warping question

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We are getting ready to build a chassis jig and I am a little stumped on how to keep the jig from warping as we weld it. We are making it out of 4"x6" .250 My main concern is welding the 3 legs on the lower end of each 16' piece. If I clamped another 4" x6" to the one I am welding is there anything else I can do to help eliminate warping. If this thing warps it is going to be pretty much useless so any help would be appreciated.ThanksMark
Reply:Without knowing exactly what your doing I can only tell you to try to weld linearly along the beam . Welding it cross section will make that part shrink (thus shortening the length) and cause piece to warp. By the way , the piece can be straightened if you did warp it by doing the opposite. You can take a torch ,run it across the width -just enough to get it to change colors , and then cool it with an airgun immediately. This would be done to the opposite side of the bow or warpage. Hope that helps.Johnny
Reply:Search this forum-type "breaking back". Both Tek and Wroughtnharv each wrote responses to minimizing distortion.  Harv response supports Johnny's suggestion, though Tek offers info on minimizing it on the front end.  Both could be useful to you.
Reply:here is pretty much what we are building, I will search for "breaking back" and see what I can find there. We are mig welding it...FYI. Thanks! Markhttp://www.irvansmith.com/catalog2/p...sis_jigs.shtml
Reply:Just welding the legs to the rails will cause a kink at each joint location.  If you were building a pair you could tack them back to back before you start.     Here is what I would do:  Tack weld everything together and I mean everything including all cross members.   Tack bracing across the bottom of the legs so that the feet cannot move closer to each other.  I would use at least 2 by 2 angle or heavier.  Also cross brace the feet so that they don't lose location.  You should have some way to turn your unit over several times.  Tack welds should be very small but at least one inch long.Stand it up on edge and weld half of each joint between cross members and rails.  Stand it up the other way and weld half of each joint again.  Repeat until you have done all the welds that would be vertical when the jig is in working position.   Set the jig right side up and weld half of the joints accessable on the top surface.  Turn it over and weld half of the opposite joints.  continue until the job is done.   I know this is a lot of handling but what you are trying to do is balance shrinkage forces.  After it is cool break the tacks holding the bracing.  You likely will have a frame straight lengthwise because the welds will balance out on each side.  My bet is that you will have a slight camel's hump over each leg connection.  To remove the humps over the legs heat the corner for about three inches with a torch duplicating the heat you would produce from a weld below at the leg connection.  I would heat for two inches on one corner over each leg then let cool.  The heat should be to a full red and travel along at the speed of a weld.  Do not heat again.  Go one to the next leg positon.  YOu can then take a wet rag and cool all the heated areas to room temp.  Don't put a wet rag on the red metal.  Let it at least cool to black naturally.  Put a straight edge on the top rail and check to see if you have to repeat.
Reply:thanks, sounds like this should work, we are going to start on this Friday, I will let you know how it turns out. Do you see a problem with just tackwelding the legs and the crossbraces in this application, or am I going to be fighting the warping no matter how much I weld? I guess the good thing is I can heat up the opposite side and eventually get it to pull back straight if needed.
Reply:I was going to mention that.  Don't bother welding horizontally on the large tube connections and limit the welds on the legs and bracing.  The frame you are building is for rigidity rather than strength.  The connections will be strong enough even if they are not completely welded closed.  If it was my frame that is what I would do.  Looking at the picture I suspect the railing on the top surface is set on level and there would be the odd location that there would be a 16th gap.  The railing need only be stitch welded.  Weldments are never perfectly true.  If you want that you weld then send out for stress relieving in a furnace before you  do your machining.
Reply:My bet is that you will have a slight camel's hump over each leg connection.