18 gauge cold rolled steel box - need fabrication tips

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I am new to welding. I have done some flux core but want to do some TIG for a special purpose (8) 18gauge steel boxes  4' by 8' by 1' (1 - 4' by 8' side open.) . My questions are as followsI know it needs to be tacked, 1. Do I tack the whole thing up first? In my mind this insures that it will be square when finished2. How far apart should the tack welds be? I assume that the closer the tacks are the longer a continuous final weld can be. 3. Should I make the continuous weld inside or outside of the box? If outside it might be 'easier' but but may not look as good, and not as strong. If internal it will be stronger but greater difficulty in corners.4. What can I do for the integrity of the corners? I am considering welding 2"*8" strips at a 45 degree angle to the adjacent inside surfaces, is there anything already made I can put in a corner and weld it in for greater intergity?Yes, I am going to be doing a lot of practice for quality of weld and heat sinking before tackling this. Thanks, any help or observation is much appreciated
Reply:I would get some aluminum angle for fit-up and to use as a heat sink when welding.Outside corner joints would be better/easierTacks every 4" or so would sufficeIf available or design permitting - roll beads in the sheet metal before assembling - adds a ton of rigidity.Failing the bead rolls - gussets would help for sure. Could also plug weld some angle into the corners too - should stiffen it up some.IMHO of course.
Reply:I weld alot of planter boxes as a hobby but mine are cnc cut 10-11ga 1/8".  I use a product called tru angles to square mine up.  On thinner stuff I would tack it all up and then finish weld.  Do you plan to use filler rod or no?  I havent done much sheetmetal so Im not much help above what I mentioned.I would tack the whole thing up with tacks every 3-5" and skip around welding strips and letting it cool.  Also if you plan to back it with angle iron on the corners maybe tack that in after you tack the box up but before final welding.  Just an idea.  Good luck and post pics.
Reply:Box and pan brake. Sheet metal is a whole other trade from welding.
Reply:You'll need a pretty healthy brake to bend 8' of 18 gauge.......
Reply:Originally Posted by weldypantsYou'll need a pretty healthy brake to bend 8' of 18 gauge.......
Reply:Originally Posted by Bosco99Standard in sheet metal shops but not DIY equipment. Might see some savings by having formed in a shop.
Reply:Originally Posted by weldypants  1. I would get some aluminum angle for fit-up and to use as a heat sink when welding.2. Outside corner joints would be better/easier.  Could also plug weld some angle into the corners too - should stiffen it up some. IMHO of course.3. If available or design permitting - roll beads in the sheet metal before assembling - adds a ton of rigidity. Failing the bead rolls - gussets would help for sure.
Reply:Just FYI, this is not a new to welding project. It is going to be frustrating and failure prone. When done it is doubtful it will be flat or square. Why don't you tell us what it is for and we can come up with a better solution. You would be much better off farming this out to a sheet metal shop, as even if you use a commonly available 5x10 sheet, you would still have and 8' weld plus the corners.
Reply:Originally Posted by scramblerjI weld alot of planter boxes as a hobby but mine are cnc cut 10-11ga 1/8".  I use a product called tru angles to square mine up.  On thinner stuff I would tack it all up and then finish weld.  Do you plan to use filler rod or no?  I havent done much sheetmetal so Im not much help above what I mentioned.I would tack the whole thing up with tacks every 3-5" and skip around welding strips and letting it cool.  Also if you plan to back it with angle iron on the corners maybe tack that in after you tack the box up but before final welding.  Just an idea.  Good luck and post pics.
Reply:Originally Posted by Bosco99Box and pan brake. Sheet metal is a whole other trade from welding.
Reply:Originally Posted by Bosco99Standard in sheet metal shops but not DIY equipment. Might see some savings by having formed in a shop.
Reply:Originally Posted by walkerJust FYI, this is not a new to welding project. It is going to be frustrating and failure prone. When done it is doubtful it will be flat or square. Why don't you tell us what it is for and we can come up with a better solution. You would be much better off farming this out to a sheet metal shop, as even if you use a commonly available 5x10 sheet, you would still have and 8' weld plus the corners.
Reply:18 ga. is .0478" thick. You cannot fabricate a 4' x 8' box using flat sheet 18ga. and expect it to have any rigidity. You will need an interior or exterior frame constructed of square tubing or angle iron to retail any shape.About the tig welding, or any form of welding thin guage sheet, always make a nice edge to edge corner fit and fuse the out side joints. If you attempt to tig the inside corners it will take too much heat and warp to no repair.Miller TrailBlazer 251Miller HF-250-1Miller MaxStar 150 STLHyperTherm PowerMax 380 plasmaLincoln PowerMig 180Millermatic 252Miller Diversion 180
Reply:Originally Posted by wagin18 ga. is .0478" thick. You cannot fabricate a 4' x 8' box using flat sheet 18ga. and expect it to have any rigidity. You will need an interior or exterior frame constructed of square tubing or angle iron to retail any shape.About the tig welding, or any form of welding thin guage sheet, always make a nice edge to edge corner fit and fuse the out side joints. If you attempt to tig the inside corners it will take too much heat and warp to no repair.
Reply:You have to make 8 of these?  How about breaking the one foot sides and having a lap joint (2-3 inch?) along the bottom.  Would save a lot of warpage.  Would this be out of sight for you?
Reply:You are talking about a very flimsy part when you get done.  You will have to take care handling to prevent bending or kinking.If I was to do it, I'd look for wider sheets, such as 8' x 20'.  I'd fold the two 8' sides.  I'd then make ends with 1" flanges on them that would be slid in place.  I would then spot weld the ends to attach them.  No way would I try to weld them full length.If all I could get is 4' x 10', I'd bend the ends up and spot weld in the long sides same as above.
Reply:Originally Posted by wagin18 ga. is .0478" thick. 1.) You cannot fabricate a 4' x 8' box using flat sheet 18ga. and expect it to have any rigidity. You will need an interior or exterior frame constructed of square tubing or angle iron to retail any shape.2.) About the tig welding, or any form of welding thin gauge sheet, always make a nice edge to edge corner fit and fuse the out side joints. If you attempt to tig the inside corners it will take too much heat and warp to no repair.
Reply:Easy, tig the outside corner joints, lay some .030" wire in the joint if you need to fill it up. the thing will be floppy as hell unless you have a structural perimeter around the open end, even then if it's holding anything the sides are going to want to bow out depending on the dimensions and load.Welding/Fab Pics: www.UtahWeld.com
Reply:Originally Posted by weldypantsNot to split hairs here but carbon steel should not be fusion welded especially in an application where there may be flex and known rigidity problems as it will crack.Stainless - whole different ballgame there though.....
Reply:You can offer two of the five sides together and hold one corner, then tack with a blast of amps through the tungsten. Work lengthwise tacking every 2". Just use your hands to coax it to a proper fit. A brake is the best way to form at least two angles. The long welds will shrink like elastic. The box won't lay flat after being welded. Would it work to use angle at your joints, and stitch weld.An optimist is usually wrong, and when the unexpected happens is unprepared. A pessimist is usually right, when wrong, is delighted, and well prepared.
Reply:Originally Posted by OldSparksYou have to make 8 of these?  How about breaking the one foot sides and having a lap joint (2-3 inch?) along the bottom.  Would save a lot of warpage.  Would this be out of sight for you?
Reply:Originally Posted by Willie BYou can offer two of the five sides together and hold one corner, then tack with a blast of amps through the tungsten. Work lengthwise tacking every 2". Just use your hands to coax it to a proper fit. A brake is the best way to form at least two angles. The long welds will shrink like elastic. The box won't lay flat after being welded. Would it work to use angle at your joints, and stitch weld.
Reply:Originally Posted by MikeGyverEasy, tig the outside corner joints, lay some .030" wire in the joint if you need to fill it up. the thing will be floppy as hell unless you have a structural perimeter around the open end, even then if it's holding anything the sides are going to want to bow out depending on the dimensions and load.
Reply:I had a chance to play around with some 14 gauge this weekend. It seemed VERY rigid but I did not do a whole mock-up.  I will have to also get some 16 gauge and see what that does.