldnt weld the inside all the way around either, their is no need.  You wont gain any strength from welding short sections then all the way around, but it will definitely warp more.Just my 2 cents.
Reply:Originally Posted by SundownIIIThink if you check out the "mig like tig" thread, you'll find everything you need regarding setup and weld sequence.As mentioned, backstepping will help to reduce warping.
Reply:why weld it continuos?is this supposed to contain leaks???
Reply:Originally Posted by weldbeadwhy weld it continuos?is this supposed to contain leaks???
Reply:Originally Posted by Mikel_24Probably not. as far as I know, underbody protection is just for preventing any sharp rock or whatever from puncturing the tank. So no, it does not need to to be welded all the way arround.Seems like all of us without a strong engineering background feel that more weld is always stronger.I am lazy so I would have tried to get most of the shape of the skid broken and then weld only the uprights and whatever piece is left. I also understand that the way the OP is doing it gets the most out of the raw metal, since you don't have to make a complicated layout to allow for bending.Mikel