Auto frame welding

I am about to take a pair of mid-70s mild steel Jeep Cherokee frames (not modern heat-treated) and use sections of one as pre-formed panels to fully
box the other. I do not have a flat concrete pad to jig it up on, but have lots of big channel to use. I am considering setting it on a rotisserie, attaching a pivot to each end of the frame so I can rotate it at will, allowing easy access to the bottom to do the welding with 2 inch stiches and spread them out to minimize warpage. Would I be better off making a rigid jig, levelling it and securing the frame to it? Would that kind of tight setup actually reduce warp, or just hide it until it is unclamped? I think the rotisserie will allow me to see what is going on and control it, but i'm a machinist, not a professional welder.
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Why would you want to bother doing something like this? All you are likely to achieve is considerable distortion (without using a pretty hefty jig), increased weight, and only marginally improvements regarding strength.
If you must do this, then you will need to make an accurate jig using something like H section steel, with properly machined mounting brackets for suspension pickup points, and ideally a couple of further mounting points on either side, to which the chassis rails can be clamped down to.
Also best to have new chassis sections folded up with press brake, which will fit nicely inside the existing rails, which may mean you would be able to get away without using a jig, if the new sections were to be stitched in rather than fully welded.
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The frames are only boxed for 1/3 of the length, and I am building a high clearance, low speed rock crawler. I want to reduce flex and prevent the fatigue cracking that goes with it, as the new suspension will do all of the flexing needed. Unless you are supplying steel, I will use what I have.
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wrote:

Just in case you're not local, I'd point out that Guy Fawkes is a resident nutter, troll and general sad case (Googling him is amusing). He's not worth listening to, not even for amusement potential.,
If I were working on this sort of chassis, even for a rock crawler, then I'd find a flat concrete base to work on. It's not that hard, and the time-saving is worth having. If I couldn't manage this, then I'd do the best I could with 4 carefully levelled paving slabs, one under each corner. You can do an awful lot of support with a stack of wooden pallets, if you have the time, but you do need some sort of datum to measure from.
A long bricklayer's level is a godsend. If you don't have one, make yourself a big wooden A-frame plumbline level instead. A "height stick" (metre rule with attached plumbline) is handy too.
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Maybe a troll/nutter..................but looking at the nonsense you have posted, at least a troll who knows what he is talking about.........................lol
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Look, guys, I don't want to play the personality game.
Simple question. Will a heavy framework holding the frame to be welded control warpage, or just disguise it until unclamped?
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Properly done, a strongback will reduce the warpage, will not elimnate it completely. But most vehicle fames are not particularly straight anyway. The trunion rig will allow you to space your welds out and keep distortion down. Just make sure any weld is compensated on the opposite side of the vhicle.
Is this a unibody vehicle or one with a sperate frame?
Stupendous Man wrote:

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A normal frame, factory boxed for less than half the length.
http://home.goldrush.com/obsoelyt/MobileCrane.jpg
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Eliminating distortion would require 'cold fusion'. Distortion can be controlled but it is more of an art than a science and experience helps. One of the most common weld faults is overwelding and this is usually a major contributor to unwanted distortion.
There are many techniques that can help to reduce or control distortion including the use of strong fixtures to restrain the work while it cools. One of the problems with many fixtures is that they tend to get in the way of welding unless the fixture is very carefully designed for weld access and this usually interferes with its rigid holding ability. It is also harder to build a heavy fixture that you can roll in order to position the welds, few people make the same size welds overhead that they do on the flat.
For your frame job the first thing you want to avoid is overwelding and excess heat input. It is easier to control distortion and overwelding by using skip or intermittent welds but if you want to weld the frame solidly then use the smallest weld that will do the job and avoid long welds. A continuous weld will also avoid the stress riser that occurs at the ends of intermittent welds. A high heat setting and rapid travel speed will often result in less total heat input. Reducing the weld cross section by grinding the weld smooth will help (especially if it is done before the weld cools) and you may wish to do this anyway for appearance. For both distortion control and stress relief, it can be helpful to peen the hot welds to stretch the weld deposit as it cools, an air powered hammer or flux chipper can make this a lot easier.
The order and progression is important and keeping the welds short and back stepping will help. Multi pass welds will increase distortion transversely and large single beads will tend to distort longitudinally. Two welders simultaneously working on opposite sides of the same frame rail can help as the simultaneous shrinkage will be self compensating. It may result in more rapid heat input than desirable and tends to rush the job more than we would like, it works best on big heavy weldments like large beams.
Some suggest heating before welding but this is a lot easier said than done and requires a lot more heating than most of us want or are able to do, it also introduces a bunch more variables than most of us are able to manage. Some localized preheating can help but YMMV. Some controlled straightening using heat and shrinkage may be possible after weld completion but do not depend on this being practical for large areas. Some commercial shops would say to just weld it as best you can and then use large presses to straighten it after welding but that is a somewhat inelegant approach and requires heavy equipment.
Some may suggest welding the side frames separately and then straightening them in a press individually before adding the cross members and bracing. It will be easier to fit the boxing material and since the boxing will be continuous it will result in a frame with more even flexing and lack of hard or rigid spots at the cross members which will tend to concentrate forces and cause cracking in service. This method is a lot easier if the frame members are straight sided as they can then be easily fastened to a large channel beam to restrain the frame rail when welded. The rail can be placed on its side to allow both top and bottom welds to be made in the horizontal position, but I would not recommend this for the fitup I think you are proposing (butt joint) but would work well for a 'nested' (lap joint) or flat plate (corner weld) type of boxing.
You should make small welds all over the frame, do not start in one corner and weld solid to the opposite corner. Do not weld the top side solid then start on the bottom.
IMHO the best way is to visualize the way you would like the distortion to happen and try to make it distort so you end up with a helpful crown that will flatten when loaded. Do not start final welding until the frame is completely tacked and restrain it as best you can. Once the tacking is complete do a careful alignment measuring to make sure it is straight before you start the final welding and allow to cool completely and repeat the alignment measuring frequently as the job progresses.
Don't get in a hurry, don't overweld, peen lots, and most important keep it cool.
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