How To Get The Job Square and Flat?

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How do you professionals get a job such as table frame - four legs sticking out of each corner of a square that the table top goes on - (1) square - so's the legs are at precisely 90degrees to the table top and the 'top frame' (if you see what I mean) is itself square and (2) get the 'top frame' so's it doesn't warp a little?I recently made a little coffee table from 20mm x 20mm x 2mm steel and the way I did it was to clamp the pieces on a piece of flat plywood about a metre square. Then I welded them.first I did the 'top frame' as I'm calling it. The square shape that would sit underneath the table top.  I would weld one corner then turn it around to bring another corner to my working position, clamp and weld.Then I used the same technique to do the legs. Clamped the frame on the board and clamped a leg to the frame at the corner and welded it.I finished up with a passable job - after all it is all under the table top and no one sees anything. (The table is about 800 mm x 800 mm x 250mm high ). But I can see that some of the legs are a couple of degrees out of square and I noticed when screwing the table top onto it that the square frame had a 'twist' in it. It wasn't perfectly flat. The screws pulled it into shape but it raised the question for me.What do you professionals do? I'm imagining I need some sort of  jig. I know I've got to stop using the plywood (it burns, doesn't it?) and I'm looking to see what I should use instead.Do I have to get an enormous piece of flat steel to work on? Do I have to make up a jig for the right angle corners?  Is there some sort of standard tool you all use - I'm thinking maybe something can be made after the style of a frame rather than a whole flat table?Or is my current practice the way to do it and I just need to use steel instead of wood and be more careful?(I've also got a 2mm difference in the length of the legs. My tiny angle grinder I used to cut the steel and put a grinding wheel on to finish the cut end. I think I made them all exactly the same so the difference would have come from the clamping method, I guess)Last edited by abrogard; 07-10-2007 at 09:34 PM.
Reply:The correct tools, skill and experience. Basically that is what it boils down to. You will learn, as you already did, that as metal is heated it warps or moves. You have to compensate when welding. Clamp down the work as best possible - tight and hard, tack one side, then the other. Once you get used to it, you can easily take your T-square, check level and then tack a piece ...without clamping. Practice, practice, practice. John -  fabricator extraordinaire, car nut!-  bleeding Miller blue! http://www.weldfabzone.com
Reply:Square it..Clamp it..Re square it..Tack it..Check it..Tack it somemore..Recheck it..Weld it all the way around without stopping on a corner...Re check it...Beat it into submission with a big hammer while hot if needed...You'll get it after awhile......zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Thanks for the super-rapid reply. So I take it my technique is good enough, I've got the 'correct tools', I just need to develop the skills and that'll come with experience.....okay... back to the workbench.  I think I might make a right angle jig and see if it helps me, though. Going to make a little chair for my four year old next, same job as the table, square of steel hollow section and four legs, only smaller and perhaps I need bracing between the legs for a chair?I'm thinking that holding the pieces in a jig will hold them firmer than clamping only the one end of the leg that is against the steel it is being welded to.But perhaps if you weld badly (too hot, warping) it'll deform as soon as it is released from the clamps?And if you weld correctly you need nothing....  as you say? Only a T-square check.
Reply:After a while you will learn to hold things a little "past" square because your first weld will pull it over a little.  Its a feel thing.For legs or long things, I measure diagnol to the corners.  I can see if its out of square better that way than any other.   X check.DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:What do you professionals do? I'm imagining I need some sort of jig. I know I've got to stop using the plywood (it burns, doesn't it?) and I'm looking to see what I should use instead.
Reply:Check, tack, check...as suggestedApply the correct amount of heat, coffee table is not a work bench so don't over weld.  Save welding the inside corners until the end.Welding magnets are nice, though no substitute for checking squareness as you go.  Don't wait until the end.It helps to work on a level, sturdy surface.  Wood works alright for a jig as long as your are keeping up with your work and not depending on things to be straight just because it is a jig.Also, plan your build.  If one side is a mirror image of the other, then build one side flat in the jig and use the first side as the jig for the other, the erect the table or chair by adding cross members.I use ratcheting tie down sometimes to pull diagonals into alignment.  Be careful because they burn.Last edited by tapwelder; 07-10-2007 at 11:03 PM.
Reply:I think I've got it, guys: tack, tack, tack, check, check, check, and a little judicious persuasion from time to time might make all perfect. I'll go back to the bench.....  thanks to you all....
Reply:It would help if you had a steel table to work off of.  But like the other guys said check, tack, check, tack, check, etc.....I don't rely on a square much.  If you're building something that is pretty much a box just cross square it with a tape, assuming your pieces are right.DewayneDixieland WeldingMM350PLincoln 100Some torchesOther misc. tools
Reply:Something that hasn't been mentioned yet is to watch the heat input.   If you have alot of welds to make, weld a little in one area and stop, let it cool there, weld a little over somewhere else, jump around abit to keep the heat from getting concentrated too much in one area and cause warping.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:Putting legs on a table can be harder than it originally seems because you have 2 angles to square up.Just measure, tack everything in, measure your bottoms that are up against the tabletop piece, and make sure those are the same measurements you get at the end of the legs.  Keep tapping them into place, make more tacks, tap into place, tack and weld.  It's basically just a lot of fidgeting.Of course, having another person really helps as they can keep it square on both angles while you tack.WElding magnets can help in situations like this too, so that your 90's are kept at 90.I've had problem with stuff like this in the past, but luckily it hasn't been stuff where a couple degrees here or there really mattered."A winner isn't someone who doesn't lose, a winner is someone who doesn't quit."
Reply:I also try and keep some flat bar 'L' brackets around, various dimensions. They'll let you clamp and still leave the corners and edges exposed for tacking and even welding. If you don't have any save your cut offs and end pieces. Make what you want when you need it. Scrap flat stock and all those odd bits of angle can make a lot of temp jigs.
Reply:Two years ago!   Guys, if any of you are still watching, I'm sorry I didn't acknowledge your posts. I don't know what went wrong. Perhaps that was when we moved house. My table and a couple of stools worked out alright but all got a twist in them.  I think perhaps the true flat steel welding table might be a good thing to have.  ab
Reply:I like to keep different length 1'-3'  size 3/8" sqaure tubing for using to brace legs on 2 angles by putting small temporary tacks till welded.Work the iron! Don't let the iron work you!
Reply:Like they all said tack,tack, tack,tack, check. Also tack all the stiffeners and braces in before welding anything, especially on the thin stuff.
Reply:I stuggled with this too and wasted a lot of materials and confidenece until I bought an aircraft construction book on making steel frame fuselages.  The guy outlined great strategies.  Basically when you weld things shrink... When molten, little expansion occurs because stuff flows and oozes against the stress of expansion and is accomodated, but when it cools it hardens, it shrinks and can lock in stresses.  The whole theory is to make allowances for the shrinkage, limit heat by squencing the welds around the frame and recognizing that stresses will build in a "constrained" shape.  Basically envisioning things that you can't see or haven't happened yet.Andy
Reply:The best way is to tack the whole table together with very tiny tacks at every corner of the square tube, squaring as you go. It is easy to cut a tack with grinder if you have to. When it is all together you can weld it and all the tacks will hold it square.
Reply:One thing to throw in with everything else, if your working with square tube frames I have found it best to weld the 2 flat surfaces first, then the outside and lastly inside.  The inside will tend to pull the most, so by welding the other joints first it will pull less.Have we all gone mad?
Reply:7 years late:  thank you everyone for your advice.
Reply:WOW !!! The little one never grew in your avatar
Reply:He's turned into a fierce warrior, mate:   I have to watch my step.  What hasn't grown is my welding ability.  p.s.  he doesn't know it but when I look at him I see the avatar.
Reply:you're probably not needing anymore advice but here goes, if the ends of the material isn't cut straight (and square) then this will further add the difficulty of ending up with a square and flat...whatever you're building
Reply:Originally Posted by abrogardHe's turned into a fierce warrior, mate:   I have to watch my step.  What hasn't grown is my welding ability.  p.s.  he doesn't know it but when I look at him I see the avatar.
Reply:Originally Posted by LtBaddyou're probably not needing anymore advice but here goes, if the ends of the material isn't cut straight (and square) then this will further add the difficulty of ending up with a square and flat...whatever you're building